The Transistor Radio Handbook

RADIO HANDBOOK Theory --" - Circuitry -Equipment THE TRANSISTOR

RADIO HANDBOOK

(First edition)

-Theory -Circuitry -Equipment \c

at - by Donald L. Stoner and L. A. Earnshaw

Published and distributed to the electronics trade by

EDITORS and ENGINEERS. Ltd.Summerland , California DiaIsm: Electronic distributors, order from us.Bookstores,libraries, newsdealers order from Baker L Taylor,Hillside,N.J. Export (exc. Canada), order from H. M. Snyder Co., 440 Park Ave. So., N.Y.16. Other Outstanding Books from the Same Publisher (See Announcements at Back of Book)

THE RADIOTELEPHONE LICENSE MANUAL

THE SURPLUS RADIO CONVERSION MANUALS

THE WORLD'S RADIO TUBES ( RADIO TUBE VADE MECUM

THE WORLD'S EQUIVALENT TUBES ( EQUIVALENT TUBE VADE MECUM

THE WORLD'S TELEVISION TUBES ( TELEVISION TUBE VADE MECUM THE TRANSISTOR RADIO HANDBOOK

1st EDITION

TABLE OF CONTENTS

Chapter One - INSIDE SEMICONDUCTORS 9 1-1 What is Matter 9 1-2 Building Blocks of the Universe 10 1-3 Atomic Structures. 12 1-4 Crystal Lattice Structures 12 1-5 Impurities 13 1-6 Junctions 15

1-7 Diode Action . 17 1-8 Transistor Action 19 1-9 Transistor Construction 23

Chapter Two - AUDIO AMPLIFIERS 28 2-1 Circuit Configuration 28 2-2 Bias Considerations 29 2-3 Stabilization 30 2-4 Transistor Impedances 32 2-5 Interpretation of Transistor Data 33 2-6 Load Lines for Transistors 34 2-7 Amplifier Considerations 36 2-8 Miscellaneous Audio Circuits 39 2-9 Semiconductor Speech Amplifier 40 2-10 A Deluxe Audio Compressor. 43 2-11The Mini -Amplifier -Modulator 46 2-12 The Mini -Amplifier #2 47 2-13 A 10 -Watt Amplifier/Modulator 48 2-14 A Kit 10 -Watt Modulator 50 2-15 A 5 -Watt Class A Amplifier 52 2-16 Sliding Bias Amplifiers 53

5 Chapter Three- R.F. CIRCUITS 55 3-1 Alpha Cutoff 55 3-2 Impedance Matching 56 3-3 Neutralization 57 3-4 RF and IF Amplifier Stability 58 3-5 Detection 59 3-6 The Mixer and the Converter 60 3-7 Automatic Gain Control. 62 3-8 Self -Excited Oscillators 63 3-9 Crystal Controlled Oscillators 66 3-10 Transmitting RF Amplifiers 68 3-11Modulating RF Circuits 69

Chapter Four- RECEIVERS 71 4-1 The Superregenerative Detector 71 4-2 Simplex Crystal Set 73 4-3 Simplex Audio Power Pack 74 4-4 The TR One 76 4-5 The TR Two 77 4-6 The Solar Two 79 4-7 A Regenerative Receiver 79 4-8 The Super Three 81 4-8a Constructing IF Transformers 85

4-9 A Four Transistor Superheterodyne . 86 4-10 The Mobileer 89 4-11The Product Detector 92 4-12 A Professional Communications Receiver 94 4-13 Crystal Filters for a Communications Receiver 103 4-14 A Mechanical Filter for the Communications Receiver 105 4-15 A Transistor All -Band Converter 106 4-16 Autodyne Converters 111

4-17 A Transistorized Six -Meter Converter.. 113

4-18 A Transistorized Two -Meter Converter . . 115 4-19 A 220 Mc. Transistorized Converter 117

6 Chapter Five - RF POWER AMPLIFIERS 120

5-1 RF Power Amplifiers. 120 5-2 RF Oscillators 123 5-3 Linear Amplifiers 125 5-4 Modulation. 127 5-5 A Transistor Phasing Exciter 128 5-6 A Transistor Filter Exciter 136 5-7 40 Meter SSB Transceiver 138 5-8 A VFO for AM, CW, or SSB 143

5-9 A 3 -Watt C.B. or 10 -Meter Transmitter . 146 5-10 A Two -Meter Transmitter 147 5-11 A Potpourri of Circuits 149

Chapter Six - POWER SUPPLIES. 156 6-1 A.C. to D.C. Conversion 156 6-2 D.C. to D.C. Conversion 161 6-3 D.C. to A.C. Conversion 171 6-4 A Low Cost Power Converter 171

7 FOREWORD TO THE FIRST EDITION

In all probability this is the first transistor manual written by two authors in two widely separated countries.Transistors are transistors the world over, and in each country they function the same. Different environments engender different ideas, however. One cannot imagine an Eskimo inventing a plow, a Bolivian inventing a kayak, or an American inventing a set of chopsticks. However, if one gave an American a set of chopsticks and said "Improve on these," no doubt he would. Through the medium of amateur radio, the authors have discussed the subject of transistors back and forth across the Pacific ocean. We sincerely hope that the synthesis of our ideas have produced worthwhile results. While we make no claim to having invented a new kind of plow or a better kayak, we do feel that by adding small bowls to the ends of the chopsticks and calling them spoons, we have improved on something.Bowls are not new, nor are chopsticks, but a combination of both is a very useful tool. The authors particularly wish to thank the following individuals and companies who have provided assistance and encouragement in this project.

Mr. Jules Ruben Allied Radio Mr. Stanley Issacs Lafayette Radio Mr. Irving Seligman Irving Electronics Mr. Bill Courtney J. IV Miller Coil Co. Mr. E. P. Kelly Stancor Mr. Ed. King Amperex Semiconductors Mr. John Vadja Motorola Semiconductors Mr. William Wilson International Rectifier Corp. Mr. C. D. SimmondsPhilco Semiconductors Mr. Frank O'Brien Pacific Semiconductors Mr. John Fischer Radio Corporation of America Major Gilbert Texas Instruments

8 CHAPTER ONE

Inside Semiconductors

The reader need not understand all the A compound, on the other hand, is a mysteries pertaining to holes, covalence pure substance which contains two or more bonds, etcetera, to construct transistor cir- elements and has a constant composition. cuits and equipment.All that is needed is It usually exhibits properties different from a good magazine article and a few clear the elements of which it is composed. A photos.However, this basic information very abundant compound is water.It is isessential either to troubleshoot equip- composed of two parts hydrogen and one ment properly or to design circuits your- part oxygen and is known by chemical self.If you are content to follow the leader, shorthand as H20. omit this chapter. Before delving into the operation of a The Elements are thought to be com- transistor,it would be useful to review a Molecule posed of molecules. A molecule bit of high school chemistry regarding the is the smallest physical part of nature of matter. an element or compound.If one divides a piece of copper many times into the smallest part possible and still has copper, this 1.1 What is Matter? small part would be a molecule.

That'ssimple. Matter isjust about The Scientists believe that molecules everything! Typical examples are the air Atom are constructed of various ar- we breathe, the water we drink, and the food rangements of atoms. The we eat.These examples also illustrate the molecule may contain one or more like three basic forms of matter: gaseous, liquid, atoms in an element or two or more differ- and solid.Matter may be found in these ent atoms in a compound.Figure 1.1-A forms as either elements or compounds.An shows the relationship between the atom element is defined as abasic structure which and the molecule.Two of the simple cannot be separatedinto substances of hydrogen atoms combine with a single other kinds.Scientists have been able to atom of oxygen to form one molecule of isolate over 100 elements.Some of the the compound water. more familiar ones are copper, aluminum, Since there are over 100 elements, it is hydrogen, and oxygen. Lesser known ele- reasonable to expect that there exist the ments, which we shall study later, including same number of atomic structures which silicon and germanium. correspond to these elements. As will 10Inside Semiconductors The Transistor

0000001 INCH ® =ONE MOLECULE Of WATER

TWO ATOMS OF HYDROGEN PLUS ONE OXYGEN A TOM . H2O Figure 1.1-A THE RELATIONSHIP BETWEEN ATOMS AND MOLECULES

MAG. 10,000,000 X soon be shown, the atom is made up of Figure 1.2-B subatomic particles.The same particles make VAGUE OUTLINE OF THE ATOMIC up all the atoms in the universe. Only the STRUCTURE OF ALUMINUM manner in which they are arranged varies, and it is the arrangement which gives each atom its own characteristics. This may be shiny, and may be drilled, punched and easier to comprehend if one compares the formed into angles.If, however, the sur- atom to a fingerprint.The fingerprint is face is magnified 100 times,it becomes composed of simple loops and whirls, but obvious that the aluminum is not smooth, they can be combined in an infinitenumber but actually has a rough crystalline surface of ways, and no two fingerprints are alike! as shown in figure 1.2-A. Criminologists are convinced that all the If the surface is magnified 10 million fingerprint types in the world have notbeen times, one begins to see vague outlines of discovered. Similarly, scientists assume that roughly spherical shape. These tiny blobs there are more elements which have not been are the atoms of aluminum, and may be identified. "seen" in figure 1.2-B. No one has seen an atom.However, with X-ray techniques and advanced mathe- 1.2 Building Blocks matics, it is possible to make some rather of the Universe accurate guesses about their appearance. Scientists believe the atom looks like the drawing in figure 1.2-C.It consists of a For the purpose of illustration, let the central core with one or more tiny particles reader equip himself with a more power- orbiting around it as planets orbit the sun ful microscope than exists in the world. in the solar system. With the microscope, examine a piece of To simplify explanations and drawings, aluminum. Aluminum is usually associated with chassis and panels.It is smooth and

MAG. 100,000,000 X MAG. 100 X Figure 1.2-C Figure 1.2-A SCIENTISTS BELIEVE THE ATOM LOOKS CRYSTALLINE SURFACE OF ALUMINUM LIKE THIS. Radio Handbook Atoms 11

.000000000001 the atom is usually depicted as shown in INCH figure 1.2-D; in this case it is the "chemical schematic" of an atom of aluminum. No- tice that this atom of aluminum is more complex than the hydrogen atom shown in figure 1.1-A.

Composition Referringto figure 1.2-D, the of the Atomcentral core of the atomis MAG. 1.000,000.000, 000 X termed the nucleus.If it were Figure 1.2-E possible to examine the nucleus of the alum- THE NUCLEUS OF THE ATOM IS BE- inum atom under the imaginary micro- LIEVED TO LOOK LIKE THIS scope, one would see something similar to It contains both positive protons, and figure 1.2-E. The nucleus is composed of neutrons which have no charge. neutrons with no charge and protons which have a positive charge.These components make up the heart of the atom. If you can comprehend a copper penny Note in figure being enlarged to the size of the earth's 1.2-D that the nucleus of the atom consists orbit around the sun, then you would be of 13 protons and 14 neutrons. Each of the tiny particles orbiting the able to visualize the electrons in the copper. They would be roughly the size ofbaseballs nucleus is a negatively charged electron. The and might be spaced approximately three electrons whirl attremendousspeeds around the nucleus in orbits or rings, as miles apart! The nucleus, thecentral "sun," would be composed of a group of neutrons they are more commonly called. Note fur- and protons about the size of walnuts! ther that the aluminum atom has two elec- All matter is composed of atoms.By trons in the first ring, eight in the second, the same token, all atoms are built up from and three in the outer ring, for a total of electrons, neutrons, and protons.Only 13 electrons. the way in which the parts are arranged and their quantities will differ.Thus, these Size It might be helpful to describe particles may be considered the fundamental these particles in definite terms. building blocks of the universe. The diameter of an electron is believedtobe 0.00000000000022 or 22 x 10'4 inches and is about three times Charges The positive charge on thepro- the size of the proton. Despite its smaller ton and the negative charge on size, the mass of the proton is about 1,850 the electron are exactly equal in times that of the electron. The comparison the aluminum atom discussed earlier. The is something like a cubic foot of lead to a 13 protons equal the 13 electrons, and this cubic foot of feathers. aluminum atomissaid to be electrically balanced. These charges are believed to be the 1ST RING. 2 ELECTRONS NUCLEUS CONTAINING 13 PROTONS AND 14 smallest that exist, and therefore they are NEUTRONS. considered the fundamental unit of electrical charge. However, this quantity is much too small to deal with, and the coulomb is 2ND RING, 6 ELECTRONS 3RD RING, 3 ELECTRONS commonly used in its place. Onecoulomb Figure 1.2-D of electricity contains more than six million, A SINGLE ATOM OF ALUMINUM million, million, or 6.28 x 1018 electrons! 12Inside Semiconductors The Transistor

1.3 Atomic Structure IST RING- 2 ELECTRONS The most important feature of the atom 2ND RING- 8 ELECTRONS 3RD RING- 4 ELECTRONS is the fickle outer ring of electrons. The atom is somewhat similar to the common Figure 1.3-8 household onion in that it is composed of SILICON ATOM (Si) many rings.It is relatively easy to remove pieces of the outer ring simply by peeling con atoms.Each atom has four valence them off.The inner rings do not fall off, electrons in the outer ring. Since the action for the outer rings hold them in place. occurring in the atom takes place in the The inner rings of the atom are tightly valence ring, the atomic schematic may be bound to the nucleus, but the outer ring further simplified by showing only the val- may be stripped off.More commonly, ence electrons, as in figure 1.3-C. When subtle modifications are made to change its reduced to this form, both germanium and characteristics. silicon appear to be identical, and as a It is also known that the electrons in the matter of fact they are equally useful in outer ring may skip from one atom to manufacturing transistors. another in a random manner due to thermal The reader should understand that dif- agitation (heat).Also, electrons may be ferentmaterials may contain a different added or removed from the outer ring. number of valence electrons. However, just Electrons able to move in this manner are because two atoms contain the same number known as valence electrons.Any matter with of valence electrons, they do not necessarily a large number ofvalence electrons is known constitute the same material. Note that in as aconductor, and matter which has few aluminum the valence ring contains three loosely held electrons is termed an insulator. electrons, as does gallium, which is also Materialsreferredtoas semiconductors lie used in manufacturing semiconductor pro- somewhere between these two extremes. ducts. Two other elements useful in pre- Germanium and silicon are two excellent paring semiconductors are antimony and examples of semiconductors.These two arsenic, each having five valence electrons. elements are not very good conductors in their pure states. When properly modified 1.4 Crystal Lattice they can be made to conduct electricity, much the same as a piece of wire. Structures The atomic schematics for germanium Most substances when examined under and silicon are shown in figure 1.3-A and a microscope will exhibit a crystalline struc- 1.3-B, respectively. For simplicity, the neu- ture, even though the surface may appear trons found in the nucleus are not shown. smooth to the eye.All crystalline sub - Since neutrons have no charge, they do not affect our discussion.Note also the similarity between the germanium and sili-

1ST RING- 2 ELECTRONS 2ND RING - 8 ELECTRONS Figure 1.3-C 3RD RING -18 ELECTRONS A SIMPLIFIED DRAWING SHOWING 4TH RING - 4 ELECTRONS ONLY THE VALENCE ELECTRONS This could be either the germanium or Figure 1.3-A silicon atom since they both have four GERMANIUM ATOM (Ge) valence electrons. Radio Handbook Atomic Structures13 stances have an identifying characteristic. The most common example would be the elusive snowflake, which is composed of an infinite number of geometric patterns made EXCESS ELECTRON up of 60° angles. Common household salt will form tiny cubes, while some materials form needles, rhomboids, or variations Figure 1.5-A of hexagonal or rectangular structures. A "DONOR" IMPURITY HAS BEEN By scientific deduction, it appears that ADDED TO THE PURE GERMANIUM the rotation of the valence electrons in one INTRODUCING AN EXCESS ELECTRON atom is closely coordinated to the electron motion in adjacent atoms. Thecoordinated rotation forms an electron pair bond or a like polystyrene is a poor conductor, and covalence bond. This bond is countermanded examination shows that its valence electrons by the repulsion of the positively charged are very tightly bound together. nucleus and a state of equilibrium exists, forming a single crystal of the material. 1.5 Impurities The lattice structure of a single germanium crystal is shown in figure 1.4-A. For simplicity, only one "layer" ofthe structure Earlier it was stated that silicon and germ- is shown. Although it is not possible to anium could be made into conductors by illustrate the fact with two dimensional draw- modifying their structure. These structures ing, each germanium atom is bonded by were considered to be pure, that is, contain- electron pairs to four adjacent atoms (or ing nothing but germanium or silicon would be if an infinite number could be atoms. To be usable in semiconductors, illustrated).This is due to the fact that germanium and silicon must be refinedto the each atom contains four electrons in the point where only one impurity will be outer or valence ring. found in 10 billion parts of the pure mater- In the element copper, the valence elec- ial. trons are not tightly bound as just des- When this state of purity is achieved, a cribed, but are free to move about. Thus, controlled amount of impurities may be added when subjected to an electric potential, the to modify the structure in a desired manner. valence electrons move in an orderly man- Such impurities might be antimony, arsenic, aluminum, or the gallium mentioned earlier. ner. Such material is said to be a good conductor.On the other hand, material A typical example might be the pure germanium structure discussed in section 1.4. Impurities "dope" the element by adding one part antimony (five valence electrons) to 10 million parts of germanium. An examination of the germanium structure >COVALENT BONOS now will show an extra atom in the group (figure 1.5-A0, a single atom of antimony. A closer look will show that the atom is accompanied by an excess electron.Re- member that germanium has four valence Figure 1.4-A electrons, while antimony has five.Since A STABLE LATTICE STRUCTURE CON- there can be no more than four electrons TAINING ONLY GERMANIUM OR inany of the valence rings, this excess SILICON ATOMS. electron must drift through the structure 14Inside Semiconductors The Transistor

looking for a gap to occupy. Fortunately for the transistor industry it never finds one!The excess electron may combine with one of the valence rings, but it will always drive out an electron, leaving one extra electron in the structure for each excess electron added. Figure 1.5-B The impurity that has donated the extra AN "ACCEPTOR" IMPURITY IS SHOWN electron is logically called a donor impurity. ADDED TO PURE GERMANIUM Arsenic could also be the donor since it Valence electrons try to fill this space, has one extra electron when compared to but a gap always remains in the germanium or silicon.Although the dis- structure. cussion has dealt with single structures and a single excess electron, the reader should be aware that millions of these excess elec- purities have only three valence electrons. trons are present in millions of lattices, What happens to the atomic structure when even in a piece of germanium too small to these impurities are combined with the be seen with the naked eye. germanium or silicon? There is no longer Germanium to which a donor impurity an excess of electrons, but rather a lack of has been added is called N -type germanium, them. Thus, this impurity is able to accept since it contains an excess of electrons which electrons from the germanium, leaving a carry a negative charge. bole in the valence rings.This type of impurity is referred to as an acceptor impurity. To illustrate what might happen in a Acceptors Just as we have added the anti- single crystal of germanium when "doped" mony and arsenic impurities, it with acceptor impurities, refer to figure ispossible to "contaminate" 1.5-B. The impurity atom might withdraw pure germanium with aluminum or gal- an electron from the adjoining germanium lium.As mentioned earlier, these two im- atom ina futile effort to neutralize the

SEMICONDUCTOR RECTIFIERS AND TRAN- SISTORS TAKE MANY SHAPES AND SIZES. These are only a few of the package styles you willfindinelectronic equipment. (Courtesy, GeneralElectric Company) Radio Handbook Junctions15

structure. However, this leaves a gap in the adjacent atom's valence ring which might be filled.Our incomplete atom is compelled to become whole again and it,in turn, withdraws an electron from another atom. This occurs continuously, and in a random manner. The hole is simply a gap in a valence ring, which has no electron to fill it.Since there is continuous agitation in the structure (due to ever-present heat) the gap or hole is continuously changing position. As before, there are millions of these impurity atoms in a tiny speck of germanium, and, consequently, there are also mil- THESE POWER DIODES WILL DO THE lions of holes seeking an electron. Although SAME JOB AS A 5U4 RECTIFIER the holes are moving, they are evenly dis- But they will provide more output volt- tributed and never leave the confines of the age for an indefinitely long time. germanium. A piece of germanium doped (Courtesy, General Electric Company) in this manner is called P -type germanium, since it lacks electrons and therefore must carry a positive charge. space (the hole) toward the entrance. An analogy which might help clarify the When a potential is applied to the P -type function of the hole is the classic garage germanium, the holes will move in a very full of cars illustration. As shown in figure orderly fashion and in one direction, which 1.5-C, the 12 -car garage is nearly filled to constitutes an electric current.In N -type capacity with 11 automobiles.Naturally, germanium, the electrons move from the the only empty space is at the rear of the negative to the positive source while in P - garage. When one drives up to store a car, type material, the holes traverse from posi- the attendant moves the front car forward tive to negative. The holes (P) or electrons into the space. He then moves the second (N) are termed the majority carriers since they car up a space.Finally, after moving each make up the majority of moving particles. car up a space, he makes way for the 12th car. Although the attendant has only moved cars forward, in effect, he has moved the 1.6 Junctions

The readerwith a fair imagination

C[21 C C CI should be able to visualize what happens

CAR MOVEMENT V. inside a piece of impurity -doped germanium or silicon, whether type N or type P. C C CCJ SPACE "HOLE" MOVEMENT Consider nowwhat happens when

C C 121 blocks of N -type and P -type materials are "sandwiched" together to from a P -N junction(figure 1.6-A.)The situation pro- Figure 1.5-C duced by joining of P and N type material CLASSIC EXAMPLE SHOWING MOVE- issomewhat complex.Remember that MENT OF HOLES when manufactured, P -type material is elec- 16Inside Semiconductors The Transistor

P -TYPE N-TYPE

<0 .0 O. a. 0 0 00 0 0 ATOMS

I 0'0 00 0 O00 o HOLES o I

0 00 I O00 ELECTRONS * (EXCESS)

DEPLETION REGION SHOWING RECOMBINATIONS Figure 1.6-A TWO "BLOCKS" OF GERMANIUM ARE JOINED TO FORM A JUNCTION

battery to force reluctant electrons across the barrier. The physical width of the barrier is ra- PROBABLY THE WORLD'S SMALLEST ther indefinite and is usually measured in SEMICONDUCTOR DIODE terms of the voltage necessary to drive an The "Tiny Tim." That's not a 16 -penny electron across the depletion region. With nail;it'sa common household pin! no external potential applied, the barrier (Courtesy, Pacific Semiconductors, Inc.) width may be as high as 0.6 volts. This is called the barrier potential. trically neutral, as is also the N -type material.Because the atoms in each are sharing electrons or holes, there is no actual surplus Forward If electrons can be made to of electrons or holes. A surplus or a de- Bias traverse the barrier with an ap- ficiency of electrons would mean that the plied potential, it is reasonable germanium (orsilicon) was electrically to assume that the P -N junction will con- charged. When the two types of germanium duct a current. are placed together the reader might im- To obtain forward bias, the positive term- mediately assume that the excess electrons inal of a battery is connected to the P -type in the N -type material would join the holes germanium and the negative terminal of the in the P -type material. This is not the case. battery is connected to the N -type german- Although a few recombinations do occur, these ium.If one could examine the inside of are caused by thermal agitation. The two these slabs of germanium (figure 1.6-B) it sides actually set up a barrier, preventing would be seen that the holes are being total recombination.If an electron crosses repelled from the positive terminal and the from the N zone to the P zone, both electrically neutral pieces will now take on a P -TYPE N -TYPE OO charge.The N zone will have lost an elec- CS. C.* 0 ATOMS O00 Co OO O tron and the P zone will have gained an 00 oHOLES 0- O 0 ; 00 -0 electron.The P zone will thus become ELECTRONS OO0 00"" O0O (EXCESS) negative with respect to the N zone. Thus, Cw, 00 ELECTRONS 00 FROM VAL- the negative P zone will now repel the entry ENCE BONDS of further negative particles. Remember that like poles repel and unlike poles attract. In Figure 1.6-B fact,a barrierisset up between the two A GERMANIUM JUNCTION WITH FOR- junctions, and it takes a potential from a WARD -BIAS APPLIED Radio Handbook Diode Action17

electrons repelled from the negative term- TYPE -P TYPE -N inal. FILTER CAPACITOR This electric pressure against the de- ACTION pletion region greatly reduces it, allowing large quantities of holes and electrons to LOAD recombine.For each recombination that occurs, an electron from the negative term- Figure 1.7-A inal of the battery enters the material. Sim- SINGLE P -N JUNCTION USED AS A ilarly, an electron breaks its bond and enters RECTIFIER the positive terminal of the battery. At the same time an electron from the negative terminal enters the junction to repair the Actually the resistance does not go to in- bond. The action is continuous. Recom- finity, for a small amount of recombination does occur. This represents bination about the depletion region occurs a tiny reverse so long as the battery is across thejunction. leakage current caused by the minority current carriers.With a small voltage applied, the Connected in this manner, an electriccur- rent flows in the external circuit. leakage current will be onlya few microamperes.If excessive voltage is applied, the atomic stability of the latticemay be im- Reverse An entirely different effect takes paired, which breaks down the junction. Bias place when the battery isre- This effect does not damage the junction versed.Fora reverse bras con- unless excessive heat is generated dueto dition, the positive battery terminal iscon- heavy current flow. ected to the N -type germanium and the negative terminal is connected to the P- 1.7 type germanium. Diode Action

Once again inspecting the interior of From the precedingexplanationthe the germanium (figure 1.6-C),one finds reader can see that the diode will conduct that the holes in the P -type germaniumare when the applied potential is in theforward attracted to the negative end ofthe structure, direction and (for all practical purposes) while the electrons are drawn to the posi- will not conduct when reverse biased. tive end.This has the effect of stretching If connected as shown in figure 1.7-A, the depletion region, which increases the the junction becomes a rectifier whenan a.c. potential needed to overcome this barrier. signal is applied. During the positive half- Inactual practice, the barrier width in- cycle the depletion region is almost non- creases to the potential of the applied volt- existent (once the barrier potential is over- age and conduction does not occur. come), and current flows virtually unop- The reverse -biasedjunction may be posed.During the negative half -cycle the thought of as having a very high resistance. conditions are identical to reversing the battery (reverse bias) as explained earlier in P-TYPE N-TYPE section1.6.During the entire half -cycle .. 00: 0 0 the depletion region expands and contracts 0- 00o in proportion to the appliedvoltage. Thus, 0 00-o during the entire negative half -cycle, the 00 01 0 00 barrier places a very high resistance in the

DEPLETION circuit.The voltage appearing across the REGION load, then, is a series of positivehalf-cycles. Figure 1.6-C These may be used to charge up an electro- A GERMANIUM JUNCTION WITH RE- lytic capacitor to supply nearly pure d.c. to VERSE -BIAS APPLIED the load. 18Inside Semiconductors The Transistor

heat and limit the operation?Only 0.6 volts is "lost" inside the diode, and at 500 Figure 1.7-B ma. one finds that only 0.3 watts is absorbed THE SYMBOL FOR SEMI -CONDUCTOR (and must be dissipated) by the diode. DIODE Actually, thisisslightly utopian, for the In the case of a junction rectifier the diode resistance does not drop to zero. arrow corresponds to the anodewhich Even so, the losses are not likely to exceed is of type -P material.The bar is the one watt, which may easily bedissipated cathode and is of the N -type. The without cooling fins. arrow points INTO the directionof The seleniumrectifier,on the other electron flow. hand, has comparatively high losses. This can be proved without an atomicexplana- tion simply by examination. A typical 500 Silicon The P -N junction makes a very ma. selenium rectifier might be three inches Power effective rectifier in high voltage square, one inch thick, and contain five or Diodes and high current supplies, par- six plates with selenium oxide spread on ticularly when silicon is used.Silicon is the cooling fins. Compare that area to the able to perform its duties in the presence tiny 0.5 ampere diodes shown in the photo- of high temperatures and therefore is some- graphs! times preferable to germanium. The P -N junction contained ina 0.5 Silicon diodes are becoming extremely ampere silicon diode might be the size of popular with amateurs and experimenters a pin head.The junction is securely fast- and have virtually replaced selenium recti- ened to the metal case (which would fit in- fiers in all but the most inexpensive com- side a thimble) to provide all the heat dissi- The reason for their mercial equipment. pation necessary. popularity is the depletion region or potential barrier mentioned earlier.It requires some 0.6 volts to cross over thebarrier. Once this potential is exceeded, the diode acts as a virtual short circuit. Examine the power supply shown in figure 1.7-A.Apply 150 volts a.c. and connect a 500 ma. (0.5 ampere)load tothe output. As soon as the barrier is overcome ( 0.6 volts ), current surges through the load on the positive half -cycle.At the peak of 150 volts, the load is drawing 500 ma. 4.* What about the diode losses which generate

GERMANIUM USED IN THE MANU- FACTURE OF TRANSISTORS MUST POTENTIAL BARRIER HAVE NO MORE THAN ONE PART REVERSE FORWARD IN TEN BILLION OF IMPURITIES. The amount of impurity then intention- ally added to "dope" the pure germanium in order to form a N or P Figure 1.7-C typeisroughly comparable to two ZEN ER DIODE VOLTAGE/CURRENT people in the population of the United CHARACTERISTICS States. (Courtesy, CBS Electronics) Radio Handbook Transistor Action 19

Zener Earlierit was mentioned that Diodes excessive reverse voltages could break down the P -N junction. Certain diode types are cultured to exhibit this effect at a very low reverse voltage. When the breakdown voltage is exceeded, the lattice structure collapses and the electrons avalanche through the junction. Thus, nearly total recombination occurs. Through careful manufacturing tech- niquesitis possible to provide diodes which break down at discreet voltages,say 3.9, 4.7, 5.6 and so on up to several hundred volts.Such a diode is known as a zener diode, or more correctly as an avalanche diode. A diode which breaks down atsome exact voltage (figure 1.7-C) makes an excellent regulating device. THIS TINY VARIABLE CAPACITY DIODE WILL REPLACE THE CAPACITORS Junction Although not particularly appli- SHOWN Capacity cable to this discussion, an in- These "Varacap" diodes will undoubt- teresting point is the capacity edly be widely used in amateur equipment. exhibited by the depletion region. As the (Courtesy, Pacific Semiconduc- reverse bias on the diode junction is in- tors, Inc.) creased, the depletion region expands. The action is similar to moving the plates ofa be used to advantage by applying diodesto capacitor farther apart.This, of course, adjust the resonant frequency of tuned cir- effectively reduces the junction capacitance cuits.Such a device is known as a variable which in turn reduces the terminal capaci- capacitance diode. tance of the diode.By the same token, lower reverse voltages narrow the deple- 1.8 tion region, thereby decreasing the capaci- Transistor Action tance.This variable capacitance effect may The diode (or P -N junction) provides an excellent foundation of blocks upon which to build a transistor.The simple TYPE- P TYPE -N TYPE- P EMITTER BASE COLLECTOR junction transistor may be thought of as a

triple -deck germanium sandwich. 000 0 000 The 000 0 000 When a block of P -type german- P -N -P 0 ium is sandwiched between two \1t Transistorblocks of N -type germanium, DEPLET ON REGIONS two potential barriers develop ---one at each junction. Figure 1.8-A The block diagram of a P -N -P transistor P -N -P TRANSISTOR WITHOUT APPLIED is shown in figure 1.8-A.From left to POTENTIAL right, the blocks are termed emitter, base, and The holes and electrons drift aimlessly collector. The recombined electrons and about. holesinthe depletion regions are not 20Inside Semiconductors The Transistor

TYPE -P TYPE -N TYPE -P trons N -P -N) cannot be propelled to the EMITTER BASE COLLECTOR collector region. 2. In either transistor type, the collector must be reverse biased to prevent all the holes (P -N -P) or electrons (N -P -N) from going directly to the collector.If the collector base junction were forward biased, thebarrier would be greatly reduced, and the emit- Figure 1.8-B ter to base junction could not control cur- FORWARD BIAS rent flow. Forward bias of the emitter -base junction allows holes to recombine in the Transistor At this point the reader may collector, creating a current flow in the Gain relegate to memory the elusive reversed -bias base collector junction. atoms, electrons, and holes to deal with the more tangible external effects created by the transistor. The illustration, shown due to space limitations.For the figure 1.8-C, shows a more schematic -like purpose of explanation, connect twobat- version of figure 1.8-B.As before, the teries as shown in figure 1.8-B. Wired in emitter base junction is forward biased and this manner, the P -N junction (emitter to the base collector junction is reverse biased. base)is forward biased, and recombination Recall that a forward biased junction be- occurs.The holes that would normally haves relatively like a medium resistance, speed off to the battery are instead driven since the base emitter circuit allows some through the P -N junction (base to collector), electrons (or holes) to exit at the collector due to the greater attraction of the negative connection. collector for the positively charged holes. Under the bias conditions just described, If the forward bias battery is reduced to an external milliammeter willshow that zero, the emitter base barrier increasesand approximately one ma. is flowing in the reduces the amount of holes reaching the emitter circuit.Further investigation will collector.The action is quite similar to show that approximately 95% of this cur- that of the grid in a vacuum tube, which rent (0.95 ma) is flowing in the collector controls the electron flow between cathode circuit. This may be hard for the reader to and plate. reconcile since it has been emphasized that the emitter base junction is equivalent to a The The action of this transistor is low resistance, and the base collector junc- N -P -N identical with the P -N -P tran- tion to a much higher resistance. Why Transistorsistorexcept that the blocks doesn't more current flow in the emitter contain the opposite type of impurities, and base junction?Although not stated prev- the majority current carriers are electrons iously, the collectoris at a much higher rather than holes.Since electron carriers have a negative charge, all the battery polarities in figure 1.8-B must be reversed. Before proceeding, it might be wise to E B (P) (N) (P)C review two points which are important to the +0.95 1.0 I following discussion and which should be MA.+ I MA. 10.05 kept firmly in mind. 1. The emitter base junctions must be forward biased in any transistor, or the major- Figure 1.8-C ity current carriers (holes P -N -P and elec- CURRENT FLOW IN A P -N -P TRANSISTOR Radio Handbook Transistor Action21

potential than is the base, with respect to Ily constant voltage v, the emitter. An inspection of figure 1.8-C 1. Zener diodes,tiler will make this fact obvious.It can be seen ed in the reverse dirt that the collector -to -emitter voltage source 1 as a voltage regulat isactually two batteries in series, while ment. there isonly one battery between emitter and base.In practice, the emitter base po- +1 tential may be only 0.2 volts, while 1.5 to 100 or more volts may exist between collector and emitter. Another consideration is the width of the base region.The base width is quite narrow, and the number of recombinations of electrons from the emitter circuit and holes from the base circuitis very small, with only about 5% of the electrons leaving the emitter. The remaining 95% of the emitter electrons will traverse the emitter collector ZEN ER DIODES ARE MANUFACTURED depletion region and reach the collector. TO "BREAKDOWN" AT DISCREET This accounts for the division of current VOLTAGE. as shown in figure 1.8-C. They thereby provide excellentregu- latingcharacteristics. Note the "Z" shape of the voltage/currentcurve. AmplificationIf, in our experimental cir- (Courtesy, International Rectifier cuit (figure 1.8-C), the bat- Corporation) ery started to weaken, the terminal voltage would drop. The current in the emitter lector circuit. Stated another way, the tran- base junction might drop from 0.05ma. sistor exhibits current amplification.Simple to 0.03 ma.We know that this current mathematics will show that this transistor drop would widen the potential barrier and has a current gain of 10. The currentgain reduce the amount of electrons (or holes) ratio is termed beta (p) and is defined as the flowing in the collector circuit. For exam- change in collector current divided by the ple, in practice, the collector current might change in base current. The beta of a trans- drop from 0.95 ma. to 0.75 ma. Thus, a istor is always more than one (assuming it change of 0.0 2 ma. in the emitter base j unc- is not defective) and may reach one hundred tion has caused a 0.2 ma. change in the col- or more. In the preceding example, when thebase

COLLECTOR COLLECTOR current dropped from 0.05 ma. to 0.03 ma., BASE BASE the collector current dropped from 0.95 ma.

EMITTER EMITTER to 0.75 ma. The emitter current, then, must have dropped from one ma. to 0.78 ma. (PNP) (NPN) Even though the margin between collector and emitter current isless,itcan never Figure 1.8-D reach unity, or one. This brings us around SCHEMATIC REPRESENTATION OF to the term alpha (a) which is expressed as P -N -P AND N -P -N TRANSISTORS the change in collector current divided by the Note that the direction of the arrows change in emitter current. Alpha can never points INTO the current flow. exceed one, and is usually closer to 0.95. 22 Inside Semiconductors The Transistor

following transistor for additional gain. The transistor emitter, which may be 100 ft 10000. thought of as emitting the electrons (or holes), is similar to the vacuum tube cathode. The base, which acts as the control Figure 1.8-E center for the electron flow, may be likened A SIMPLE EXPERIMENT DEMONSTRAT- to the tube's grid. The collector, which gath- ING GAIN IN A COMMON -BASE CIR- ers up the electrons (or holes), acts much CUIT. the same as a vacuum tube plate. The transistor may be compared to the vacuum tube in yet another way.If the Voltage To illustrate how voltage amp li- Gain fication may be obtained, con- positive peak of the driving signal is too nect the transistor as shown in great,it opposes the bias current to the figure 1.8-E.It is similar to the previous point where the transistor collector current figure, but the schematic symbol is used. will be cut-offIf the negative peak is high, Resistors have been inserted in series with it aids the bias in forcing almost all of the the emitter and the collector.Also, a electrons (or holes) into the collector, pro- source of alternating current is connected ducing saturation. This section has presented some of the inseries with the forward bias battery. Since the emitter -base junction represents highlights of transistor action. Voltage and a low resistance, this series resistor issmall current applification, bias, stabilization, cir- (100 ohms ). The collector -to -base junction cuit configurations and impedance match- appears to be a greater resistance, and there- ing are much too diverse subjects to include fore this resistor is larger (1000). Assume in this chapter. These subjects are covered that additional batteries have been added to more fully in Chapter 2. compensate for the resistors, and the current flowing in the circuit is the same as before ( 0.05 ma. base and 0.95ma.collector ). The a.c. is at very low voltage, however. The negative peaks aid the forward bias current causing the transistorto draw more collector current. Positive peaks oppose the forward bias, which in turn reduces the base current. As before, this changing current is amplified by the transistor and appears as a large variation in the collector circuit. The transistor has a current gain ( p ) of 10. For every unit of current change in the base circuit there will be 10 units of change in the collector circuit.Similarly, the transistor will exhibit a voltage gain. In other words, one unit of current through A CRYSTAL BEING GROWN BY A the base resistor will cause one unit of HIGH TEMPERATURE -METHOD voltage drop across it, and 10 units of cur- The molten material is cooled as it is rent through the collector resistor will cause withdrawn from the crucible to form a 10 units of voltage drop across it.The germanium crystal ingot.(Courtesy, amplified voltage can be used to drive a CBS Electronics) Radio Handbook Transistor Construction 23 1.9 Transistor Construction

The physical construction of transistors is interesting, but knowledge of the subject is not necessary for design of circuits using transistors.

PurificationThere are so many difficulties and variables connected with manufacturing a transistor that it is difficult to believe that one can be manufactured to sell for less than $1.00. But it is done, and the manufacturer, distributor, and supplier each make a profit! Take, for example, the purity required. To make a transistor, the impurities should be reduced to one part in 10', or about CLAIMED TO BE THE WORLD'S LARG- one grain of sand in a boxcar load of sugar! EST TRANSISTOR This is accomplished by zone refining the This demonstrator dynamically shows germanium or silicon. The contaminated the flow of holes and electrons in a material is placed in a carbon boat orsup- junction transistor.(Courtesy, Texas ported between carbon holders. Carbon for this purpose is inert, and its impurities do Instruments.) not affect the material being refined. The boat containing the slender rod of Point The invention that heralded the semiconductor material is passed through Contact dawn of the semiconductor era Transistor an induction coil which melts a narrow zone in 1948 was the point contact of material. The boat moves slowly through transistor. The junction transistor was not the coil allowing the melted zones to solid- invented until late in 1949. Point contact ify again. The impurities are forced along transistors are not now used or manufac- ahead of the melting area much the same as tured.In fact, they are considered collect- one might wipe dirt off a mobile whip an- or's items. tenna with a cloth. The end result is a prac- Internally, the point contact transistor tically pure ingot of germanium or silicon. looks very much like the 1 N2 1B radar diode, but it has two very closely spaced "cat- whiskers", as shown in figure 1.9-A. The contact points rest on a slab of N -type germanium.By itself, this device would not work at all.However, in manufacture, a very short pulse of high current is passed through the contact and the germanium. This is known as forming and producesa film a few atoms thick which exhibits the same characteristics as a type -P germanium. E B C Since the contacts are very close together, Figure 1.9-A the emitter base junction is capable of con- SIMPLIFIED CONSTRUCTION OF A trolling the current flow in the base col- POINT -CONTACT TRANSISTOR lector junction. 24Inside Semiconductors The Transistor

In this operation dots of impurities are INITIAL INTERMEDIATEFINISHED CONDITIONS STAGE STRUCTURE placed in contact with the pure material and heated.The applied heat melts the dots =MB \i 1 ALLOY and the impurities alloy into thebasemater- (IMPURITY CONTACT) WWW/71.. If W ial. IE=IDOTS When the dots recrystallize, we find DOTS APPLIEDHEAT MELTS DOTSRECRYSTALLIZE the impurities in the formerly pure material. t PI ...- DIFFUSION ., (IMPURITY CONTACT) .0f The impurities may also be added in a DOUBLE ETCHING GASEOUS DOPING DIFFUSION EXPOSES AGENTS APPLIED COMPLETED BASE gaseous state as in the diffusion method. The impurities are allowed to envelop the RATE IJ''''k.-J1 -- pure material.It is then cut up and etched GROWING ..:"--''' 7... IF i t..fk.k o (GROWN) HEATREMOVAL HEAT REAPPLIED to expose the base region. CYCLE JUST GI RAPI D T O FORM COMPLETED V GROWTHES JUNCTIONS Another way to make transistors is to MELTBACK grow them! The pure material is placed in (GROWN) e a graphite crucible (also inert) which is DOUBLE "f0 TIP FREEZING DOPED PELLETHEAT MELTS TIP FORMS JUNCTIONS inserted in a crystal -growing furnace. The mater- A GROWN ial is melted by induction and apull rod with DIFFUSED - ,... ,,H1 :7;'' . a seed on the end is dropped into the molten (GROWN) ?'. t Al. i tt k'. ir:H.'W:til MOLTEN METAL BASE IMPURITY EMITTER metal, then withdrawn slowly.The heat DOPED wITH DIFFUSES REGION ALONE EMITTER AND RAPIDLY INTO CONTINUES BASE IMPURITIESCOLLECTOR TO GROW may be appliedintermittentlycausing spaced junctions on the single crystal pulled Figure 1.9-B from the molten metal. A more common ALLOY AND GROWN TRANSISTOR method is to add impurities to the molten JUNCTIONS area just below the crystal.With careful timing and temperature changes, it is pos- (Courtesy, General Electric Company) sible to grow a long series of P -N -P or N -P -N junctions. These early point contact transistors Upon completion of the growing cycles, have apas low as 2.5. They have the further the ingot (containing the junctions) is cut disadvantage of being very delicate, since a into thin wafers.The wafers, in turn, are sharp blow could dislodge the fragile cat - diced up into tiny blocks and mounted in whiskers.As they could not be made in the transistor case. Leads are connected to production quantities, they were priced very the elements, usually by a system of thermo- high. compression bonding. Finally the airisre- To its credit, the point contact transistor moved, an inert gas inserted, and the tran- could operate to approximately 100 Mc. sistor is sealed. The input impedance was ofmediumvalue, while the output impedance was of the or- The completed transistor is then tested der of 20,000 ohms. and graded.The grading process is necessary, since not all transistors from the same Alloy andImpurities may be added to the batch will test the same. The best units are Grown pure germanium or silicon in labeled with a particular 2N number, while Transistorsone of several ways.Some of the remaining transistors are again graded. the methods are shown in figure 1.9-A. The transistor run is sorted into specifica- The alloy transistor technique was probably tion groups, such as the maximum fre- a major contribution to the mass produc- quency of amplification, and the remaining tion of transistors. few are either destroyed or sold as salvage. Radio Handbook Transistor Construction 25

The Drift The drift transistor, pioneered Transistorby Radio Corporation of Amer- ica,is manufactured in such a manner as to accelerate the electrons on their journey through the junction. This short- ens the time it takes for the electrons to traverse the material. This shortened transit time increases the high frequency performance much the same as in a vacuum tube.

Surface The surface barrier transistor was Barrier developed by the Philco Corp- Transistor oration. In this device, two pits are etched into opposite sides of the base material. The spacing between the pit bot- toms is only a few tenths of one -thousandth of an inch (a fraction of one mil). Next, the surface of the bottom of the pit is electro- plated with emitter and collector material and electrodes are attached.This close NEW METHODS OF GROWING SILI- spacingbetweenemitter and collector CON AND GERMANIUM ACCOUNT greatly reduces the transit time, which im- FOR MUCH OF THE PROGRESS IN proves the high frequency performance of SEMICONDUCTORS the unit. Silicon diodes are the end products of the crystal ingot pictured here. (Court- esy, CBS Electronics) M.A. D.T. The micro -alloy diffused transistor, a product of the Philco Corp- oration,is similar to the sur- emitter area, leaving these elements on a face barrier transistor.The base width, plateau or mesa.The mesa transistor ex- diffusion layer width, and collector dia- hibits low saturation resistance, high cur- meter are individually controlled by electro- rent beta, and, in addition, many useful chemical etching.Since these factors are v.h.f. characteristics.Its high dissipation largely responsible for determining the makes the design of high -power amplifiers transistor's high frequency properties, pre- quite feasible. cise control results in a device of excellent uniformity. Several current M.A.D.T. type transistors, such as the Philco 2N1745, are very inexpensive, yet have an f max as high Epitaxial The epitaxial transistor is a late Transistors as 1 300 Mc. development in semiconductor technology. The device is similar to the mesa, but the semiconductor layer Mesa Themesa transistor is well suited is formed by an epitaxial (Greek for "set- to high frequency, high power tling") process and evaporated on a base applications. Using photolith- substrate.These transistors feature all the ograph techniques, gaseous impurities are favorable characteristics of the mesa, but diffused into the semiconductor wafer to are somewhat easier to construct.They form two P -N junctions.The semicon- should play an important part in future ductor wafer is etched away from the base high -power communications equipment. 26Inside Semiconductors The Transistor

GEOMETRICAL In section 1.6,it was mentioned that SHAPE CROSS-SECTIONAL PROCESS B. BAR VIEW thermal agitation was responsible for the D. DOUBLE SHOWING DESIGNATION SIDED WAFER JUNCTIONS actionof the minority carriers (leakage S. SINGLE (Nor TO SCALE ) SIDED WAFER current).This points up a basic flaw of E B C the transistor. Changing temperature will RATE GROWN B I II I affect the transistor by influencing both the majority and minority carriers.To illus- MELT BACK B ED trate the point, bias a transistor to draw one milliampere in the collector circuit. As MELTBACK-DIFFUSED B ED a soldering iron or match is brought near

GROWN -DIFFUSED B I II the case, the heat causes increased thermal agitation in the unit and the collector cur-

DOUBLE DOPED B I I rent increases beyond one ma. This effect is inherent in transistors and must becom- ALLOY D pensated for in circuit design. Thetemper- DIFFUSED ALLOY(DRIFT)D ature influence on current is minimized in the better transistors but is always present. may ALLOY DIFFUSED S Abnormallyhightemperatures CE make the transistor useless or actually des- DIFFUSED BASE B troy it. Germanium transistors are usually (MESA) S rated to 85° C (185° F. ), which is morethan DIFFUSED EMITTER -BASE S B Military (MESA) E L adequate for most applications. equipment, on the other hand, must have a SURFACE BARRIER D g wide margin of safety. This type of equipment almost always employes silicon tran- MICRO ALLOY D sistors which may be used in external temperatures as high as 350°F. MICRO ALLOY DIFFUSED D #

Figure 1.9-C High The high frapency tinnsirtor CHART SHOWING JUNCTION PRO- Frequency places special emphasis on FILES FOR VARIOUS TRANSISTOR Performance reducing the base width to TYPES shorten transit time and to increasethe high (Courtesy, General Electric Company) frequency performance.In addition, the emitter and collector junctions are made as tinyas practicable to minimize junction capacity, which also inhibits the high fre- Power The construction and manu- quency performance. Transistorsfacturing techniques for power These techniques do increase the high transistors are similar to those of frequency operation but have one serious alloy transistors.In this device, however, drawback. As the junction size is reduced, more material is used to increasethe power so is the heat -dissipating ability.For this handling ability.In addition, several steps reason most high frequency transistors have are taken to remove heat from the junctions. very small dissipation rating. One exception Since the power transistor has more mass, is the mesa transistor. its high frequency performance is reduced. A power transistor has a relatively large The transistors discussed represent only mass and junction area.Although it can a few of the popular types. A more com- be made to dissipate hundreds of watts, the plete listing is shown in figure 1.9-C. operating frequency is reduced when com- Radio Handbook 27

pared to smaller transistors.Some high - power audio transistors do not exhibit flat response between 20 cycles and 20 kilo- cycles, and feedback correction in amplifier circuits using these devices must be incorporated.

Noise The early transistors were very noisy. Although this gave transistors a very bad name, modern devices are the equal of tubes insofaras noise generation is concerned. In theory, at leastit should be possible to make a transistor which is quieter than a vacuum tube, due to the lower operating temperature of the transistor. Currently, transis- HIGH POINER ATHIGH FREQUENCIES torsare replacing tube preamplifiers in These transistors have 125 watts dis- applications where quality and performance sipation and an alpha cut-off of 50mc. are of paramount importance. and are the smallest types in theser- ies.These triple diffused mesa trans- TransistorItis impossible to make a flat istors are available up to 1200 watts Life statement as to the life of a dissipation. (Courtesy, Pacific Semi- transistor.If itis adequately conductors, Inc.) sealed to prevent external contamination and operated conservatively it might last actually been shot from a gun into a tele- forever! The fact that it has no filament phone book to advertise dramatically how weighs heavily in its favor. well they are constructed. When the tran- Transistors are far more rugged than sistors were reconnected into the circuit are vacuum tubes. Military transistors have they worked perfectly! CHAPTER TWO

Audio Amplifiers

Fundamentally, the transistor is a valve Common -Base Many have used the cir- whichcontrolstheflowofelectrical Amplifiers cuit in which a carbon charges. We call thesechargescutrentcarrters. microphone is connec- As current carriers pass through the tran- ted in series with the cathode ofa tube amp- sistor they are controlled as easily as the lifier.This is a simple grounded -grid audio current carriers which pass through an elec- stage. tron tube. Itis sometimes advantageous to com- transistor equivalent, shown in pare transistors with vacuum tubes, for in figure 2.1-A, the signal is introduced into many ways they are similar, but there are the emitter -base circuit, is then amplified major differences to be pointed out as we by the transistor, and is extracted from proceed. the collector -base circuit.Since the base The transistor may be compared with a element is common to the input and out- vacuum tube by considering the actions of put, this circuit is termed a common -base the two devices.The emitter, which may configuration. The emitter requires a pos- be thought of as emitting electrons (or itive voltage, and the collector requires a "holes"), is similar to a tube cathode. The negative potential (for a P -N -P transistor), base, which influences electron flow, may and, in this circuit, two separate batteries be likened to the tube grid. The collector, are required. which gathers the current carriers (electrons or holes), acts in a mannersimilar to the vacuum tube plate. 2.1 Circuit Configurations

We can make further comparisons between the transistor and vacuumtube by examining the circuit configurations. There Figure 2.1-A are several circuit arrangementsfor introducing a signal into a transitor, and for A COMMON -BASE AMPLIFIER COM- extracting the amplified signal from the PARED TO THE GROUNDED -GRID VACUUM TUBE AMPLIFIER circuit. Bias Considerations 29

Figure 2.1-B A COMMON -EMITTER AMPLIFIER COMPARED TO A GROUNDED -CATH- Figure 2.1-C ODE VACUUM TUBE AMPLIFI ER A COMMON -COLLECTOR AMPLIFIER, OR EAMTTER FOLLOWER, IS SIMILAR Common -Emitter This is the most com- TO THE VACUUM TUBE CATHODE Amplifiers mon configuration in FOLLOW ER transistor (and vacuum tube) equipment and is shown in figure 2.1-B.Not only does it exhibit more 2.2 Bias Considerations power gain than the other circuits, but like the vacuum tube equivalent, it is the only configuration which provides signal A transistor which is designedto have phaseinversion between the input and out- a negative d.c. voltage applied to its col- put circuits. lectorisa P -N -P type (positive -negative- positive).An N -P -N transistor has apos- In this configuration both the base and itive potential on the collector (withrespect collector are biased with negative poten- to the emitter). tials. Since both potentials are negative with respect to the emitter, a single battery The circuit of figure 2.2-A isone of the may be used.Internally, the base -emitter simplest transistor audio amplifiers. Ignor- junction forms a voltage divider in con- ing resistor R ,for the moment, the sig- junction with R1, which provides the nec- nal is applied between the base andemitter essary low value of forward bias voltage. of the transistor.This is equivalent to driving the grid and cathode ina vacuum tube circuit. The emitter (cathode) is Common -Collector Thecathode follower grounded, and the collector (plate) iscon- Amplifier isa circuit which nected through the headphonesto theneg- operates with its ativeterminal of the battery. plate "grounded" for the signal frequency. The transistor equivalent is a common collector stage oremitter -follower,and is shown in figure 2.1-C. Contrary to other circuits, PNP NPN TRANSISTOR a cathode or emitter follower may have TRANSISTOR powergain but has novoltagegain.

E NOTE ARROWHEAD Symbols In figure 2.1-D will be found REVERSED (A) (B) the symbols for P -N -P and N- P -N transistors.Note that in both cases the arrows point in the directionof the Figure 2.1-D holeflow, and opposite to themovement STANDARDIZED SYMBOLS FOR THE of electrons. PNP (a) AND NPN (b) TRANSISTORS 30 Audio Amplifiers The Transistor

The input signal controls the current current rises.This larger current flow will flow between the emitter and collector. in turn heat the junction further. (A com- Unlike a vacuum tube, a transistor requires parable phenomenoninthe economic current bias. In other words, the base is world is termed inflation, which if allowed made to draw current so that it can control to continue unchecked will cause a struc- the flow of carriers between emitter and tural collapse!)Most of us have, at one collector.This is accomplished by apply- time or another, inadvertently madethe ing a negative potential to the base until it plate of a tube glow cherry red, yet have draws the correct current. This is the pur- seen the tube go onworking year after pose of R1, which is known as the forward year. One cannot do thiswith a transistor. bias resistor, and its value may be determined Once a transistor is over heated, its charac- by Ohm's Law. The required base current teristics are altered and the unit often is is given in the transistor data and charac- rendered inoperative. teristic charts. If the radio equipment containing the transistoris heated (in a closed car, for If the audio amplifier must handle a example, that has been standing in the hot large input signal, the transistor is biased sun) the current flow through the transistor near the center of its collector current curve may well rise to startling figures.Some collector poten- so that the instantaneous means must be adopted to check the current tial may swing an equal amount in each di- and heat rise.This means, or measure of rection.If the amplifier is biased near the bottom of its curve it will operate class -B. control, is known as stabilization. The circuit of figure 2.2-A is a simple and practical one, and many similar cir- D.C. Feedback The circuit of figure cuits will be found in this and other books. 2.3-A is identical with It might be considered as an audio "pack- that shown in figure 2.1-B, except that the age" which could be added to crystal sets earphones have been replaced with a load or transistor regenerative receivers. resistor.When the current through the transistor increases, voltage is dropped a- 2.3 Stabilization cross the resistor R2.If the forward -bias resistor R ,is connected to the collector as It is an unfortunate characteristic of the in figure 2.3-B, a measure of stabilization transistor that when it is heated the collector is obtained.As the current through the

PNP TRANSISTORS 0071,2N109, 2N117, GT109, 2N1380 (-IA OUT.

IN. 2

SIGNAL INPUT

+ 1.5 TO 3V.

Figure 2.2-A Figure 2.3-A A SIMPLE TRANSISTOR AMPLIFIER This circuit is similar to fig. 2.2-A, but A simple transistor amplifier showing uses a collector load resistor.It is sus- the easiest method of applying bias. ceptible to thermal disturbance. Radio Handbook Stabilization 31

Capacitor C, is used to bypass the signal OUT. around R3, for without it severe signal de- I N. a generation would result.The value of R3 is chosen to create a voltage drop of one- 8- third to one -sixth of the supply voltage. This isa simple Ohm's Law calculation obtained by dividing the required voltage Figure 2.3-B drop across R3 by the collector current of Connecting the bias resistor to the the stage concerned. collector provides a degree of stabil- zation. Bias Voltage In figure 2.3-D, another Dividers resistor, R4, has been ad- transistor rises and voltage at the collector ded to the circuit. A volt- decreases,the available base biasis de- age divider is formed by the two resistors, creased.As a result, the flow of current R 1 and R4, and the junction is connected through the transistor is decreased. This to the base. Due to improved regulation circuit (or variations of it) is used to a con- of the base voltage the bias is unable to siderable extent in small radios and hearing change. The system shown in figure 2.3- aids where the number of components D will provide superior stabilization.It is must be held to a minimum. the most widely used bias system of all and will be found extensively in this book. Whether they are audio amplifiers,os- Emitter The circuit of figure 2.3-C is cillators, or class C amplifiers, most tran- Bias identical with that of figure 2.2- sistor circuits will have a means of obtain- A except that the components ing stabilization and bias as depicted in C1 and R3 have been added. When the figure 2.3-D. The larger the value of R3, collector current rises, an increased voltage and the smaller R and R4, the better will drop occurs across R3.This, in turn, be the stabilization. A point to remember has the effect of lowering the difference in is that if R 3 is made too large, the voltage potential between the base and the emitter. between the collector and emitter will be- In other words, an increase in collector come too small. This causes the transistor current reduces the bias on the base and to saturate (as explained later in this chapter) "drags" the collector current down again. and will create severe signal distortion.

OUT. OUT.

IN. IN.

Figure 2.3-D Figure 2.3-C PREFERRED CIRCUIT FOR BIASING A An emitter resistor also helps stabil- TRANSISTOR ize the transistor to temperature vari- This circuit is almost immune to tem- ations. perature effects. 32 Audio Amplifiers The Transistor

Th erm istor The resistor,R4,may be a a step-down transformer is not used, it is Stabilization thermistor.A thermistor necessary to employ a different input cir- is a special resistor whose cuitry. An example of this is shown in fig- resistance decreases when heated.When ure2.4-A. This circuitis an emitter fol- used in the circuit of figure 2.3-D (inplace lower which is the transistor equivalent of of R )itwill reduce the base bias when the cathode follower which was discussed the temperature increases.Thermistors in section 2.1.Like the cathode follower, are used when very precise stabilization is it has a high input impedance, approxi- required or when transistors are working mately 100,000 ohms in this version. The very close to the maximum collector dis- shunting effect of the transistor on the mi- sipation rating. crophoneisfurther reduced by placing a resistor, R1, in series with the microphone. 2.4 Transistor Although some gain islost, the over-all Impedances result is improved.

Capacitive To obtain maximum transfer One of the major differences between Coupling of power from onetransistor transistors and tubes is that transistors are stage to another, itis neces- current amplifiers and tubes are voltage amp- sary to match the output impedance of the lifiers. A small increase in current in the one to the input of the other.This may base of a transistor will cause a large in- readily be done with transformers. Trans- crease in current in the collector circuit. Since formers, however, are expensive and usually the transistor base draws current, its impe- occupy considerable space.Often it is ad- dance will be low. This is quite the oppo- vantageous to use resistance- capacitance site of a tube. coupling andacceptthe reduced gain brought about by the impedancemismatch. The transistor may have a high output impedance.The common emitter circuit Negative If feedback is used around an (figure 2.1-B) may have an input imped- Feedback amplifier, the input impedance ance of 1,000 ohms and an output imped- may be raised. A cathode fol- ance of 50,000 ohms.In some instances lower has a high input impedance because the output impedance may be more than a of its complete feedback. megohm. Impedance Matching The differing im- In Transistor pedances may be Circuits used to advantage. 680 A low impedance microphone may be fed directly into the emitter of a transistor whose high imped- 1-0 AUDIO OUT. 2 N408, 2N1380, ance output will match the grid of a follow- 0071. GT109 101( 81.IF ing tube.Not only does the transistor act 1 - as a matching device, it provides considerable gain.The use of a high impedance crystal microphone creates an immediate problem because of the low transistor input impedance. When shunted across the mi- Figure 2.4-A crophone, the transistor will reduce con- AN EMITTER -FOLLOWER STAGE siderably the output voltage and distort the Suitable for use with high impedance frequency response of the microphone. If microphones. Radio Handbook Transistor Impedance 33

0075, 2N408, ETC. 8-, a-120. quivalent to grounded -grid) hasa low in- putimpedance anda high output impedance.Quite often it is advantageous to 1G. IN. C follow one configuration with another such 8.1.18 100 K ISO OUT. as following a common base with a common

1 collector circuit. ) Z 0 100K r. 47K (LOAD) 2.5 Interpretation Of Transistor Data In the figures published by transistor Figure 2.4-B manufacturers concerning their products, An emitter -follower stage with feedback the following information is usually listed: to further raise the input impedance. (a) Maximum collector voltage. (b) Maximum collector current. (c) Maximum collector dissipation. Figure 2.4-B shows the circuit of an am- If the maximum current is multiplied by plifier which employs feedback to raise the the maximum voltage, the result will be input impedance. Capacitor C, is thefeed- much greater than the maximum collector back component. Without C, (and with- dissipation figure. All values given out a load) the input impedance is around are absolute maximums, andif the maximum 100,000 ohms.With C, the impedance voltage is used, the maximum current is around 500,000 ohms. But, if a 1,000 cannot be used, and vice versa.The figure ohm load is placed across the output term- produced when the operating voltage is inal of this circuit, the input impedance multiplied by the current must not exceed the again drops to 100,000 ohms. Without dissipation figure. C ,it drops to about 51,000 ohms. These If there is a transfomer, pair of head- figures tell an interesting story. They re- phones, coil, choke, or any inductance in veal that the input impedance ofa transistor the collector circuit, the peak voltagepresent stage is dependent upon the outputload. will be twice the battery voltage. Asa fur- Therefore, the stage following this circuit ther caution,it must be realized that the should also have a high input impedance. sudden make or break of the circuitmay produce voltage "spikes"many times the The table of figure 2.4-C shows the var- maximumcollectorbreak -down rating. ious impedances which may be obtained For this reason, never remove or insert a from different circuit configurations. The transistor in a circuit when voltage is ap- common -base configuration (which is e- plied.

Figure 2.4-C A CHART COMPARING THE VARIOUS TRANSISTORCONFIGURATIONS

Tube Analogy Grounded Grid Common Cathode Cathode Follower Transistor Equiv. Common Base Common Emitter Emitter Follower Current Gain Approx. Unity High, 40 to 60 High, 40 to 60 Input Impedance Low Medium High Output Impedance High Medium Low Max. Frequency Higher than Lower than Varies with Alpha Alpha Circuitry 34 Audio Amplifiers The Transistor

make an Current If the current in the collector off of 6.8 Mc. and will therefore efficient grounded -emitter amplifier up to Gain circuit rises one milliampere as the base current is made to rise a little more than 1 Mc. ten microamperes, the transistorhas a current gain of .100. This assumesthat there 2.6 Load Lines isno resistance in thecollector circuit. For Transistors The symbols for current gain are alpha ( a ) or beta (13).Both terms are used, but alpha applies to common -base circuitry The good designer does not sit down and will be less than unity. Beta applies to with a box of resistors and capacitors be- common -emitter circuitry, withtypical fig- fore him, trying different values in the tran- ures being between 20 and 100.In the sistor amplifier until the speaker gives off common -base circuit, a onemilliampere the desired sound. Rather, he obtains charts rise in emitter current mightproducea cor- containing characteristic curves, and from responding rise of 0.98 ma. in the collector. these curves he is able to work out the var- In themselves, these figures appear to give ious circuit component values. Thedesign- no gain.The impedance of the emitter er may do all this without having even seen circuit in which the one milliampere change the components. The collector voltage -current takes place is very low (possibly 50 ohms). curves yield much information to those who However, the 0.98 milliampere change in take the trouble to read them. Figure 2.6-A the very high impedance output circuit shows the circuit of a simple transistor am- shows that the grounded -base circuit is plifier.Follow the step-by-step process of capable of very high values of current gain. circuit design, just as a designer might do. is wished to operate the Alpha Alpha cut-off, or lab as it is written, Dissipation It amplifier class A and obtain Cut-off is the frequency at which the gain Intersects of the transistor decreases to the maximum possible sig- 0.707 of the gain at 1,000 cycles. Asthe nal output from it.Figure 2.6-B is a plot term implies, this ratio is measuredin the of the collector current for different collect- common -base configuration.The transis- or voltage values at various base currents tor will perform beyond itsalpha cut-off fre- for a transistor. Assume that the transistor quency, but at that frequency, itsgain is has the following ratings: maximum col- falling. Consequently, from this figure, one lector voltage = -12 volts; maximum collect- is able to ascertain with some assurance or current = -40 ma.; maximumcollector whether or not a certain transistor will do dissipation at 25°C = 80 mw. (milliwatts). a certain job. Suppose onedesires to build an r.f. amplifier at a frequency of 4 Mc., with either a 2 N 3 31, a 2 N 1 3 8 0, or a 2N371. Which would be the best transis-

tor to use?The data on these transistors OUTPUT shows that the 2N331 has an alpha cut-off of 1.6 Mc. The 2N1380 has an alpha cutoff of 2 Mc., and the 2N371 has an alpha cut-off of 30 Mc. Obviously, the 2N371 is the transistor for this application. If the transistor is operated as a ground- Figure 2.6-A ed emitter, it is well to choose a device with A TYPICAL TRANSISTOR AMPLIFIER an alpha cut-off of five timesthe operating The circuit values are determined in the text. frequency.The 2N1 39 has an alpha cut- Radio Handbook Load Lines 35

100

COMMON -EMITTER CIRCUIT, BASE INPUT AMBIENT TEMPERATURE = 25.* C V) DISSIPATION -60 t4/'MAXIMUM LINE I-1.2i a.U.1 717-7.7 F.--'.i...71-is0 _J I"--"-t-"-----0.9 5 60 0.6 I. 0 -0.7 I - U -0. is

-40 0-1 -04 IC MAX. ..--. ...OPERATING 20 LOADLINE POIN7- 4.- BASEMI L1AMPERES4-0.2 e rHuro POINT SECONDPOINT F/ SrPOINT

---"'---,-..rr

NO SIGNAL 4,,,SUPPLV VOLTS 01 OPERATING PD/NT VC MAX I ../.. -2 -4 -6 -6 -10 -12 -14 -16 COLLECTOR -TO -EMITTER -VOLTS

Figure 2.6-B BASE CURRENT CURVES FOR A TYPICAL TRANSISTOR A "family" of base current curves fora typical transistor showing how to determine the operating conditions for the transistor. Thecomponent values may also be determined by correctly interpreting this data.

First, draw intersects showing the max- line crosses the collector (Y)axis of the imum dissipation of the transistor. Starting graph at approximately 37.5ma. Had the at 10 volts, 80 mw. divided by 10 volts = 8 line higher than this been taken thetran- ma. Find and mark the point on the graph sistor would operate in the overload where the 10 volts and 8 portion ma. intersect. This of the graph.By just touching the maximum is marked "First point" on graph. Do the dissipation line,it is possible to realize the same at 8 volts. 80 mw. divided by 8 volts maximum output from the transistor.Had = 10 ma.(Second point) Repeat at 6ma. and so on along the graph. maximum output not been required, the Connect the line could have been takento a lower col- points with a dotted line. Thearea to the left of the dotted line lector current point. This would havelow- represents all the ered the standing collectorcurrent and, of points which are within the rated transistor course, limited the collector swing. dissipation (under the maximum dissipation line). Operating Now selecttheoperating Next, decide upon a supply voltage.A Point point.This should be ap- 9 -volt battery is a common size, so in this proximately halfwayalong example assume 9 volts. Mark9 volts on the load line.The left-hand limit is set the base line ("supply volts"). Draw aline by curvature of the "base milliampere"line from this point toward the verticalcurrent at the left hand edge of the graph and the scale so that it just touches the dotted line.This right-handlimit by theI cc, or collector 36 Audio Amplifiers The Transistor current that continues to flow even when 2.7 Amplifier the base current is zero.If the operating Considerations point is at the center of the load line, the collector voltage can swing from approxi- In figure 2.7-A, with the component Four volts mately 0.8 volts to 8.5 volts. values shown, the collector current has been has been chosen as the operating point. calculated to be one ma. with a collector Thisis not exactly the center of the load voltage of 6 volts. line, but near enough for the purpose. By selecting a figure of 4 volts, the collector to Input To calculate the input imped- emitter potential may swing down to 1 volt Impedance ance use the formula: and swing up to 8 volts without running into non -linearity. The base current will fluc- Input Impedance = (1+0 ) hib tuate from 0.04 ma. to 0.4 ma. The static collector current will be -20 ma. The load The term h feis the forward current transfer The e resistor equals: ratio with the output shorted for a.c. after the h, signifies common emitter configuration.The forward current transfer RI Collector voltage Collector current ratio is the base current gain and maybe found inthe transistordata. For the 4 volts 2N190, this figure is 43. 0.02 amps The term h ,bis the input resistance with = 200 ohms output shorted for a.c.The b after h. indicates common base operation. This figure is also given in the transistordata and The base current at the operating point is 29 ohms for the transistor in question. = 0.22 ma. FromOhm's Law, resistor Rb At one ma. of collector current, the in- is: put resistance for the circuit shownin figure 2.7-A is 43 x 29 or approximately 1250 ohms. Rb 9 volts(supply volts) 0.22 ma. Output The outputimpedanceis Impedancegiven by the formula: = 41,000 ohms.

From the graph, one may obtain the current gain for d.c. The base current varies from 0.04 to 0.4 ma., or a total variation 12 V of 0.36 ma. The collector current varies OUT. from 4 to 32 ma., or a total of 28 ma. The d.c. gain of the transistor is 28 ma. divided by 0.36 ma. or a Beta of 78.

+12 V Thus, from one graph we have obtained the following:static collector voltage and current, minimum and maximum collector Figure 2.7-A currents and voltages, collector loadre- A PROPERLY DESIGNED AUDIO AM- sistance, forward bias resistance, staticbase PLIFIER STAGE current, minimum and maximum base cur- Impedance calculations are discussed rents, and the d.c. current gain. in the text. Radio Handbook Amplifier Considerations 37

This is simply Ohm's Law and says in this case that the voltage on the collector divided by the current flowing in the collector will give the resistance of the load, R.,.

Power The maximum power output is Outputagain Ohm's Law.

Power output -V" x i` 2

The When the load is a trans - Transformerformer, the turns ratio has to be adjusted to suit the speaker impedance or the impedance of

the following stage.The formula for this XR-2P Right View ratio (N) is: HEATH XR-2P The Heath XR-2P uses a class B push- N-.slOutput impedance Load impedance pull audio output stage.(Photo by Heath(

Assume the output impedance is 4,000 ohms and the load is 1,000 ohms. The optimum in order to give the signal source formula becomes: a higher impedance.This has the effect of reducing the variation in input resis- 4000 N - -2 tance at low current values, which in turn 1000 makes a sizable reduction in distortion. Thus a ratio of 2:1, in the step-down di- TransformerThe output impedance for rection, is required. Output a push-pull Class B stage Impedance isgiven by the formula: Class B Unlikeatube, a transistor 2 (1,7')2 - p0 Amplifier cannot be completely cut off. Even when the base current is Power The maximum power output of zero there will still be a small flow of col- Output a push-pull Class B stage for a lector current. This is known as the lc.. given load and supply voltage is: Class B transistors must be biased a little above the cut-off point. This is the no -signal operating point shown on the graph po 2(V`e)2 of figure 2.6-B. The mer Input The calculation of the input The transformer -turns Transformer Impedanceimpedance is more difficult ratio may be calculated in the same manner as for a class A stage. now because the base impedance is varying widely with signal.The Efficiency The efficiency of the class B greater the input signal, the lower will be stage is considerably higher than the base impedance. The impedance of the for class A configuration.The standing driver transformer may be made other than collector current issmall, and the peak 38 Audio Amplifiers The Transistor

tor impedances, and as a consequence capacitors may be larger and resistors smaller. Inverse This technique also follows Feedback tube practice.The resistor Rin figure 2.7-C feeds back

O INCORRECT ® CORRECT energy from the output stage to the driver stage. Its value in this instance may be about 33K. Inverse feedback is helpful in Figure 2.7-B reducing distortion and in stabilizing the AUDIO -AMPLIFIER VOLUME CON- gain of a transistor. TROL CONNECTION Correct and incorrect methods of con- An unbypassed emitter resistor will pro- necting the volume control in a tran- duce negative feedback.If carried too far, sistor audio -amplifier are shown. as with tubes, the stage gain will suffer severely. As a rule, the collector current is large. Saturation This is more important than class B stage may be designed to deliver might be supposed.The power equal to five times the collector dis- termsaturationcould well explain the reason sipation of one transistor. That is, a tran- theamplifier or receiver that was built sistor with a dissipation of 80 mw., when "didn't work the way the book saidit operated in push-pull class B (with another should."In figure 2.7-D, assume that the transistor, of course), may deliver 400 mw. forward bias resistor R, has a resistance of power output. of 10,000 ohms. Under these conditions Class -B operation is preferred for port- the 2N1380 will draw about 8 ma. when able receivers so that the standing current the output transformer has a d.c. resistance drawn from the battery is low. of 300 ohms. The potential between the collector and the emitter will be about 1.5 The VolumeThe gain control circuit of volts. If a pair of earphones whose d.c. Control figure 2.7-B(a)ispoorly designed because the current flowing in the control will make it noisy.The circuit of (b) should be used. If the amplifier follows a receiver, the gain control may be the diode detector load.

The Tone Transistor tone controls will Control differ little from tube designs. A variable resistorin series a- with a fixed capacitor connected from base to ground of one of the audio stages will Figure 2.7-C give ample treble cut.There are so many systems of bass and treble tone controls A SIMPLE METHOD OF APPLYING itis impossible, in the space available, to INVERSE FEEDBACK TO A TWO - cover them.In general, they are identical STAGE TRANSISTOR AMPLIFIER to those used with tubes. Differences will This technique increases thefrequency be brought about due to the lower transis- response and decreases the distortion. Radio Handbook D.C. Amplifier 39

V -9V

Figure 2.7-D An amplifier such as this will saturate easily if the collector load resistance varies too far from the nominal value. Figure 2.7-E A COMPLEMENTARY AMPLIFIER The amplifier operates class B, yetre- resistance measures 2,000 ohms iscon- quires no input or outputtransformers. nected in place of the transformer thecur- rent will drop to 3 ma. and the collector- normal position and the result is inferior emitter potential will drop to 0.1volts. operation. A complementary amplifier config- The large current flowing through the high uration such as that shown in figure 2.7-E resistance earphones has reduced all the causes the two collector currents to oppose available collector voltage.The result is each other, and the speaker cone is not dis- severe distortion and very low output. placed. The two transistors must be close- If Ris increased to 47K, the collector ly matched in their characteristics. current will fall and the voltage on the collector will rise, allowing the transistorto operate normally.The rule to be inferred 2.8 Miscellaneous from this study is that the d.c. resistance Audio Circuits of the output transformer must bethesame as thatspecifiedtoobtain the proper results. Otherwise, the experimentermust There are many transistor circuits found be prepared to change the bias condition in communications equipment whichare on the transistor.The lower the d.c. re- difficult to classify. Some of themare dis- sistanceof the output transformer, the cussed here. greater will be the power output that may D.C. Amplifier be obtained from the stage. A dc. amplifier is exactly what the name implies: d.c. voltage applied to the input is ampli- Complementary N -P -N and P -N -P tran- fied and presented in the output circuit. Amplifiers sistorsmay both be It may or may not appear in thesame used in the same am- phase.The a.g.c. circuit of figure 3.7-A plifier stage.Figure 2.7-E shows how a (Chapter 3) is really a d.c. amplifier. The push-pull complementary circuit that does r.f.is converted to d.c. by the base -emitter not use an input or output transformer diode, and the d.c. causes.the collector volt- may be constructed. age to vary.The voltage drop across the Although single -stage amplifiers may resistor at the collector will be consider- drive a speaker directly, without an output ably greater than the voltage whichpro- transformer, the steady d.c. collector cur- duced it.In this case it will be of the op- rent displaces the speaker cone from its posite polarity. 40 Audio Amplifiers The Transistor

i.f. amplifier is reflexed it cannot be con-

0-1 trolled with a.g.c. voltage without depre- ciating the audio signal.

2.9 Semiconductor Speech Amplifier Figure 2.8-A A SIMPLE TRANSISTOR D.C. AMPLIFIER High impedance crystal and dynamic microphones are the phone operator's stock -in -trade.But, as pointed out in sec- The d.c. amplifier of figure 2.8-A is a tion2.4,such microphones are heavily current amplifier. A small increase in cur- loaded due to the low input impedance of rent at the base (input) of the transistor a transistor.When the microphone does willcausealargeincrease in current not "see" the proper load impedance, the through the meter.This circuit may be low frequencies are severely attenuated. The used to increase the sensitivity of a meter. net effect makes even the most resonant D.c. amplifiers are considerably more voice sound like a young boy. Even under difficult to temperature -stabilize than are the best possible conditions, the mismatch a.c. amplifiers. Temperature changes may will cause a loss of gain. make necessary frequent adjustments to the The solutiontothis dilemma, also zero setting in circuits such as infigure shown in section 2.4, is to connect the in- 2.8-A. put stage in a grounded collector (or emitter -follower) configurationandemploy S -M eter The circuit of figure 2.8-A is feedback toraise the input impedance. Amplifier shown as an S -meter amplifier in figure 2.8-B. This am - How It A speechamplifier featuring a plifierislittleaffectedby temperature Works high -input impedance is shown changes. The potentiometer R adjusts the in figure 2.9-A. The input tran- meter zero, and R adjusts the sensitivity of sistor, TR1, is an emitter followerwith feed- the meter. The circuit has been redrawn in back between emitter and base. The output B to show its operation. From this, it will of this stage drives a common emitter am- be seen that the transistor and its associated plifier, TR2, which has the necessary com- components are a bridge circuit, of which ponents for proper temperature stabiliza- TRis a variable arm. Any change of TR causes unbalance of the bridge and a con- TR, Ri sequent meter reading.

Reflex Areflex stageis one which ampli- Circuits fies one signal twice, on two separate frequencies. For example, an i.f. amplifier may also be an audio amplifier.After the signal has left the i.f. amplifier,itis detected and then fed back through the same transistor again as an Figure 2.8-B audio voltage. Reflexing can only be done S -METER AMPLIFIER when the two frequencies are sufficiently A circuitsimilartofig.2.8-A,but far apart that no interaction between the modified for use as an S -meter am- two signal components can occur.If an plifi er. Radio Handbook S -Meter Amplifier 41

12*

25 F 100K 10K 100K 10 K

HI Z TR I TR2 TR3 MIC. R1 500 500 1)--M."--ff47-100K 8(1JF 100 K 2K

005 81JF 81JF

100 K 47K IOK I 8LF OK 1 +T F

+12V.

Figure 2.9-A MICROPHONE PREAMPLIFIER A properly designed microphone preamplifier which will bring a -54 db microphone up to about one volt. The coupling and bypass components are selected to amplify only those frequencies in the voice spectrum (300-3,000 cycles). Any of the transistors inthe partslist may be used without component substitution. Parts List T, - Interstage audio transformer, 10K to 2K c.t. (Triad TY-56X or Stancor TA -35). TRH, TR2, TR3 - Use either 2N217, 2N406, 2N1380, GT109, or 0071. tion.This stage, in turn, drives a similar in the base circuit of TR3. The electrolytic stage, TR3, with a matching transformer coupling capacior would connect to the arm as its collector load.A 10K resistor is of the potentiometer.This is shown in connected across the primary of the match- figure 2.9-A by dotted lines. ing transformer to help provide a constant load for TR3.This resistor may not be Construction The amplifier can be con- necessary in some audio applications. The structed on a piece ofterm- output winding will match a transistor inal strip material. The transistors mount push-pull load such as the Mini -amplifier by their leads to the terminals, and thecom- described in section 2.11. This speech ponents are laid across the board, between amplifier may also be used to drive an audio the terminals.The layout is entirely up to phase shift network or diode balanced mod- the builder, and is not particularly critical. ulator ins.s.b. phasing and filter trans- There is no tendency for the speech ampli- mitters. fier to oscillate due to proximity of components. The builder should be cautioned Audio feedback through the power sup- to use shielded cable for the microphone ply circuit is prevented by decoupling the lead, however. Due to the high input- im- first and second stages from the output pedance, the transistor speech amplifier is transistor.A 1K resistor and a 25 Ai. just as likely to pick up stray a.c. as is its capacitor are used for this purpose. Feed- vacuum tube counterpart!Noise on the back due to stray fieldsis minimized by supply line should not be a problem due using an R -C filter network in the micro- to the large decoupling components (1K phone lead. and 25 //id. ). The amplifier has more than adequate Note that the amplifier is designed with gain for most applications, and it may be both a common plus and a common minus necessary to install a volume control. This line.This will allow the builder to use the may be accomplished by substituting a 10K unit in a mobile installation with either a potentiometer for the stabilization resistor positive or negative electrical system. 42 Audio Amplifiers The Transistor

SEMI -CONDUCTOR SPEECH AMPLI- pleasing balance between highs and lows. FIER In most cases you will find that no resis- Thistransistor speech amplifier has tance is necessary, but a minimum of 4.7K an input impedance of approximately should be used to act as an r.f. filter. This 100K and provides an excellent match is necessary to prevent stray r.f. picked up to crystal and dynamic high impe- by the microphone lead from being rec- dancemicrophones. Itoccupies a tified by TRI and producing audio feed- space of only 4" X 2" x 3/8" when back. mounted on the etched circuit board. When you are satisfied with thefrequency response, disconnect the divider which was temporarily placed across the output. Measure the audio voltage across the 470 - Operation The best way to test this am- ohm resistor while speaking into the mi- plifieristo connect it to a crophone. You should be able to obtain transmitter with known audio characteris- almost one volt of audio before noticeable tics.Connect a 470 -ohm resistor across distortion takes place.This is more than one-half of the secondary of T1. In parallel adequate drive for s.s.b. audio phase -shift with this, connect a 100 to 1 divider (a networks and is just about ideal for diode 100K and 1K resistor will do nicely). Us- balanced modulators which operate with ing shielded cable, connect the 1K resistor about 6 volts of r.f. signal level. across the high impedance microphone input on your transmitter. Ground the speech The noise generated by the input tran- amplifier to the transmitter (either plus or sistoris approximately 30-40 db down minus line) and connect the microphone from full output.This figure may be im- you will be using with the unit. Have some proved considerably by substituting an in- reliable person check your audio quality put transistor intended for low -noise audio while you adjust resistor R for the most applications. Radio Handbook Audio Compressor 43

2.10 A Deluxe Audio Compressor ...

An audio compressor, when used ahead of public address amplifiers or commercial and amateur transmitters, will provide a major gain in "talk -power." The average voice level may be held high, but the peaks (which would normally overload an amplifier or transmitter) are compressed to a safe or desirable level. In tube compressors, use is made of the variable-fi characteristic of the control grid to adjust automatically the gain of the amplifier. Ordinarily, the transistor gain is varied by (a) reducing the emitter current, or (b) by reducing the collector voltage. The two systems are discussed in Chapter 3, under' the heading automatic gain control.It will be realized that automatic gain control is much the same whether the signal being controlledisaudio, r.f., ori.f.energy. Both of these control systems drive the transistor into a non-linear portion of its characteristic curve in an effort to reduce stage gain.This, while perhaps permissible in r.f. applications, is not desirable in audio circuits.Considerable experimental work (both with characteristic curves and compressors) based on the above principles was undertaken, and the conclusions con- THE COMPRESSOR firmed that distortion of a serious nature The compressor was constructed ina did take place. small L-shaped chassis box. The knob is the compression control. A New Principal- Apartfrom the two How It Works methods mentioned, the gain of a stage may be controlled by shunting its output element In this case the resistor is a nearly -sat- with a low -value resistor. A resistor placed urated transistor connected in parallel with across the output transformer primary of the load resistor of an audio amplifier. The a tube receiver will reduce the audio voltage control transistor is biased to operate just across the transformer to an extent depend- above the knee in its curve.Signal is ex- ent upon the value of the resistor.If the tracted from the amplifier, further ampli- resistoris reduced in value whenever the fied, converted to negative d.c., and applied signal in the transformer exceeds a pre- to the base of the control transistor. The determined voltage, the audio (as delivered operating point of the transistor "slides by the speaker) will never rise above a cer- down" the curve, and the internal resistance tain loudness level. rapidly falls.As the connection between 44 Audio Amplifiers The Transistor

OUTPUT

TR, TR 3 PRI. SEC. ION 2 N,C.T. 100n P 811F 5.66 TI BLUE LO Z YELLOW M IC. 81JF )31

COMPRESSION RED CONTROL Ill

110011E -100 'UP 2255 11j:' T o +12V. - TRa

RI TON APPROX. (SEE TEXT)

Figure 2.10-A SCHEMATIC DIAGRAM FOR THE DE - LUX AUDIO COMPRESSOR overdriven. A large signal at the collector of the controlled stage may conceivably be Parts List rectified by the controlling stage. However, Di - General purpose germanium di- with normal operation this undesirable con- ode, 1 N34A, 1 N64G, 0A85, etc. dition does not exist (figure 2.10-A). R -Approximately 70K. See text. T - Interstage matching transformer, Construction Theunit may beconstruct- 10K to 2K c.t. (Stancor TA -35 or ed in any convenient form. Triad TY-56X) The main point to observe is the matter TR12 3 4- General purpose transistors, of shielding, which must be complete. R.f. prototype model worked success- from the transmitter should not be allowed fully with 2N1380, 2N408, GT- in the compressor unit. 81R, and OC 75. The resistor R may be left out until Note that the two 25 /Ad. electro- the unit is completed.Transistors con- lyticsare connected in the manner siderably different from those suggested shown to eliminate polarization, not may require a different value for this resis- to increase breakdown voltage. tor.A potentiometer could be substituted and then replaced with a resistor of the A single 15 /dd. capacitor cannot be measured value. used as a substitute.Ceramic, mica, paper, and electrolytic capacitor no- Adjustment Afterthewiringhas tations are given below the values. And Operation been completedand checked, measurethe battery current, which should be between the control and the controlled transistor is 3 and 4 ma.Next, turn the compression via a large value capacitor, the a.c. part of control off and connect an audio generator the signal is shunted to ground, leavingthe to the compressor (you can whistle into the d.c. working point of the controlled stage microphone).Adjust the temporary po- unchanged.Clipping of the signal cannot tentiometer R ,until the output signal just occur unless the amplifier is considerably starts to fall.This indicates that the bias Radio Handbook Audio Compressor 45

Ta TR t will become excessive, and a voice control- CRYSTAL OR DYNAMIC MIC led rig will trigger every time a fly moves a foot on the ceiling! C The transformer T,, if different from that suggested, may require phasing. That Figure 2.10-B is, the input or the two output leads will MATCHING CIRCUITRY FOR have to be connected in the rightmanner. HIGH IMPEDANCE MICRO- Incorrect phasing will show spikes at the PHONES beginning of each audio cycle and a rough Schematic showing how a transformer may be connected to al- hum in the monitor. The remedy is simple: low crystal or dynamic micro- reverse either the primary or the secondary phones to be used. Transformer leads, but not both. T2 may be either a Stancor TA- 46 or a Triad TY-50X. The input impedance to thecompressor is low and suitable for a dynamic micro- has been adjusted to place the transistor phone without a matching transformer. A on the knee of its characteristic curve. A crystal microphone will require a matching further reduction in resistor R willvery transformer or a matching stage suchas rapidly reduce the output which indicates the one shown in figure 2.4-B of this that the correct point has been passed.If chapter. the resistor is too high in value, and the bias too low, the rapid change in the collector voltage of the control stagewillcause a pronounced thump at the beginning of INSIDE VIEW OF TRANSISTORIZED each word and will show on the scope as SPEECH COMPRESSOR a "blip" that exceeds the compressed level. The size of the unit could have been Excessive compression isto be avoided. greatly reduced if subminiature com- An overcompressed signal will become ponents had been used. harsh and unpleasant. Background noise 46 Audio Amplifiers The Transistor

2.11 The Mini - Amplifier -Modulator

The Mini -amp -modulator is designed to follow a receiver which in itself has insufficient amplification for comfortable listening. The amplifier may be used in conjunction with many of the receivers described in this book.It is also a permanent 9-12V part of the communication receiver de- Figure 2.11-A scribed here -in.By substituting a suitable THE MINI -AMPLIFIER transformer,it may be used to modulate The Mini -amplifier has a power out- a 1/2 to1 -watt walkie-talkie.The Mini -- put of approximately 350 milliwatts, amplifier should do much to dispel the which is more than adequate for ama- incorrect impression gained through listen- teur radio work.The various taps on ing to too many 2 -inch speakers.Tran- T2 provide correct speaker matching. sistor quality can be as good as tube quality! If the amplifier oscillates when R2 is connected, connect it to the other collector. How Refer to figure 2.11-A.The It Works signal is fed to the bases of the Parts List two transistorsviathestep- - Interstage transformer, 10K to 2K down transformer T1.The input impe- c.t. (Triad TY-56X, Stancor TA -35, dance was calculated for transistors with a or equiv.). T2 - Speaker matchingtransformer, rather low hfe so that a variety of transistors may be used with only small alterations 500 ohms c.t. to 4, 8, or 16 ohms to the circuit. (Triad TY-45X, Stancor TA -42, or equiv.). The transistors operate in class B and TR1-TR2 - 2N1380 (Texas Instruments). are biased just sufficiently to prevent cross- Other transistors may be used if over distortion due to curvature of the tran- R'is adjusted for minimum cross- sistor characteristics. A lower output trans- over distortion.The 2N109, GT former primary impedance would allow 109, 2N408 and 0072 were all greater output without saturation and dis- used in the prototype with good tortion, but the transistors might then ex- results. ceed their ratings.Output is much more than sufficient for the average room. Operation If transistors other than the Negative feedback is applied over the stage 2N1 380's are used, the re- via feedback resistor R2. This resistor may sistor R should be adjusted to give an be omitted if desired. The result will be a idling collector current of around 3.0 ma. slight increase in gain and distortion. If the idling current is too low, crossover distortion will be apparent at low volume levels. If it is too high, battery power will be wasted Construction Any practical layout may for no useful purpose. be followed.Printed cir- If the amplifier is operating from a bat- cuit boards as used in the prototype make tery to which the other equipment is also for compactness, yet allow open, easy -to - connected, "motorboating" or oscillation get -at wiring. may take place.This is particularly true if Radio Handbook Mini -Amplifier 47

essary to add decoupling resistors as shown in figure 2.11-B. Modulator A small r.f. power amplifier (either tube or transistor) can be modulated by substituting a 500 - BATTERY to -5,000 ohm transformer (such as the the Triad TX -49X or Stancor TA -4) for T2.It may be necessary to re -adjust the bias Figure 2.11-B divider network for minimum crossover It may be necessary to add decoup- distortion. ling components as shown when the Mini -amplifier is used with accessory equipment. A 100 pfd. capacitor is 2.12 The Mini- already built into the Mini -amplifier. Amplifier #2 Many applications call for more gain the battery is partly discharged or if there than provided by the Mini -Amplifier alone. isresistance in the battery circuit.The When thisis the case, a suitable driver class B current of the amplifier may reach stage (figure 2.12-A) may be added. The very high peaks which can pull down the driver should draw approximately 1 ma. battery voltage.This, in turn, will reduce of collector current to match correctly the signal. The result is a "phut -phut -phut- transformer T1. phut," not unlike the sound of a motorboat.If there is very little capacity in the Note the polarity of the coupling capaci- circuit the "motorboat" may reach jet speed tor C1.If the input side of this capacitor and give forth the appropriate sound! As is connected to the collector of a preceding the cause is almost always due to coupling stage, the capacitor must be reversed. Since through the power supply, adequate de- the collector will be more negative than the

.coupling is essential. A 100 /dd. capacitor base of TR1, the negative end of the capaci- directly across the battery should be suf- tor must be to the input side. ficient to prevent the voltage variation.If The motorboating condition described the motorboating continues, it may be nec- earlier cannot occur in the Mini -amplifier

THE MINI -AMPLIFIER The completed Mini - Amplifier with driver mounted on an etched circuit board 48 Audio Amplifiers The Transistor

CI TR I driver stage (TR2, TR3-2N1380) by a min- 81JF iature transformer.The driver stage is in

Si MINI- turn direct -coupled to the class B amplifier. Toll AMPLIFIER The emitter current of the driver is actually the base current for the class B stage. Ac- tually, both stages operate class B in order to deliver the maximum amount of output power. The driver stage operates very close toits maximum dissipation rating, Figure 2.12-A and a heat sink is recommended if the am- MINI -AMPLIFIER plifier is to be operated in high temperature Many other transistors may be used environments. The heat sink may consist in this circuit without component value of a small piece of copper platebent around alteration. Resistor 121 may b e adjusted the transistor case and bolted to the chassis. to give a collector current of approxi- The amplifier may be used as a modulator mately 1 ma. when the transistor char- simply by substitution of the output trans- acteristics are considerably different former. No component value changes are from the types specified. required.

AdjustmentBefore the amplifier is con- No. Two due to the added 100 ohm/100 nected to the battery ascer- gfd decoupling network in the supply lead. tain that the potentiometer P, is at maximum If the amplifier is used with accessory radio resistance.With a tone fed into the input, equipment( receiver,i.f.strip, etc.), the and an oscilloscope connected across the B-buss should be connected to the point output, adjust r, for minimum crossover marked "A" in figure 2.12-A. distortion consistentwithminimum 2N554 collector current. If a scope is not available, adjustP1until the two driver tran- 2.13 A 10 Waft Amplifier/Modulator TOP VIEW OF AMPLIFIER/MODULA- TOR Because it uses so few components (and The remaining transistors are mounted these are inexpensive), this amplifier should on terminal strips under the chassis. be very popular for public address or modulator applications. The amplifier has been designed around the Motorola 2N554, a $1.35 transistor.Transformers, usually the most expensive part of an amplifier or modulator, have been kept to a minimum and direct coupling has been employed between the driver and the output stage.

How TR is a conventional common It Works emitter amplifier and requires very little mention. If an audio gain control is required on this stage it may be wired as shown in figure 2.13-B. Tran- sistor TR, iscoupled to the push-pull Radio Handbook 10 -Watt Amplifier 49

TR1 T Ra.TR3 TR4.TR5 2N1380 2 N1380 2N 554 T1 Tz

SPEAKER INPUT OR CLASS "C. STAGE

Figure 2.13-A A 10 -WATT EMITTER -FOLLOWER AM- T2 Collectorto speaker, 24 ohms PLIFIER AND MODULATOR c.t. to 4,8, or 16 ohms (Triad TY- Experimenters could use this same 29X).Collector to 20 -watt modu- circuit as a 40 -watt amplifier/modulated stage, 32 ohms c.t. to 4K, lator by using the 2N554's as emitter 6K, or 3K (Triad TY-65Z). followers to drive a pair of 5 ampere A universal push-pull plate to speaker transistors. A Triad TY-66A (modu- transformer can be reverse -connected lator) or TY-67A (amplifier) transform- as a modulation transformer. er would be used, and T2 would be eliminated. If such a transformer is used, connect the 16 ohm tap to one collector, the PARTS LIST 4 ohm tap to B-, and the common lead P1 - 4 -watt wire -wound potentiometer. to the other collector. The secondary T -Interstage transformer, 10K to 2K will then be twice the original value -c.t.(Stancor TA -35 or Triad TY- of impedance. 56X). sistors draw 7 ma. of idling current. This Other Othertransistorsmay be should provide a 2N554 idling current of Transistors used in place of those sug- approximately 100-150 ma. The idling cur- gested. However, caution rent may fluctuate a little, according to tem- should be used in the selection of substi- perature, but should not differ greatly from tutes. The two driver transistors must have this figure. a dissipation of at least 150 mw. each if they Application A phonograph cartridge fed directly into the input stage TR will provide plenty of output if it offers a 8.UF reasonable impedance match to the tran- INPUT 0 -14. - sistor. A crystal pickup will require matching, which can be provided by an emitter- follower or 100:1 stepdown transformer. A dynamic microphone will drive the amplifier to full output with close talking, and Figure 2.13-13 when a matching transformer is used it VOLUME CONTROL CONNECTION will provide a reserve of gain. A method of connecting a volume The input impedance of the amplifier is control to the 10 -watt amplifier/mod- around 1,000 ohms. ulator. 50 Audio Amplifiers The Transistor

are to drive a pair of 2N554's, or similar Figure 2.13-C transistors.The pre -amplifier transistor Typical currents for the 10 -watt ampli- may be readily replaced with a great number fier/modulator of similar transistors, for itis in no way critical. The driver 2N1380's and the out- NO SIGNAL put 2N554's are particularly suited to each other, anditis recommended that this TR1 1.0 ma. combination be retained. TR2, TR3 7.0 ma. total TR,s, TR5 100 -150 ma total For maximum output, the 2N554's (or (measured atT2 C.t.) substitutions) must be matched to each other and to the load.If the output tran- SIGNAL sistors are unbalanced, one will run warm Maximum current while the other remains relatively cool. The recommended transformers provide the

TR1 I0 ma. proper impedance match either to a speaker TR2, TR3 60 ma. total or to a 20 -watt class C final r.f. amplifier. TR4, TR5 1 ampere total (meas- 2.14 A Kit ured atT2 c.t.) 10 -Watt Modulator

Figure 2.14-A

THE HEATHK1T PUBLIC ADDRESS AMPLIFIER MODEL AA -80

TO 8 OR 160. SPKR

R,6 as MALE CONN. T1 72 +I Go +IC9 +IC8 50 1JFD Rs 50 LIFO R72 50 .1.1F D sn 2.70 Is V. 1 K- 15V. rc, R4 R6 R9 AUXILIARY 3680 [4.7K 22K R,4 IpF INPUT 6.6K L COM. Q, CS 02 Q3 FEM. RI R2 C3 CONN'S. 500 K 100K .11.1FDs .6 101/FD vill48.4 2 118 AUX 0 0 GAIN" 11;7047 e1 .1%W42 2 Cs swot, RIB Q4 33011 CI Rs R7 - CaRI° R e13 R,s - C7 40.UFD + "C6.80 10 +TOLIF4.7 K 470 3.3K 2211 -117501./FD. 25V. .n. 15V. P c.2 V. r;I LIFO ON CHASSIS AC OFF GROUND KI

3 A SLO-BLO 12-I5VDC

MALE FEM. R3 CONNECTORS K2 MIC. GAIN * PAIR MATCHED FOR POWER GA/N. ** Q3 MUST BE INSULATED FROM SUBCHASSIS. r -- 1. Q I, Q2 2N 408 DYNAMIC 2. Q3,Q4, Q5 CDT 1337 MICROPHONE I SHIELD 3.ALL VOLTAGES ARE MEASURED /N RESPECT TO THE `(YELLOW POSITIVE BUS BAR USING A HEATHMIT MODEL MM -I VOM. 4. R,7 IS A 2 OHM THERMISTOR USED FOR TEMPERATURE COMPENSATION. PUSH -TO 5. VOLTAGES SHOWN ARE AVERAGE READINGS OBTAINED US/NG A 72 V -TALK 1 J BATTERY AND WITH NO SIGNAL APPLIED TO THE AMPLIFIER. Radio Handbook AA -80 Amplifier 51

HEATHKIT AA -80 PUBLIC ADDRESS AMPLIFIER Top view showing the layout of components.

Those who like to integrate commercial matching is required, however, between this kits into their equipment will be interested stage and the very low base impedance of in the Heathkit AA -80 10 -watt Public Address the driver transistor, Q3. The driver stage Amplifier.It can be used to modulate a 20- supplies sufficient power to swing the class watt transistor transmitter with no modi- B output stage into saturation. fications.In addition, it contains interesting circuits which can be adapted to existing equipment. The circuit can be duplicated easily by experimenters. Transformer T1 is similar The circuit diagram of the AA -80 is to the Triad TY-61X, while T2 is almost shown in figure 2.14-A. No input impe- identical to the Triad TY-29X. An experi- dance matching stage is included since this menter's transistor, such as the 2N234A, unit is designed for use witha low -impe- 2N307, or 2N554, is suitable for Q3.A dance dynamic microphone.Impedance matched pair of transistors, rated at 10 52 Audio Amplifiers The Transistor

FRONT VIEW OF HEATHKIT AA -80 PUBLIC ADDRESS AMPLIFIER Amplifier can be used as a modulator with no modifications.

watts or more, can be used in place of Q age collector current of a class A stage is and Q. 4 constant.Therefore, audio excursions are Intransistor -transmitter applications, less likely to frequency -modulate the local the 16- ohm connection would drive the final oscillator in a communications receiver. amplifier, while the 8 -ohm winding would supply modulation power to the transistor The circuit shown in figure 2.15-A was driver stage.This technique is described not constructed or duplicated, but appeared in more detail in Chapter 5. in the Pbilco Application Report #309, a de- pendable source of construction information.The input impedance of this ampli- 2.15 A 5 -Watt fier is 100 ohms, and at 5 watts output it Class A Amplifier has a total distortion of 5.4% (combined 2nd and 3rd harmonic). The 3 db bandwidth is 110 cycles to 14 kc. The 39 -ohm When battery consumption and output resistor across the secondary of the driver stage efficiency are not considerations, con- transformer is necessary to reduce distor- structors may wish to use a class A ampli- tion at high levels of power output. This fier in receivers and modulators. Theaver- distortion occurs because the input impe- Radio Handbook 5 -Watt Amplifier 53

dance of the T-1041 output transistor rises sharply when the collector current is reduced to very low levels. The effect causes a peaking of the output waveform on one- half of the audio cycle.At output levels below 4 watts, the effect of this resistor is negligible.The 0.27 -ohm resistor in the emitter of the T-1041 output stage is necessary to insure thermal stability of the unit. The prototype of this amplifier employed a U.T.C. type LS -33 five -to -one interstage transformer and a Freed type QGA-37 for the 2:1 output transformer. CENTERLAB AUDIO AMPLI- It would ap- FIER pear that the more common Stancor TA -48 This Centerlab audio amplifier, which and TA -50 would make excellent substi- is about the diameter of a dime, tutes for these transformers. con- The output tains 4 transistors and associated transistor, which is not commonly available, components.It features 80 db of voltage can be replaced with a 2N234A (Bendix), gain. 2N301 (RCA), or 2N554 (Motorola).

2.16 Sliding A more efficient "pseudo -class A" am- Bias Amplifiers plifier is shown in figure 2.16-A.This trick circuit, developed by Phillips of Hol- The 5 -watt amplifier just described land, is called a slidhig-bras amplifier. Some draws a larger amount of collector current of the audio output signal is rectified and since it is biased for class A operation. This fed back to the transistor in the form of ad- means the transistor must dissipate large ditional forward bias. Thus, at higher audio amounts of power in the form of heat. levels, the forward bias increases, permit-

Figure 2.15-A -12 V A 5 -WATT CLASS A AMPLIFIER Transformer commercial equivalents are given in the text.

INPUT LOAD 100 500 3.6

500-I2V 27 -13

1000 I T -13 54 Audio Amplifiers ting the transistor to handle the larger amplitude signal with a minimum of distor- 2N301 tion. The bias, and therefore the operating point, slides up and down thecharacteristic slope of the dynamic curve in proportion to the signal amplitude.The sliding -bias amplifier is much more efficient than class

A for it consumes only enough power from -12 V. + the battery for proper operation.In this regarditis similar to the class B amplifier. This circuit produces a higher level of distortion products than the class A or B Figure 2.16-B systems. A SECOND SLIDING BIAS CIRCUIT Principleissimilarto that of Fig. In figure 2.16-A the rectified audio volt- 2.16-A. age adds to the bias.Thus both the bias value and amount of audio must be adjusted for best performance. A 2N301 or any experimenter power transistor can be used in place of the OC-16. Note that the phasing of the output transformer is important for proper operation. Figure 2.16-B shows an adaptation of the Phillips circuit which is used in the Auto- crat (New Zealand) car radio. A separate winding is used for the sliding -bias source permitting the speaker to be grounded. Note that the phasing of the bias winding is important.It should be pointed out that the sliding -bias circuit should always be adjusted with the aid of an oscilloscope and audio oscillator.

0C-16

2200 T.

Figure 2.16-A SLIDING BIAS CIRCUIT The switch Si adjusts the point at which rectification occurs. CHAPTER THREE

RF Circuits

In principle there is very little difference High frequency performanceisalso between an audio and an r.f. amplifier. The limited by the base -emitter capacitance and comments made in Chapter 2 regarding the base resistance, Rb, which is in series bias, stability and so on, are equally appli- with the source.These two components cable to r.f. circuits. form a 'built-in' phase -shift network which cannot be compensated for externally. The effect of the capacitance can be minimized somewhat by reducing the source imped- 3.1 Alpha Cutoff ance. If a transistor is driven with a 1,000 cycle audio signal, the circuit will exhibit acertain amount of gain (expressed in One factor to consider when dealing decibels).If the applied frequency is then with r.f. circuits is the alpha cutoff frequency increased, a decrease in the output voltage of the transistor. No doubt you have seen will be noted. At some frequency the out- this term in technical articles and transis- put of the amplifier will have decreased by tor literature.What exactly is this myster- 3 db from the signal level at 1,000 cycles. ious attribute which separates sometransis- This is known as the alpha cutoff frequency. tors from other transistors? However, this is not an absolute frequency above which the transistor quits working. Transistors will work well beyond the alpha Transistors, like vacuum tubes, exhibit cutoff frequency, but above this point the reduced performance as the operating fre- output declines. For this reason a transistor quency is increased. There are two major should not be used in r.f. amplifier cir- reasons for this. Like the tube, the current cuits much beyond alpha cutoff. A trans- carriers of the transistor (whether holes or istor will usually oscillate up to 4 or 5 electrons) take time to traverse the region times its alpha frequency. between the emitter and collector. This is known as the transit time. When the length R.f. transistors are constructed specially of time is appreciable in relation to the to minimize the transit time and junction period of a full cycle at the frequency being capacitance. The construction of these amplified, the high frequency performance various transistor types is discussed under of the transistor is impaired. section 1.10. pro- to impractical be may It system. down stages. step- capacitive a such from obtained easily transistor between match impedance more be often may it stage), -base grounded interstage an provides coupling Link a as (such source impedance low very a 3.2-A Figure requires stage following the If pedance. im- lower a assumes two the of junction the resonance), maintain (to smaller C and larger made is 2 C As tap. center coil a to equivalent be will two the of junction 2 the value, capacitance same the have C and C2 If transmitters. of stages output the in used often so coupler pi the to similar being as recognized be will circuit This 3.2-B. Fig. in shown is ratio - trans -tuned double a If selectivity. and impedance step-down pacitive Coupling gain reduces This circuit. tuned the of - ca a obtaining of system The Capacitive both part across shunted are components bias the arrangement coil tapped the in whereas respectively. detector, diode a and winding, the of bottom the at in fed is tage base transistor a for typical are ohms 5,000 vol- bias the that is link the of advantage and 600 figures, quoted two The detector. The capacitor. coupling suitable a through diode a feeds secondary the if than lower be coil the on tapped be may base the link, a will impedance the stage, following a of a of Instead winding. link from fed is base the feeds transformer the of secondary stage following the of base the and loading, the If mixer. a following ohms 100,000 reduce to circuit tuned the down tapped of primary a transistor driver the of collector The trans- i.f. an use to common is It high. be 3.2-A. Fig. thus will impedance the and low, be often in shown is This action. -transformer auto will current collector its mixer, r.f. an is y b accomplished e b ily stage preceding the If transistor. driving read- may another to istor Coupling - one the of collector the of impedance output trans from Coupling Transformer the by determined is impedance primary The ohms. 600 to ohms 100,000 from gain. reduced to addition in ity and ohms 5,000 and ohms 25,000 tween selectiv- poor cause would which circuit, be- range figures step-down Common tuned the loading from transistor the of circuit. bias the to returned secondary the impedance low the prevents also matching on of bottom the and secondary the down circuits, r.f. In gain. possible greatest tapped be may base the used, is former the obtain to provided be must match ance imped- an amplifiers, audio the with As cerned. base. to collector match con- frequency the at amplification efficient a also will divider voltage capacitive A allow will cutoff alpha whose transistor 3.2-B Figure select to necessary is It transformer. audio the of place in circuit tuned a has plifier am- r.f. An same. the essentially are fiers ampli- audio and r.f. earlier, stated As

Matching Impedance 3.2

Transistor The Circuits RF 56 tap. primary a of instead secondary link the from taken C2. of place in used be may be may voltage neutralization The capacitance junction the high 3.3-B Figure internal is capacitance transistor the When 3.2-C Figure

quite is voltage feedback source the case high which through terminals output the this in for small, be will capacitor ization and input the between capacitance ternal neutral- The accomplished. be will ization in- have each They common. in thing one neutral- and itself, transistor the through have transistor the and tube triode The passed has which that of opposite the be will energy -back fed the of phase The NC. capacitor neutralization the through Neutralization 3.3 base the to back connected is end lower the and tapped, is T transformer 3.3-A to Fig. In tubes. vacuum to as well as istors coil. the coupled winding impedance low a provide or cuit trans- to applied be may Neutralization cir- tuned the tap either to preferable is it very a neutralization. called is technique This ratio, L to C high using by circuit circuit. input and output the between path the of Q the raise to possible is it Though external an via component -phase-of out an required. not is selectivity and exists cuit very a may back feeding by prevented be could cillation cir- Q low where done be This os- that found experimenters tube Early 3.2-C). (Fig. circuit tuned previous the of 'top' the to connected be desired. may stage following the Down Step nor required neither is it when oscillation of base the that noticed Without circuit causes feedback This input. the to put be will it circuits some In Circuits Tuned out- the from flow may energy frequency - transformation. impedance job same the voltage. accomplish Both similar. fact, in are, They neutralization for tap a provide (T) places. proper the in used when cellent transformers I.F. commercial Many ex- are systems Both favor. wins system 3.3-A Figure capacitive the circuits, switched in cially espe- frequencies, higher the At efficient. and simple is it because quencies fre- radio lower the at used ally System? gener- is system inductive The Which

frequencies. high very at true particularly be may This 3.2-A. Fig. in system ing match- inductive the in required are which turn, a of part or turns, few very the vide

57 Neutralization Handbook Radio be would emitter the to back path the is This oscillation. cause to place takes ground, to returned If C3. and C, ignated feedback insufficient and capacitance lector des- are components These 3.4-A. Fig. -col- base internal low very a have sistors in shown as emitter the to back nected tran- modern Many unilateralization. or con- be should circuits collector and base neutralization require transistors all Not bypass to used Capacitors stable. cuit unilateralization. cir- the keep to used be must bypassing as known is This act. to designed was and decoupling Adequate circuits. ternal it as act will amplifier the and phaseshift, ex- the through elements transistor the internal the for correct will NC with series between coupling get still may one But in resistor a of inclusion The mismatch. circuitry. tube with is there as ponents impedance through gain of loss to lead then com- between coupling by caused cillation will which impedance different a exhibit os- toward tendency much as not is there to base the cause however, will, It gain. impedances transitor low of Because high very has amplifier the unless cillation os- to lead not may cancellation complete Stability Amplifier in- The NC. through back fed deliberately IF and RF 3.4 is that energy the by out cancelled pletely com- be not will which shift phase a ergy en- feedback the to imparts capacitance and signer. resistance of combination The pacitance. de- to the of specifications the follow best ca- as well as resistance some contain may is It or unilateralization. neutralization emitter the and base for call do transistors using circuits these the between path The Unilateralization some However, others. many and sistor, tran- mesa the series, Drift RCA the types, manufacturer. the by provided MADT Philco the are nature this of sistors data transistor the in found be may pacity tran- Among frequencies. intermediate ca- -collector base The capacity. collector and broadcast lower the at so particularly - base the times five be therefore will NC

load. 3.5K a into Mc. 1K 5 20 is bandwidth nominal The db. 5.5 of figure noise a and db 14 of gain power a has preselector Mc. 220 This AMPLIFIER R.F. NEUTRALIZED A OF be: will ratio the 1K to 25K is impedance EXAMPLE PRACTICAL A transformer the if example, For former. 3.3-C Figure trans- the of ratio turns the by multiplied then is value This collector. the and base END. COLLECTOR FROM T. 2.5 AT TAP OUTPUT END. COLLECTOR the between capacitance the of value the FROM AT. AT TAP V. -10 LONG. 7/6 LD., 3/8" 14, # =6T. L3 . to you 3.3-B. I L OF END COLD AT WOUND ENAM. #78 IT L2= know need NC calculate To FORM. COIL 4"1/ SLUG BRASS ON WOUND CLOSE IA ENA 418 r 3 = Li Fig. in shown is circuit This accordingly. value in increased be then must NC ary. second- impedance low a is which winding thebase utilize may we purpose, this for tap primary special a providing of Instead

T. voltage. back feed- same the obtain to value in increased be to need will NC capacitor the coil, the 2N1742 down moved is tap the however, If, high.

Transistor The Circuits RF 58 rejected. are portions negative but diode, emitter. the to the through flow to able are wave r.f. the returned be should capacitors bypass of portions Positive rectified. is and ode stability, circuit best For di- the to applied is r.f. 3.5-A, Fig. In 3.4-A Figure

- a AGC Detection 3.5

increased. be must necessity by tance capaci- decoupling the circuits, impedance low with constants time long sufficiently obtain To voltage. supply power the half drop well might large too is that resistor decoupling a through milliamperes few a trans- In critical. not is R1 of value the because value resistance low very a has tor particular, in circuitry tube With 3.4-A. resis- decoupling the Usually, pfd. 1,000 Fig. in R1 and C2 are components These as as may high be values Typical ards. this. prevent to necessary is decoupling stand- circuitry tube by large be may cuitry adequate and stages, between impedance cir- transistor for capacitors Decoupling supply power common the of result a as critical. be might value its place take may oscillation Similarly, ator! and later), explained (as system a.g.c. ward oscill- excellent an into amplifier the vert a for- of part be well may it for care, with con- to sufficient often is this and reactance i chosen be should R1 however, circuits, istor some have will C Capacitor C1. through

DuKane) of courtesy (Photo chapter. this in discussed circuits the to similar amplifier, pushpull and oscillator overtone an uses transmitter This TRANSMITTER SATELLITE

59 Detection Handbook Radio input two the of difference or sum the either DETECTOR is which resultant, The oscillator. local the E EDANC -IMP FINITE IN TRANSISTOR A energy antenna from the and the from signal 3.5-B Figure a be might these example, For mixed. are components two signal which in transistor - a a be to considered is mixer a Generally

Converter The OUT. AUDIO (-0 and Mixer The 3.6

circuit. emitter sig- a between Mixing curve. characteristic the into injected is signal mixing The dynamic transistor's the of portion curved DETECTOR PRODUCT TRANSISTOR A a on operated is It mixer. a as regarded 3.5-C Figure be may detector product The it. detect to as well as signal received the with carrier a mix to designed detector, of form special a is 3.5-C) Detector (Fig. detector product The Product The handbook. this in found be to are detector of type this using receivers Several well. as amplifier audio an as detectorbut a as only not acting therefore is transistor The vary. to circuit collector the in current the causes R1 sistor 3.5-B. Fig. in shown kind the of detector re- the through and junction the through -emittercommon a as functioning actually flowing current d.c. Pulsating load. diode upset. is detector product The is dition the is ft, and junction, -emitter the is con- appear base biasing transistor the because diode the Here detector. diode the of that output will audio off, turned is b.f.o. the If to similar is operation Its istor or detector. correctly. functioning not is overloaded trans- -emitter common a is 3.5-B Fig. either is detector the that indicates it output audio is there If ceases. output audio and component. d.c. the stores which detuned considerably is b.f.o. the if indicated C1, capacitor by audio the from arated is action detector product Correct ter. sep- is R.f. control. gain the through flow emit- the via detector to is product the fed will voltage d.c. pulsating resultant The voltage b.f.o. The amplification. further for amplifier audio the to passed is band - side kc. 1 the and component) kc. 911 (the image unwanted the bypasses ground radios. transistor in used Commonly to circuit collector the from C pacitor DETECTOR DIODE SIMPLE A Figure ca- The kc.). 456 plus kc. (455 kc. 911 3.5-A and kc.) 455 minus kc. (456 kc. 1 be will sidebands two circuit. to 0 The base the fed C a C is kc. 456 at signal and kc. 455 to tuned AUDIO TO IN. RF is b.f.o. the Assume created. are bands -side and occurs, carrier local the and nal

Transistor The Circuits RF 60 INJECTION EMITTER - oscillator The configuration. converter ent WITH CONVERTER TRANSISTOR A differ- a of circuit the show 3.6-B Fig. chapter. this in later discussed 3.6-B Figure be will oscillators Local collector. the to directly coil oscillator the joins it that seen be will it resistance, low very a as moment a for considered is transformer LE the If base. the in is winding feedback the and lead, collector the in also is circuit tuned oscillator The collector. the from taken is frequency intermediate the and base, the into injected is signal the 3.6-A Fig. In grid. the into injected was nal sig- a and circuit cathode the in oscillate to made was tube triode the which in circuit autodyne old the to similar is converter The injection. emitter and injection base are troublesome. quite be might these stations used generally methods two The effective. strong near areas in and 'birdies', called ucts are systems three All circuit. collector the spurious generate to likely more are prod- or 3.5-C), (Fig. circuit emitter converters antennas, large to Coupled ents. through the through circuit, base the through ways: compon- and space in saving the of because three of one in mixer the to coupled be radios portable in popular are Converters may oscillator external The range. audio mixers. of that than critical more is verters the in of instead frequency intermediate con- of adjustment the general, In other. the at now is frequency output the that is the than better is one that state to able not here difference only The detectors. are authors the and used, are systems Both product on paragraph preceeding the in plained coil. oscillator the on made is tap a circuits ex- was mixer The -versa.vice and mixer, a some In link. a through coil oscillator called often is converter A confused. often the to coupled be it that necessitates emitter are terms These oscillator. own its is istor the of impedance low very The collector. trans- the but above, the to similar is stage the to connected is winding feedback the converter A transistor. another in place and circuit, emitter the in is circuit tuned takes usually oscillation local The circuit. collector the from recovered is frequencies, Radio) Allied of courtesy (Photo 3.6-B fig. to similar circuit converter a INJECTION BASE uses portable -transistor 5 Knight The WITH CONVERTER. TRANSISTOR A RADIO -TRANSISTOR 5 B- BIAS KNIGHT THE

3.6-A Figure

COIL OSCILLATOR

. T. F. I COIL ANT.

61 Converter Handbook Radio current collector in increase An voltage. agc. detector diode by Controlled supply the to equal about is collector the at STAGE I.F. potential the and cutoff, almost to biased is 3.7-A Figure transistor the signal a of arrival the Before current. collector in increase an causes tage vol- This 3.5.) section (See R1. resistor 10K load the across up build to voltage d.c. a causes it rectified is signal a When voltage. amplifuda.g.c. an supply to used be also may 3.5-B Fig. of detector transistor The 8.UF circuit. detector the in diode the versing (.0 a.g.c. re- by simply obtained is forward OUT. bias negative The a.g.c. reverse AUDIO for required I.F.T. TRi F.T. I. as known is method previous The a.g.c. forward as known is This reduced. is gain the and voltage of deprived thus is lector - ex be may system This gain. in duction across a col- The X. resistor drop voltage re- further a about bringing upset, been cause turn, current in will, which collector has transistor the and transformer i.f. the or, heavy drawing by react will transistor the between matching impedance the changed, a.g.c. detect- diode the from line the along been have transistor i.f. the on ditions a fed is voltage negative and 3.7-A Fig. con- bias the since time, same the At rent. in `X' marked point the at inserted is tor cur- collector the thus and bias standing the resis- a If ways. two in done be may This reducing stages, or stage amplifier i.f. the of voltage. collector its varying by controlled base the to back fed is voltage This sistor. be also may transistor a of gain The re- load the is instance, this in which, trol action. a.g.c. effective additional viding con- gain audio the across up built is tage pro- thereby sensitivity, in loss siderable vol- d.c. positive A diode. the by rectified con- a causes diode This circuits. tuned is signal strong A ma. 1 approximately the of one with parallel in connected is of current emitter an have to biased is 1 TR which diode a bias forward to resistor this 3.7-A, Fig. In bias. the reduce to is current across drop voltage the use may circuit A emitter the reduce to way simplest The stages. oneofthe only in 'X' marked lead the in resistor a putting by but stages, i.f. two reduced. to it applying only not by further tended be also will gain the reduced, is current ter emit- the If ma. 1 about of current emitter an at gain maximum exhibit transistors here. shown Most controlled. be may amplifier istor as detector impedance infinite the trans- i.f. or r.f. ofan gain the whereby ways from obtained be may agc. Amplified two are There amplifiers. i.f. and r.f. the 3.7-B Figure in level signal the control automatically to used be may voltage This resistor. load the across voltage the is greater the signal the stronger The detection. of process the in signal incoming the from recovered is tage vol- d.c. a 3.5, section in discussed As

OUT. AUDIO FLO Control Gain Automatic 3.7

Transistor The Circuits RF 62 collector. and of total a be will coil the Across capacitor. emitter between feedback with mid. 100 a with tuned is which circuit ant oscillator coil tickler A reson- a to connected is base transistor the 3.8-B Figure and 100Nifd. of capacity input an has istor trans- the that Assume permit. will cillator os- the as circuit tuned the into capacitance external much as introduce designers dent Pru- transistor. the of capacitance output and input higher the of because tubes with than transistors with important more even is This transistor. or tube the to possible as coupled loosely as be circuit tuned the that suggests design oscillator Proper be may circuit tuned the desired, If ner. circuit. the man- normal the in resistors bias forward in loss the by limited is it until up builds the with obtained easily very is Bias sistor. process The on. so and amplified, input, tran- the with used be not should capacitor the to back fed amplified, is it where input and leak grid usual tube's vacuum The the into back fed is transistor a of output circuit. base the in coil the to back fed and r.f. the of portion A only. principle one collector the from taken is r.f. 3.8-A, Fig. and principle one on work Oscillators In transistor. a by replaced transistor. a of heat been merely has tube The Oscillator internal the with compared when increase readers. most to familiar be Coil large a is it tube, a of heat internal the with will circuit "old-time" This Tickler The compared increasewhen small a is perature tem- in increase -degree one a Whereas tage. altogether, oscillation prevent however. vol- supply and temperature by changed much Too stability. cillator will C external be will and variable are elements transistor os- the upon effect less have now will itance the between exist which capacitances The capac- -emitterbase in variations Obviously, circuitry. transistor to common pacitances -emitter up capacitance. base the of made ca- and impedances differing the by about is 10% than less which of Nefd., 1,100 be are , brought differences The oscillators. to then will capacitance thetotal ,000Rifd. 1 tube to similar are oscillators Transitor increased is capacitance external the fore, there- If, variation. to subject is capacitance -base emitter The capacity. input transistor Oscillators of up made is which of 50% mfd., 200 -ExcitedSelf 3.8

base. and collector between feedback with is. relationship the what just oscillator coil tickler A show soon will analysis little a and scribed, 3.8-A Figure de- already those to related are others Most use. in systems main the are These again. system a.g.c. reverse the is This gain. system the reduce thus and stages, or stage, plifier am- i.f. the on bias negative the reduce to used be may voltage -going positive This positive. more swing will which load or collect- the across drop voltage a cause will

63 Oscillators Handbook Radio capac- a form capacitance) junction -base to and CI of size the concerning remarks The - 2 emitter the representing latter (the C and circuit. tuned -parallel series a with placed C, potential. ground r.f. the at is which re- is circuit tuned parallel the oscillator base, the and collector the between nected this In 3.8-D. Fig. in trated con- is circuit tuned the which in oscillator illus- circuit Clapp the is pitts Oscillator Audion Ultra of form a is 3.8-F Fig. Col- the of version popular A Clapp The ). C 3.8- 2 (Fig. oscillator Colpitts the of C and C of equivalent the are -emitter) -tobase and base OSCILLATOR COLPITTS THE - -to(collector transistor the of capacitances 3.8-C Figure internal The circuit. tuned the to made is tap center capacitive no but Colpitts, the to similar cillator scillator os- an is 3.8-E, Fig. Audion, Audion Ultra the circuitry, tube In Ultra The

fed. parallel or RFC series be may oscillator The circuit. tuned the from isolation better obtain to coil the may down tapped be oscillators, discussed frequency. oscillation stable a maintain to as previously the with base, the and lector required is circuit L -low C high a general, col- the except that said be In voltage. supply in variations with value need Little circuit. favorite Oscillator in changes capacitance collector The cuit. an is too, old Hartley, The Hartley The cir- tuned the upon capacitance collector the of effects the reduce to large made be also ent. C1should Capacitor circuit. tuned the upon cur- base in change compensating a causes capacitance transistor the of influence the turn in which thermistor the in change reduce to large made be should C2 Thus resistance a cause temperatures Fluctuating C2. with parallel in effectively is capacitance required. is stability ifbetter thermistor a be -emitter base The glance. a at apparent be may 3.8-D Fig. in R1 Resistor time! same will 3.8-C, Fig. circuits, pitts the at grounded be not would lines ter Col- transistor the and tube Oscillator emit- and base the Obviously, desired. if or the between similarity The Colpitts The collect- the ground even may One ground. to not one connected be should point coil. the down any reason no why good is there circuitry tapped be may collector the loading, duce transistor In oscillator. this in changed re- further To emitter. the to connected been has point ground that that Note 2 winding feedback the and lead collector the oscillator. Clapp the to apply also above C in circuit tuned the shows 3.8-B Fig. 3.2. section in fiers OSCILLATOR CLAPP THE ampli- r.f. under discussed was This base. 3.8-D Figure the to connected junction the and coil the across placed be may divider capacitive a Alternately, oscillate. to refuse may sistor tran- the loading, reduce to coil the down tapped is base the unless and impedance, base low the by damped is circuit tuned the 3.8-A, Fig. to Referring base. the to coil tickler the and collector the to connected

Transistor The Circuits RF 64 (MULTIVIBRATOR) transistor second the of output The other. GENERATOR -WAVESQUARE A the of input the to fed is transistor one 3.8-H Figure of output the wherein sistors tran- two of arrangement an is vibrator 3.8-H) (Fig. multivibrator The Multi- The network. phaseshift the in losses the overcome to order in potential supply lector ETC. 0071, col- permissible highest the with junction 2811380, 2N408, TRz TRI, con- in employed be should transistor beta high a operation, proper For quencies. fre- audio at used mainly is oscillator This oscillation. of frequency the it pulsed, be transistors both that quires determines components network the of constant time re- oscillator the If element. stable one have to said is it operation, into pulsed be to needs which transistor one has oscillator the OSCILLATOR If running. five be to said is operation start AUDIO -SHIFTPHASE A to pulse no requires which oscillator The 3.8-G Figure not. do which those and pulse triggering a require which those classes, two into vided di- be may multivibrators However, here. describe to numerous too far are which properties special or different with each evolved, been have configurations of host OUT. vs, A machines. calculating of multitude a and receivers, and transmitters TV equipment, test in application special finds vibrator - multi The waveforms. sawtoothed and The oscillation. sustain to base the to back square generating for useful especially are fed and N, network shift phase the through and waveforms of variety a produce may passed collector, the from Multivibrators oscillation. of state a into taken is output 3.8-G, Oscillator up builds rapidly multivibrator the balance, Fig. oscillator, this In Phaseshift RC im- circuit inherent Once by started first. meters. dip grid in input to useful the of the back returned then is extremely been has circuit This possible. as value a large as have should C, pacitor ca- 1 circuit. tank the complete external the and C stability, of terests 2 to transistor the of capacitance in- the In oscillation. sustain to C with in a -emitterbase the uses oscillator This parallel capacitor place to necessary OSCILLATOR ULTRA-AUDION AN becomes it frequencies lower the At L1. coil the across connected divider voltage itive 3.8-F Figure OSCILLATOR ULTRA-AUDION THE 3.8-E Figure

jig 65 Oscillators Handbook Radio to path. eedback thef with series in crystal cir- The oscillation. sustain and begin determining thefrequency with family, required resistance) (negative state the is Colpitts the of oscillator Another This decrease. to current the cause will age 3.9-D Figure volt- in rise a diode tunnel a in current, in rise corresponding a cause will voltage in rise a which through wire, copper of piece a Unlike resistance. negative a exhibits which conductor a is it Briefly, transistor. the or tube the either from principle different entirely an Diode on operates diode tunnel The Tunnel The

flip-flop. term the on feedback tickler the of circuit the shows Hence again. back it flops pulse other an- 3.9-A Fig. control. frequency vernier of until rests it where condition, -off ased a a means a as provided be may capacitor a and bi- to condition saturated from flip to cause may one to operation of frequency the on effect some it transistor of base the to have will circuit the in Capacitance crystal. fed pulse trigger A bi-stable. be said is the by wholly almost determined now is oscillation of frequency The crystal. a use to adapted be may oscillator of type back Radio) Allied by (Photo feed- tickler the Even section. previous the 3.8-A. Fig. to similar oscillator audio a in described oscillators L/C the of most in coil tickler simple is oscillator This circuit tuned the replace may crystal A OSCILLATOR PRACTICE CODE KNIGHT THE

Oscillators Controlled Crystal 3.9

response. signal weak in amplifier parametric the to only second places noise low diode tunnel the figure KEY relatively the for frequencies, -highultra the of true particularly is This it. know we OSCILLATOR CODE as radio benefit greatly will diode tunnel PRACTICE the that likely is It 3.8-I. Fig. in shown is TRANSISTOR diode tunnel a using oscillator an of cuit

OSCILLATOR R.F. DIODE TUNNEL A 3.8-I Figure

OUT. R.F.

Transistor The Circuits RF 66 the control to used be may C1 tor Heath) by (Photo feedback. Capaci- oscillator. transistor popular a is chapter. this in discussed and Colpitts the to similar is 3.9-C Fig. circuits the of many uses receiver This resonant lel RECEIVER COMMUNICATIONS mode. in operating crystal the by placed TRANSISTORIZED paral- the re- Colpitts. a is 9-B -1A GC HEATH THE been has coil The 3. Fig. in shown circuit oscillator The crystal. the as frequency same the to tuned are C and L coil. the down tapped is collector the if obtained be -Grounded configuration. -base grounded will stability Better crystal. the of frequency in operates transistor The 3.9-D. Fig. in resonant series the by determined is operation shown that is family Colpitts the to belongs of frequency The path. feedback the in tal also which oscillator popular Another crys- the with oscillator transistor of type

OSCILLATOR OSCILLATOR OSCILLATOR -CONTROLLEDCRYSTAL CONTROLLED CRYSTAL CRYSTAL COLPITTS THE COLPITTS MODIFIED A COIL TICKLER A Figure 3.9-C Figure 3.9-B 3.9-A Figure

67 Oscillators Controlled Crystal Handbook Radio AMPLIFIER R.F. figures. high very reach C CLASS PUSH-PULL A might voice) (on peaks the but low, is rent 3.10-A Figure cur- standing the s.s.b., handling is plifier am- B class the If current. collector static high a cause will carrier the of presence the as reduced, considerably be will stage the of ratings maximum the carrier, a with nal our. sig- a handling is stage the If point. cutoff the near set is stage Amplifiers a transistor B class on bias The B Class

2. Chapter in cussed r.f. where point the to advanced has the-art dis- were amplifiers A Class bias. the state-of- the However, transmitters. ductor is operation semicon- power high of development the by entirely determined of class anypoint or B, on check severe a been has transistors The in-between. C, A, class r.f. in operated may suitable of unavailability the past, the In be amplifier r.f. An neutralization. require not does circuit of type This r.f. for potential ground at base RF the with operated is stage The R2. and Amplifier R1 resistors by determined is bias the A, Transmitting 3.10 3.10- Fig. in shown amplifier the In cutoff. beyond biased are amplifiers C Class frequency. harmonic tal percentages. modulation higher crys- the to tuned are C, and L1 path. back in results and drive r.f. increase to done is feed- the in placed is crystal the This stage. C class -modulatedcollector a which in oscillator overtone an Oscillator by followed stages, driver modulated have of circuit the shows 3.9-E Fig. Overtone often transmitters transistor power High interference. television severe cause may justed. is antenna, the reaching from prevented not if ad- properly when excellent oscillator to which, harmonics in rich be will amplifier C this of stability The oscillate. refuse will too squegg. or class A waveform! modulated a of portions transistor the small, is C1 If clipped the restore cannot circuit, -regeneratesuper may oscillator the large, too however, 1 tuned A circuit. tuned the of action wheel is C If frequencies. higher the for ferred fly- the by restored are portions clipped pre- is and operate will transistor the which the but waveform, r.f. the clips amplifier at frequencies the extends operation base an of operation C Class in-between. point some at or B, A, class operated be should crystals. overtone fifth with It C. class operated not usually is it nal, used be may circuit the neutralized, a sig- modulated of level the increases fier properly is capacitance crystal the If ampli- an If circuit. collector OSCILLATOR OVERTONE the in modulated usually are Amplifiers 3.9-E Figure amplifiers transistor C Class C Class

amplification. modulated amplitude for required those as same the be not will tions condi- operating the signal, the in carrier of absence the to due but signal, sideband a single- amplify may amplifier linear A ETC. 00515, 2N247, L

Transistor The Circuits RF 68 MODULATOR BALANCED DIODE MODULATOR LEVEL HIGH 3.11-C Figure 3.11-A Figure

C OUT. AUDIO IN. AUDIO

TzMein Li Di (z) 2N301

- con a As connected. are they which to cuit should T1 of Theprimary modulates. it fier cir- the load they devices, impedance low ampli- the of impedance the match should are diodes the Because diodes. two the of T2 of secondary The T2. characteristics unequal the for compensates transformer modulation the Modulator . watts Level control This P1 potentiometer balancing from audio of 12 deliver the into fed are audio the and carrier the will 3.11-A Fig. of circuit The High The both 3.11-C, Fig. In out. balanced is rier car- the and modulation (b) overmodulated. or operated incorrectly is the with mixed is r.f. the (a) Modulator amplifier the unless place takes sort the of two performs lator ed Balanc functions: nothing for misnomer, a of something is modu- balanced diode The Diode The `amplitudemodulation' term The process. modulation the during amplitude in change not does itself carrier The carrier. the amplifier. linear to addition in transmitted are which bands a by followed be should stage modulated -side creates modulation of process The The 3.11-B. Fig. in shown is circuit The efficient. more much are oscillator. r.f. the replaces cuit thus and C class operated be may plifiers cir- audio The mixer. tional Modulator am- R.f. reason. good very a for plications conven- a from different little Level ap- communications in favor considerable is modulator level low The Low The found has modulation) or ( mixing level high or plate However, signal. audio an with carrier r.f. an modulate to used be may 2. in Chapter ered circuits mixer usual the of any and process, cov- operation its been has and amplifier, mixing a as regarded be may Modulation audio B class a is This driver. the match

Circuits RF MODULATOR LEVEL LOW 3.11 3.11-B Figure Modulating

5. chapter in detail greater in described are circuits Transmitting emerge. now might supplies power suitable providing of problem new a that seem would It megacycles. hundred eral sev- to operation of capable are transistors These watts. 125 as high as figures sipation dis- collector have style planar and mesa layer diffused triple as known type a of transitors

69 Modulators Handbook Radio MODULATOR BALANCED P1. potentiometer the by out balanced is TRANSISTOR carrier The audio. for unbypassed left be 3.11-D Figure should 1 R resistor the stability, better For carrier. the out balance other each opposing transistors two The mixer. a of that to similar is transistor Modulator each of action The 3.11-D. Balanced IN. AUDIO Fig. in shown is circuit lator Transistor modu- balanced transistor A The

OUTPUT BAND SIDE IN. CARRIER resistance. reverse high a have not need diodes the low, is circuit the of impedance the Because resistance. forward low have should diodes the and direction, forward the in matched be should values. smaller diodes the of resistance The each. ufd. have may C2 and C1 capacitors the and 0.002 is mc. 4 at C2 and C1 for figure cal 3.11-C, Fig. of that than higher siderably typi- A circuit. the of Q the raise to effort con- is circuit tuned the of impedance The an in large made are C2 and CI sequence,

Circuits RF 70 CHAPTER FOUR

Receivers

Several of theconstructionprojects a radio frequency signal from two or three which follow are not really of a radio am- millionths of a volt up to headphone vol- ateur nature.However, the authorsfeel ume! Truly, the sup erregen erative detector that the construction of these simple broad- creates a minor miracle! cast radios will be good preparation for One of the first high -frequency tran- the more elaborate equipment to follow. sistors available was the Philco surface barrier Not only quite sim- type, and many circuits have been devised ple, but the authors have tried to build -in to employ this unit.The Philco SB-100, guaranteed performance. The little radios the AO -1, 2N232, 2N346, and the T-1768 have been duplicated on both sides of the globe, using different components, and 2 .001 they work as advertised. 1 -COLLECTOR In getting circuits to work properly, 2- BASE Lt 3- EMITTER all too often overseas readers are left in OZ root doubt regarding the exact description of RFC

components. Componentdescriptions Tt needing clarification are the correct num- 20K HIGH Z AUDIO OUTPUT ber of turns on coils, turns ratio of trans- PRIMARY formers, and approximate inductive values. Sufficient information of this type is in- T T cluded here so that reasonable substitu- +1 5V - + 3V.- tions can be made. Figure 4.1-A A SUPERREGENERATIVE 4.1 The "Super Regen" DETECTOR Detector This detector uses a surface barrier transistor.CoilLiis a two turn link placed at the lower end of 12.Coil One of the most popular circuits con- L2, for 28 Mc., is 7 turns, #22, 1 inch structed by amateurs and experimenters diameter, L1/8 inch long. Transform- alike is the superregenerative detector. With er T1is a 10K to 2K transistor inter - only one transistor it is possible to amplify stage transformer. 72 Receivers The Transistor

also set for maximum sensitivity.How- ever, once the best setting has been found, no further adjustment is required. Capa- citor C1 controls the quench frequency and may have values between 0.015 /ad. and 0.1 /Ad., depending upon the transistor used.This circuit has been operated with a 12 -foot antenna and provides excellent performance,even without anexternal ground. Amateur stations on the10-meter band were received from distances as great as 2,000 miles. !' Figure 4.1-B Another superregenerative detector is A S ENsmvE shown in Fig. 4.1-C. Notice the similarity SUPERB EGEN ERATIVE DETECTOR to Fig. 3.8-F (the Ultra Audion oscillator). For 10 meters, coilLiis 7 turns, #22, This circuit differs from the previous super - 1 inch diameter, 1-1/8 inch long. Coil regenerative detectorsin the method of 12, the antenna link, is 2 turns at the antenna coupling.Note that the antenna

bottom end of Li. Coil 13, the feed- is connected to the emitter through a trim- back winding, is 3 turns at the top end mer capacitor. of coilLi . Transformer Ti can be a The circuits just described will work on 10K to 2K transistor interstage 11, 10, and 6 meters.But what about 2 transformer. meters (144 Mc.)?The circuit shown in Fig. 4.1-D was designed by Philco for use with their 2N1742 transistor for operation all work on the 27 Mc. Citizens' band and between 85 and 185 Mc.This includes the 10 -meter amateur band. The SB-101, the f.m. aircraft, 2 -meter, mobile services, SB-102, SB-103, T-1324, T-1767, and the and television bands. 2N588 all operate as high as 6 meters, In this circuit the bias controlis set with several varieties operating as high as for a collector current of 1.1 ma.Capa- 90 Mc.The 2N500 and 2N588 will op- citor C1is the tuning control, and capa- erate in the 2 -meter band. The newer Phil - co MADT devices, such as the 2N1742 Li ANT. series, will also work in circuits specifying S 131 - surface barrier transistors. Fig. 4.1-A shows a simple superregenerative detector which works well with most L AUDIO surface barrier transistors. With thevalues OUTPUT.7) shown, the receiver will tune both the JJF Citizens' band and 10 meters. The potentiometer is adjusted for best sensitivity, as are the antenna coupling coil andthe emitter tap. Figure 4.1-C This circuit will work on the 6 -meter A more sensitive circuit is shown in amateur band with the Philco T-1324, Fig. 4.1-B.Potentiometer R1 is adjusted T-1657, and 2N299 surface barrier for maximum sensitivity and should be transistors. Coil Li is 5 turns, #16 wire reset for each station. Potentiometer R2 is 3/8 inch inside diameter, 1 inch long. Radio Handbook Simplex Crystal Set 73

2 N1742 "G/MMICK. 1.0 ULF

C C2 5 25UOF 4 -30 ILUF 2 TUN/NC REGENER. CONTROL Hi' .005 . 27

AUDIO OUTPUT 0 1 -COLLECTOR 2 -BASE 3 -EMITTER

Figure 4.1-D THE SUP ERREG EN ERATIV E DETECTOR Shown here is designed for operation REAR VIEW OF THE on the two -meter band and will work SIMPLEX CRYSTAL SET to at least 180 Mc., using the Philco CoilLi is 2N1742 MADT transistor. How it The antenna couples a multitude 3 turns, #12 self supported, 1/2 inch Works of signals to the coil and variable inside diameter, and 1 inch long. Coil. L2 consists of 18 turns, #30 space wound capacitor, which together select on a 3/16 inch form, 1/2 inch long. the desired station.The chosen signal is The "gimmick" is a few turns of wire then rectified by the diode.This means wound around the collector lead. that the audio component is recovered from the radio wave and passed to the headphones.In the headphones the fluc- citorC2is adjusted for maximum sensitiv- tuating current (the recovered audio)flows ity.Although not specifically stated in the through a coil of wire, creating a mag- Philco Application Report #505, this circuit netic field.This in turn vibrates a soft could be moved up to 220 Mc. for use in iron diaphragm. The end result is a move- tiny hand-held transceivers. ment of air which creates sound.It is im-

4.2 Simplex Crystal Set FRONT VIEW OF THE SIMPLEX CRYSTAL SET

Excluding the headphones, this crystal set uses four inexpensive components: the coil, the tuning capacitor, a germanium diode, and a 0.005 pfd. fixed capacitor. Yet the circuit offers much in the way of knowledge and experience to those new to the fascinating hobby of radio.There is much to be learned from the humble crystal set. The circuit is shown in Fig. 4.2-A. 74 Receivers The Transistor

Construction The tuning capacitor may Hints be obtained from a discarded broadcast set.The ca- PHONES pacitor in the photograph is a two gang unit, which means there are actually two capacitors on a common shaft.Only one Figure 4.2-A of the capacitor sections is used. THE SIMPLEX CRYSTAL SET Parts List Don't forget to scrape the enamel in- C1 - Tuning capacitor, any value be- sulation from the wire whenever a con- tween 300 and 385 Mufd. nection is made to the coil! CR1- Any germanium diode, such as Experiments If the antenna is connected 1 N34A, 1 N48, 1 N60, etc. to Perform near the top of the coil, it Li - One inch closewound winding will detune the circuit. The of #34 enameled wire on 1-1/4 stations will cover large portions of the inch form.Tap 1/4 inch from dial, one station often interfering with an- ground end of coil.(J. W. Miller #2004 or similar loopstick may other.If the antenna is connected near the also be used.) bottom of the coil, the selectivity of the circuit will be immensely improved but the volume will be less.The tap shown has portant to realize that the flow of current been adjusted to give a compromise be- must complete the circuit.It must flow tween selectivity and gain. from the tuned circuit (coil and capacitor) through the crystal, through the head- 4.3 Simplex Audio phones, and back to the tuned circuit. Power Pack The direction of flow is governed by the way the crystalis connected.Reversing the crystal will reverse the direction offlow. A capacitor, a resistor and an inexpen- The sound, however, will be unaffected by sive transistor are all the components re- the reverse connection. The capacitor quired to add an amplifier (power pack) across the phones bypasses any r.f. which to the Simplex Crystal Set. The amplifier manages to get past the crystal. will provide pleasant listening, and even

GROUND ANTENNA Figure 4.2-B CHASSIS LAYOUT OF THE SIMPLEX CRYSTAL SET Note thatthe phone jack and the tuning capacitor are connected together through the metal panel.If an insulated panel is used, a wire will have to be connected from the phone jack to the ground post as shown (dotted lines).

GROUNDED TO PANEL Radio Handbook Simplex Audio Amplifier 75

WIRING CONNECTIONS FOR THE SIMPLEX AUDIO POWER PACK

weak stations will be brought up to good a small amount of bias on the transistor headphone strength. The circuit is shown base. This bias is necessary for correct in Fig. 3.4-A. operation. Construction The transistor may be sol- How It In this receiver the crystal diode Hints dered directly into the cir- Works is connected to the antenna tap cuit, or as an alternative, it on the coil.If the diode is con- may be plugged into a transistor socket. If nected to the top of the coil, the stations the wires are soldered, leave the leads long, will have a very broad tuning range and but cover them with vinyl tubing toprevent will interfere with each other to a consid- shorts.Be careful not to let the heat from erable extent.The other end of the diode is connected to the base of the transistor, the iron flow up the leads and ruin the tran- which amplifies the signal and passes it on sistor.One endof the crystal will be to the headphones. The resistor R1places marked with a red, white, or black dot or 76 Receivers The Transistor

TR I Figure 4.3-A 81,476-220N THE SIMPLEX AUDIO POWER PACK Adjust value of Rito suit transistor PHONES used. The tuned circuit is the sameas inFig. 4.2-A.The transistor isnot critical and a 2N107, 2N109, 2N408, +1.510 60 - TRANSISTOR BASES 2N1380, or 0072 may be used. line.This end connects to the base of the point in using more than 1 1/2 volts sup- transistor.If the panel is made of metal it ply with a receiver of this nature! is important that the headphone jack be insulated from the panel to avoid shorting Using The TR ONE will give excellent out the battery. A Solarresults when powered with a sin- Cell The transistor will operate from a dis- gle solar cell.The results are carded flashlight cell, but best results will comparable with those from a be obtained from a small 6 -volt battery. battery, even in the shade.The cell used No switch is used in the amplifier cir- with the prototype receiver was in Interna- cuit because the current drain from the bat- tional Rectifier Co. #SAS -M, costing less than $2.00. tery is so small it will last the shelf life. The receiver works equally well when the cell is under an electric light,so ExperimentsTry connecting the antenna batteries are not necessary. to Perform and the base of the transistor to different taps on the How it The transistor operates without coil.Sometimes different transistors will Works bias and is a common -emitter de- work better on different taps. tector. Its operation has been Try a different value of R but do not use described in Chapter 3. The base to emit- a value smaller than 47,000 ohms. A low ter junction behaves exactly like the diode value of R will cause heavy collector cur- in the Simplex Crystal Set, and the remain- rent which may ruin the transistor.Al- der of the transistor acts as an amplifier. most any type of PNP audio transistor will work in this ciruit.If you connect a 0-1 ma. meter in series with the battery, resistor R may be adjusted for a current of approximately 0.25 ma.

4.4 The TR One

This receiver uses only one transistor. Yet isis "a receiver with a difference" be- Figure 4.4-A cause it will give good results with only a SCHEMATIC DIAGRAM fraction of a volt of power supply. Even if OF THE TR-ONE the battery circuit is opened and one lead The tuned circuit values are the same held in each hand, allowing the circuit to as in Fig. 4.2-A.Almost any transis- be completed through the body, the receiv- tor, except the drift types, may b e used er will still give good volume! There is no (see Fig. 4.3-A). Radio Handbook The TR One 77

Construction The same baseboard, pan- #a Now hold the transistor allowing heat Hints el,tuning capacitor, and from the fingers to flow into the transistor. coil used in the previous Watch the collector current rise rapidly. receivers may be used again. The transis- Note how little time it takes for the heat tor is mounted on a small terminal strip. to affect the current flow. This experiment No special layout is necessary.Complete indicates the reason why, in later receivers, extra resistors are used in heat stabilizing the receiver before soldering in the transis- networks.Without a means of limiting tor.It is always agood ideato have a meter the current caused by heat, the transistor in the battery lead so if anything is wrong could destroy itself.Imagine how high it will immediately show on the meter. For the current would rise if the receiver were example, if the meter indicates an excessive left in a closed auto that had been standing current after the battery isconnected, some- in the hot sun! thing is amiss.Prompt disconnection of The effects of rectification may be ob- the battery would likely save the transistor. served by tuning in a station with the meter With this receiver, as with the last, the still connected in the B -lead. A strong sta- battery current is so low that a switch is tion will cause the battery current to rise unnecessary. considerably. A weak station will cause it to move only a little.In fact, the meter may Experiments As in the case of the previ- be used to measure the relative strength of to Perform iously described receivers, the stations received. the coil tap may be varied to advantage.It is also interesting to ob- 4.5 The TR Two serve the effects of heat on the transistor. Connect a 0 -1 ma. meter in the minus A transistor, two resistors, and a capac- battery lead.The current will normally itor are the only parts required to add an be very low, possibly between 30 and 150 amplifier to the "TR ONE". Taking advan-

WIRING VIEW OF THE TR-ONE 78 Receivers The Transistor

tage of the fact that the "TR ONE" will give TR2 Ci TRi AUDIO excellent results from a very low battery DETECTOR

voltage, the detector transistor has been PHONES coupled directly to the amplifier as shown in Fig. 4.5-A. tie 0-1 How It By examining the circuit and

Works ignoring the amplifier for a mo- 5 TO 6V ment, it can be seen that the detector transistor is supplied with collector Figure 4.5-A voltage through resistor R1.Remember THE TR-TWO that in previous experiments when a station The tuned circuitis the same as in was tuned in the collector current increased. preceding receivers.The transistors In this circuit, when the collector current may be 2N139, 2N408, 2N1380, GT- risesit causes a voltage drop across R1. 109, or 0072. The changing d.c. voltage is applied to ExperimentsA0-1 ma. meter inserted the base of the transistor amplifier.The signal in the headphones is considerably to Perform in the battery lead at "X" will show that heat does amplified and will even drive a speaker not affect the current flow in this circuit. on strong stations.Resistor R2 is neces- The reason for this is simple. When the sary to raise the emitter voltage to nearly applied heat causes a greater flow of cur- the same as the base voltage. The capaci- rent through resistor R1, the voltage on tor C1is necessary to provide a path to the detector decreases so that over-all cur- ground for the signal. rent consumption remains the same.If Apart from the need to provide another the amplifier transistor is heated, a larger terminal strip on which to mount the few amount of current will flow through R2, extra components, there is little more to decreasing the bias between the emitter and the construction of this receiver. the base and thus reducing the collector

REAR VIEW OF THE TR-TWO Radio Handbook Regenerative Receiver 79 current.This action, known as temperature impedance headphones may be connected stabilization, is a very important function in through a step-up transformer to obtain transistor circuitry. The total current drawn optimum performance. When the receiver by the TR TWO is approximately 0.5 ma. is operated in bright sunlight or under an at 6 volts. electric light its performance is remarkable. AmplifierIf more output is required, the Mini -Amplifier in Chap- 4.7 A Regenerative ter 2 may be added to this circuit with ex- Receiver cellent results. This is a recommended circuit whether 4.6 The Solar Two the would-be constructor is a young experimenter or an old timer new to transistors.Selectivity is such that two broad- The SOLAR TWO is an adaptation of cast stations may be copied alongside each the TR TWO. This little receiver has been other without interference.Although it designed to operate from one or two In- uses only two transistors, the receiver has ternational Rectifier Co. #SA5-M solar cells. an audio output equal to that of many su- In this circuit the values have been changed perheterodynes which have more transis- to suit the high current, low voltage output tors and additional components. Thehigh of the solar cell.Note that R1 has been sensitivity, selectivity, and gain are obtained reduced to 10K and that the resistor and through the use of regeneration applied capacitor in the emitter lead of TR2 have to the detector stage and the incorporation been eliminated. The SOLAR TWO must of band-pass tuned circuits. operate with low impedance headphones such as dynamic types. Alternatively, high FRONT VIEW OF THE REGENERATIVE RECEIVER A National dial was used for ease of TR2 LICI TR' AUDIO IK 100K tuning and a metal panel eliminates DETECTOR hand capacitive effects.The volume control is to the right of the dial, and the regeneration control is below the dial.The phone jack is located to the right of the regeneration control.

5.45- AI SOLAR CELL

Figure 4.6-A THE SOLAR TWO The tuned circuit and transistors are the same as in preceding receivers. The solar cell is made by International Rectifier Corporation and is available at most jobbers or mail order supply houses. Transformer T1is a 100K high impedance microphone to 1K transistor base transformer,reverse connected. For additional output, two solar cells connected in series could be used. 80 Receivers The Transistor

REAR VIEW OF THE REGENERATIVE RECEIVER Note that the coils are at opposite ends of the chassis to prevent any undesired coupling.The remainder of the components are mounted on the fiber terminal strip.

How It A tuned circuit,L1 and C1, is to build up to a high value.If the signal Works top -coupled via capacitor C3 to is increased too much, the stage oscillates, another tuned circuit, L2 and C2. and the speaker emits sounds akin to a The value of C3 has been carefully chosen pet shop!The audio stage TR2 is a con- to give high gain consistent with good ventional amplifier designed to feed into selectivity. Transistor TR1is a detector high impedance headphones. similar to that in the TR- ONE receiver. In this case, however, regeneration is ad- Construction The construction may take ded to increase the gain of the stage. This Hints almost any form.In the means that a little of the output has been photograph it will be no- returned to the input of the TR1 via the ticed that the authors used a 5" x 7" chas- tickler winding on L2 , causing the signal sis to support the tuning capacitor and the Radio Handbook Super Three 81

ANT. 4 REGENERAricw C3 L, 51111F VOL UME 3 -8JJF+ TR2 -6 8 V.

PHONES

3 4704

1. 0 MA EARTH GROUND ON OFF 2 Lt 4 5 2 o -F 9 V. - ITI IIII11111

4 3 3

Figure 4.7-A strip of phenolic material 'containing most SCHEMATIC DIAGRAM OF THE of the components. A metal panel is an im- REGENERATIVE RECEIVER portant adjunct to a regenerative receiver to Parts List prevent hand capacity effects when the re- C1, C2 - Tuning capacitor, 365 ,ufd. ceiveris near oscillation. The coils are per section (Miller #2112). wound according to the data given. Note L1 Primary (1and 2) 40 turns, that it is essential that all the coils be wound #32 enamel.(Use this size in the same direction. That is, winding 4-5 wire for all windings.) Secon- on L2 must be in the same direction as dary (3 and 4) 85 turns, leave winding 3-1, in order to permit the de- 1/8" between windings. tector to regenerate.If the windings are 12 Tickler(4 and 5) 35 turns. in the opposite direction, it will be neces- Secondary (1 and 3) 80 turns, sary to reverse wires 4 and 5 to sustain os- tapped at 28 turns from #3 cillation.Keep the coils away from metal end, spaced 1/4"between a distance at least equal to their diameter. windings. TR1 2N247, 2N274, 2N371, 2N- 1745, 0C44. Operation Setthe trimmer capacitors on C1 and C2 to about center TR2 2N408, 2N1380, GT81R, OC- and tune a station in with the regeneration 75. control P1 turned down (arm of potentiometer at the emitter end). Now adjust It will be found that the setting of P1 the trimmers one at a time while rocking which gives maximum output is just below the tuning capacitor back and forth. Sev- the point at which oscillation commences. eral maximum settings will be found, but Thissettingis also the point at which one setting in particular will be louder greatest selectivity occurs.If the receiver is than the others and this setting is the one allowed to oscillate, output will be low and which should be chosen.The alignment distorted. should be done with the tuning capacitor somewhere near the center of its range. 4.8 The Super Three A drift, MADT, or other type of high frequency transistor, will give superior re- As its name implies, the Super Three is sults at TR1. Ordinary i.f. amplifier types a three -transistor superheterodyne which are not recommended. Almost any general willdrive a speaker on strong stations purpose transistor may be used for TR2. and will operate from theloopstick antenna 82 Receivers The Transistor

EXTERNAL ANT. 331F_ANv2 K I FTa TR, I FT1 TRa F-4- - C R TR3 5- 1- 51 Ti Li si 2 SPKR 12 ,

1_00 + 61JF-

all3 ' 3:10KI .05 C2 I if 0 .01 3 2 4 6/ 20 1(70T. / 470 10 105

AV 5.6K R 322K 476 100 8- 101

ABC 100 .1J F

9 V. Figure 4.8-A SCHEMATIC DIAGRAM FOR THE SUPER THREE Resistor R 1 may be adjusted to suit different transistors in the output stage.Its value will determine the collector current of TR3, which should provide a collector voltage of approximately 4.5 volts. Thetotalbattery current is 6 to 8 ma.If phones are used in place of T1, resistor R1 may needadjustment to provide the proper collector voltage.

alone when the stations are not too far Parts List away.With an external antenna, stations C1, C2 - Two gang tuning capacitors, more than 500 miles away come through antenna section 290 at good headphone strength. The receiver Oscillator section 130 pfd. is equipped with an automatic gain control (Miller #2112 with plates re- circuit so that stations maintain a fair aver- moved to track, or Alps #-72)

age loudness. CR1 -1 N64, 1 N295, 0A85, or similar germanium diodes. How It stage (TR1inFig. The first ohms i.f.trans- 4.8-A) is a converter.It is both IFT1 - 25K to 600 Works former (Miller 2041 or simi- mixer and oscillator combined. The oscillator beats against the incoming lar). 25K to 1,000 ohms i.f. trans- signal and produces an intermediate fre- IFT2 former (Miller 2042 or simi- quency. Transistor TR2is an i.f. amplifier of lar). For wind- 455 kc. which increases the signal fed to Li - Antenna loopstick. ing data see text. (Miller 2003 the diode detector ( CR1). Here the signal a few turns removed is rectified and fed back to the i.f. amplifier with as a.g.c. voltage.It also is used to drive from the primary) for winding the audio stage.The i.f. amplifier is neu- L2 - Oscillatorcoil, tralized as explained earlier in Chapter 3. data see text. - Collectorto speaker trans- Construction A little more care is neces- former, 500 or 1,000 ohms to 4, 8, 16 ohms (Stancor TA - Hints sary in the construction of the Super Three than is 9 or TriCKI TY-44X). necessary with some of the simpler re- TR1,TR2- 2N1380, 2N371, 2N410, GT- ceivers. While layout is not critical,it 139, 00615 or similar. should follow some sort of order so that TR3 - 2N408, 2N1380,GT109,0C- the input and output circuits are reason - 72 or similar. Radio Handbook Super Three 83

TERMINAL STRIP

TO SPEAKER TO CI MEM

6 UNDER -CHASSIS LAYOUT

Figure 4.8-B UNDER CHASSIS LAYOUT OF THE SUPER 3 RECEIVER ably wellseparated. The receiver may be condensed into a very small package CLOSEUP VIEW OF THE of about the size of a pack of cigarettes if HOME-MADE I.F. TRANSFORMER the Miller subminiature i.f.transformers The performance is comparable to the are used. The prototype model was delib- miniature Miller series, but this ver- erately spread out so that the construction sion requires a shield. could be clearly seen and followed (fig. 4.8-B). It is advised that constructors use jumble would to a width of about 1/2". the larger layout unless they have had con- Melt a little candle wax over the coil to pre- siderable experience building receivers. vent it from coming undone. The chassis is bent up from a piece of The antenna coil L1 is wound on a loop - #18 gauge aluminum to a length of 6 1/2" stick which is a long ferrite rod or slab. a width of 3", and a depth of 1", but a Different loopsticks will call for a different commercial 5" x 7" chassis may be sub- number of turns.As a guide, closewind stituted. The panel is made from a scrap of 28 turns of #32 enameled wire about one laminated plastic 31/2" high. A small ply- third of the distance from one end.The wood cabinet, covered with imitation lea- ends of this winding are 1 and 2. Wind- ther, is used to give the receiver a profession- ings 3 and 4 consist of 5 turns of hookup al appearance.The gain control was later wire wound alongside the larger winding. changed to a switch -type potentiometer, and The two windings should both be wound the switch is used to disconnect thebattery. in the same direction and 2 and 4 should be adjacent to each other.The loopstick is The Coils The coils are wound in the connected by 6" leads so that is may be following manner.On coil L2, a 1/2" slug -tuned form, wind 4 turns of #32 enameled wire. Space the winding out to occupy about 1/4".This is the emitter winding marked1 and 2 on the SEC. schematic.Next, closewind 13 turns over the previous winding.The ends of this winding are 3 and 4. On top of this wind Figure 4.8-C 130 turns. The start of this winding is #6 Construction details for th ehome-mad e on the schematic.The latter winding is 455 kc. transistor i.f. transformers. 84 Receivers The Transistor attached to the top of the chassis away from setting.If no carrier is heard, reverse the metal work.The J. W. Miller #2022 can be two leads 3 and 4 on coil L2. An r.f. probe, used for the oscillator coil L2 and the ifavailable,will determine whether this #2004 can be used for the loopstick Li stage is oscillating.If the heterodyne is if you prefer not to make your own. heard on 1500 kc., the oscillator tuning capacitor should have the plates approxi- AdjustmentAfter the wiring has been mately half way out, indicating a receiving finished andthoroughly frequency of about 1050 kc.If no signal checked,inserta milliameter in one of generator is available, the i.f. transformers the battery leads and check the current. will have to be peaked on a weak station. Once a signal is received, the rest will be Ifitis greater than 6 to 7 ma. switch off the receiver and check the wiring.If the easy. All stages should now be aligned current reads correctly, place the receiver with the tuning capacitor set at approxi- near a broadcast radio and listen for the mately center position.If the trimmer is carrier from the transistor oscillator as the fully meshed and cannot peak the signal, tuning capacitor in the SUPER THREE is the loopstick secondary winding requires rotated. Place the receiver near the antenna more turns.The loopstick may be pre - for this.The oscillator beat note should adjusted to frequency by using it and its tune 455 kc. higher than the capacitor associated tuning capacitor as a wavetrap and nulling out a station on the broadcast receiver. For example, if the nulled -out UNDER CHASSIS VIEW OF THE station is on 1200 kc., then the setting of THREE -TRANSISTOR the tuned circuit is also 1200 kc. BROADCAST RADIO Radio Handbook IF Transformers 85

will be the same in all cases for a frequency of 455- 465 kc.This winding, in conjunction with the 200 mfd. resonating capacitor will have an impedance of approximately 90,000 ohms (based on a reasonable working Q). Drift transistors are designed to work into an impedance of approximately 100K %kir and therefore may be connected across the primary without using a tap. Other junc- REAR VIEW OF THE tion transistors generally will require an THREE -TRANSISTOR impedance of 10K to 25K for reasonable BROADCAST RECEIVER impedance matching.

Constructing IF Transformer Place a tap on the180-turn Designed primary at 60 turns up Transformers to Match from the battery end of J. W. Miller i.f. transformers have been 10,000ohms the winding (terminal #1 ). used in this section of the book because Fora 600 -ohm secondary, they are inexpensive, compact and readily to match the transistor base, use a 9 -turn available.However, overseas readers may link of the same wire.For a 2,000 ohm have trouble locating these units orany secondary, to feed the diode detectors, transistori.f.transformers.When this wind an 18 -turn link over the primary. is the case, home -built transformers can yield the same performance as can com- Transformer Place a tap on the 180 -turn mercial units. Designed primary at95 turns up Remember that core material and size to Match from the battery end of may change the inductance of the windings 25,000 ohmsthe winding (terminal #1). considerably. Therefore the data to follow For a secondary, use the must be considered only a guide.The same turns ratio as for the previous trans- important parameter of these transformers former. is the turns ratio, and if it is necessary to in- The secondaries are wound with the crease or decrease the number of turns on same gauge wire as the primaries and are the primary, the secondary and also the taps should be varied in the same proportions. FRONT VIEW OF THE SUPER THREE The home -built transistor i.f. transfor- BROADCAST BAND RECEIVER mer shown in the schematic (Fig. 4.17-A) and accompanying photograph was wound on a Neosid 3/8" form.These forms are generally available throughout the British Empire.Similar forms should function as well. The The primary should consist of Primary180 turns of fine wire, #32 to #38swg enamelled,jumble wound to a width of approximately3/8" on a 3/8" form. Subject to the conditions mentioned above, the total primaryturns 86 Receivers The Transistor wound directly over the primaries. The metal chassis will eliminate many of the end of the primary nearest the core is ter- troubles that otherwise may be encoun- minal #4.The end of the primary farthest tered (Fig. 4.9-A). from the core is terminal #3. If Litz wire How ItThe loopstick and oscillator coil is used to wind the primary, thetransfor- Worksoutput links are in series, and mer will have a higher Q.The transfor- oscillator injection is to the base mer should be shieldedby a can with a of transistor TRi .Feedback takes place diameter of at least one inch. via the lead from the collector circuit to the tap 5 on oscillator coil L2 . 4.9 A Four Transistor The forward bias resistor RI, on the Superheterodyne second i.f. stage, is' fed back to the emitter of the first stage. The voltage drop across The four transistor superheterodynehas R2 supplies the forward bias toTR3. been designed around the popularmidget The When positive a.g.c. voltage is fed to the MILLER coils and i.f. transformers. base of TR2 , the current through TR2 de- cans of these components areabout 1/2" With care, it is creases. The voltage across its emitter square and 3/4" high. swamping resistor R also decreases, and possible to build a receiver into a very the forward bias to TR3 decreases.This small space indeed. The wristwatchradio reality! is a means of offering good a.g.c. action of the comic strips is almost a over two stages with aconsiderable re- However, the compression of a receiver duction in the number of components in into a 2" x 4" area calls for acertain the a.g.c. line. amount of experience if unnecessary coupling and consequent undesired oscillation The neutralizing capacitors, C2 and C39 transis- are to be avoided.A larger layout on a may not be necessary with many tors, particularly the drift type. However, with other transistors, neutralization may TOP VIEW OF THE FOUR -TRANSISTOR be desirable even though oscillation is not BROADCAST RECEIVER taking place.The presence of feedback A piece of insulating board was used through a transistor may modify the input for the chassis. impedance of the stage and reducethe gain. This subject was discussed in section 3.3. Radio Handbook 4 Transistor Superhet 87

C2 -.SEE TEXT- C 3

I( 3 TR, IFTI TRa IFT2 TR3 IFT3 TR4 r r-

I CR, 51 1011 8 I I `,=4 12 211

_ _1 3 L4_ - -13 4 L J3 )F-5 / 05 .05 .05 R, 5.6K 27 16 81JF 470 75 S 1.2 K -u+ 1R.26 220

5 22 6 ACC

120K 10K

K Q9r 1, Figure 4.9-A = ÷ 10011F SCHEMATIC DIAGRAM FOR THE + 9V. - SPKR FOUR TRANSISTOR SUPERHETERODYNE

Parts List CI 0,b-Tuning capacitor, antenna sec - The i.f. signal is rectified by CRi and C2 Lion 10-130 /4ufd., oscillator sec- fed to TR4, which, in turn, passes the tion 10-78 mfd. (Miller #2110 amplified audio signal via the output trans- or similar) former to the speaker. C3 - Varies with transistor type. For Construction The prototype receiver was 2N1380, CK760, 2N139, GT139, Hints built on a piece of 1/16" use 15 NM. For 2N136 use 5 thick phenolicboard meas- Nifd. For 2N371 no capaci- ing 2" x 4".The oscillator coil and i.f. tance is required. transformer are mounted along the rear CR,-R.f. diode, 1N48, 1N60, 1N64, from left to right.The tuning capacitor 1 N295, 0A85, or similar types. is at the left in front of the oscillator coil, !FT] - 25K to 600 ohms (Miller #2131 and the 2" speaker is at the right. There or similar type). is just room for the frame of the speaker IFT2 - 10K to 600 ohms (Miller #2031 to project over the i.f. transformers. The or similar type). midget audio gain control is under the IFT3 - 25K to 1,000 ohms (Miller#2042 board at the left and does not show in or similar type). the photograph.The oscillator and i.f. Li - Loopstick antenna (Miller #2005, cans are mounted with their lugs fore and or see Fig. 4.8-A). aft. The lugs are pushed through small 12 - Oscillator coil (Miller #2021, or holes in the board and bent over. Two circuitry of Fig. 4.8-A may be pieces of #22 tinned wire are run along substituted for handwound coil). the board in line with the lugs to which - Collectorto speakertrans- they are soldered.A third wire is run former, 500 or 1,000 ohms to along the front of the board. The ends 4, 8, or 16 ohms (Stancor TA -9 are bent over and pushed through small or Triad TY-44X). holesin the board material.A couple TR1,2- 2N136, 2N139, 2N371, of cross members are now added form- TR3- 2N1380, CK760, GT139. ing a grid, and all the cross -over points TR4- 2N107, 2N408, 2N1380, are soldered.The purpose of the grid is GT109, 0070. three fold:(a) it holds all the cans firmly 88 Receivers The Transistor to the board (they tend to flop about otherwise); (b) all the cans are connected to the common line; (c) the tuning capacitor, gain control case, resistor and capacitor, returnsare connected to the wire grid. This is really a busbar hook-up similar to those used in receivers years ago, before the point-to-point wiring technique be- came popular. A tie point is made by pushing component pigtail wires through holes in the Figure 4.9-B board, bending the ends over on the other THE FOUR TRANSISTOR side, cutting the ends off so that they are SUPERHETERODYNE not too long, and then soldering them Under chassis drawing of the Four together.Some of the solder knobs may Transistor Superheterodyne. The be seen on the top side of the board in heavy lines are the ground busbar. the space normally occupied by the speaker. The output transformer feet are soldered it easy to try different transistors and com- to two of the ground point knobs on the pare their performance. top of the chassis. It is strongly recommended that unless you have had considerable experience build- The receiver may be installed in a plastic ing compact receivers, you construct the case. The case must not be metal, as metal receiver first on a chassis similar to that will render the loopstick useless.The used to house the Super Three previously original case used once housed a woman's described.Then, when you feel that you necklace. The interiorof the receiver have mastered any problems which may (though a thing ofbeauty to theperson who arise, have the feel of the alignment op- builds it) may not necessarilybe considered eration, and are confident enough to go beautiful by those who know nothing about further, remove the parts from the chassis radio.Consequently, the transparent case and start the condensed model. Draw the was sprayed inside with a paint bomb. Be- layout on a piece of paper the same size fore the painting, a small dial card was glued as the phenolic board and try various lay- to the inside, face up.Also, a number of outs until everything fitsin without the small holes were made in the lid to let the receiver taking on the unkempt appearance sound come out.The shaft was cut off of of a sparrow's nest. the gain control potentiometer. Both ends were tapped, and then the shaft was replaced through a hole in the side of the AlignmentA signal generator is inval- cabinet.A case a little larger than the pro- uable when it comes to the totype would make this step unnecessary. alignment of a superheterodyne.Those who do not possess one may follow the It is not essential that you use transistor alignment procedure suggested for the sockets.It is a simple matter to drill three Super Three. holes underneath the transistor and make To proceed with the alignment, couple tie points as described above.This was the generator (tuned to 455 kc.) through done with the output transistor in the a small capacitor of 5 to 20 Nrfd. to the prototype model and may be seen in the stator of Cia . Switch on the generator photograph.Sockets were generally pre- tone and peak the i.f. transformers for max- ferred in the prototype because this made imum volume. Reduce the generator gain Radio Handbook The Mobileer 89 until the signal is just audible. A strong generator signal will create an excessive a.g.c. voltage which will cause false indica- tions. Next, set the generator to 1500 kc. Set the receiver capacitor to approximately this frequency and adjust the trimmer on the oscillator section of Cl for maximum signal.Set the generator at 600 kc. and adjust the oscillator coil slug until Cl peaks at the correct position. Set the generator and receiver to 1100 kc. and adjust the trimmer TOP VIEW OF THE on the loopstick portion of Cl for maxi- MOBILEER CONVERTER mum.It may be necessary to repeat this This simple device will convertthe3.5- procedure once or twice because there is a 4.0 Mc. ham band down to the broad- certain amount of interaction between the cast receiver range. The knob at the various adjustments. Finally, with the loop - leftis the antenna trimmer. Above stick in position and the generator output thatisthe antenna coilslug. The lead held near the loopstick, adjustCia crystal (near the center of the chassis) trimmer and the i.f. transformers again for isbetween the mixer coil and the maximum volume. oscillator coil. In an area far removed from stations, an external antenna may be connected to the fixed plates of Cia through a 5/4Ufd. capacitor. Volumeshouldthenbe sufficient The selectivityorabilitytoseparate for all normal listening. stationsis determined by the receiver to which the MOBILEER is connected, and 4.10 nothing can be done in the converter itself The Mobileer to improve this. The MOBILEER is designed to operate Many have wished to copy 75 meter with 9 to 12 volt batteries, although good stations on the auto radio, or perhaps on performance, with reduced gain, is obtain- the XYL's broadcast band receiver.The able from as low as 6 volts. wish can be fulfilled for the price of a week's supply of cigarettes and the time How It The heart of the MOB ILEER is you would have spent smoking them. Works the combined mixer and crystal Here area few of the MOBILEER's oscillator. Considerable work excellent qualities.(1) In a side by side went into the design of this important part test with a commercial 75 meter commun- of the converter. A great number of cir- ications receiver the MOB ILEER held its cuits were tried but all suffered from very own.There was no noticeable difference bad image and spurious signal break- in sensitivity between the two.(2) If the through.So bad in fact were these un- battery voltage is dropped from 12 to 6 desired signals, that the whole idea of a volts the crystal oscillator will move only converter using only two transistors was a few cycles.(3) Image and spurious sig- nearlyabondoned. Carefulthought nals are negligible and certainly no more brought the realization that the spurious than a tube counterpart. (4) The MOBIL- signals were due to incorrect operation of EER uses only two transistors, employs a the mixer -oscillator stage.The transistor war surplus crystal, and draws 3 ma. from had been operating over the entire part of a 12 volt battery. the dynamic transfer characteristic curve. 90 Receivers The Transistor

ANT. .---WAVETRAP 001 ' (SEE TEXT) Ll TRI

CAR RADIO

- 6 T012 V.1 - TR1 -2 2N1745, 2N371, 0C170, 00615

Figure 4.10-A pacitive voltage divider.This is explained SCHEMATIC DIAGRAM FOR THE in section 3.2.The values of C2 andC3 MOBIL EER CONVERTER should not be changed.The oscillator Thisis a 3.5 - 4.0 Mc. converter for operatesin the common -emitter config- car or house radio.The Mobileer uration.The mixer part of this stage op- uses only Iwo transistors, yet has a erates common base with the signal inject- performance equal in every respect ed into the emitter. to its tube counterpart. The r.f. amplifier also operates common Parts List base.This configuration was chosen for C1- 35 to 50 Aufd. antennatrimmer. simplicity and to prevent the need for neu- C2 - 100 nafd. mica. tralization, which might have been necess- C3 - 0.001 pfd. mica or ceramic. ary if transistors other than those suggest- Li- 70turns,#32 enamel jumble ed were used.If other transistors are in- wound on 3/8" form, 3/8" long. corporatedinthepresent circuit they Antenna tap 5 turns from bottom, should have a high alpha cut-off frequency. emitter link 2 turns, small hook- Both stages are fully stabilized to allow up wire over primary. (Miller safeoperation in the high temperatures #4400 or similar slug tuned form) often encountered in closed automobiles. 12 40 turns #32 enamel, same as Li.Emitter link 3 turns of hookup The The crystal for the MOBILEER wire over primary. Crystalneed cost no more than a dollar. L3 90 turns, #36 s.c. wire, otherwise A vast number of crystals are same as L1.Link 3 turns hookup available on the surplus market for much wire over primary. less than this.Any crystal between 4550 and 5100 kc. will operate in this circuit. If a 4550 kc. crystal is chosen, the broad- The peaks of the oscillator waveform were castreceiveris tuned between 550 and being rectified, producing exactly the same 1050 kc., which represents the frequencies effects one gets when a diode is connected between 4.0 and 3.5 Mc. respectively.A in series with the antenna lead in the pres- 5.1 Mc. crystal will place 3.5 Mc. at the ence of a strong station (cross modulation). high end (1600 kc.) of thebroadcastband. With this realization, the circuit design was A crystal frequency between these limits modified, the cross modulation stopped, will cause the 75 -meter band to cover the and the MOBILEER took its present form, middle portion of the receiver dial. Fig. 4.10-A. A point to note isthat the broadcast The oscillator is a Colpitts circuit with receiver tunes backwards. That is, the low the capacitor's C2 andC3forming the ca - frequency end of the 75 -meter band will Radio Handbook The Mobileer 91

the receiver dial will hold for 75 and 80 meters. L,1 COLOR Construction DOT Construction may take any Hints desiredform. The main MILLER CO/L TERMINALS points to observe are that the input and output circuits should not Figure 4 10-B be placed too close to each other, and com- Ifstrong broadcast stations "break- plete shielding of the whole unit is requir- through" the converter, a wavetrap ed,includingthe lead to the broadcast re- may be inserted in the antenna lead. ceiver. Incomplete shielding will allow The best position for the trap is inside broadcast stations to be picked up and to the Mobil eer case close tothe antenna compete with the 75 -meter signals.If the connection.The coil may be any os- unit is constructed on a flat plate, as shown cillator coil, such as the Miller #2022. in the photograph, it may be easily dropped Tune the slug until the offending sta- into a small U-shaped chassis and secured tion nulls out. in place. If desired, a switch may be built into be on the high frequency end of the re- the unit for bypassing the converter and ceiverdial and vice -versa.However, if allowing normal operation of the receiver, either of the suggested crystal frequencies Fig. 4.10-C.If a switch is used, take care are used, the stated calibration marks on with the lead dress and shield the wires

UNDERCHASSIS VIEW OF THE MOBILEER Note that the components are mounted on a length of terminal strip stock.It should be possible to use the Mobil eer on 40 meters by using a 6.0 Mc. crystal and removing a few turns from the coils. 92 Receivers The Transistor

Here is another interesting possibility. If coil L3 is replaced by a 2.5 mh. r.f. choke, the MOBILEER can be used with the fa- mous "Q-5'er" Command Setreceiver (BC -453A), which tunes 190-550 kc. Not only will this make an excellent home station for the Novice or General Class Am- Figure 4.10-C ateur, but it will also provide a stable mo- A double pole, double throw switch bile receiver.The MOBILEER could be will remove the Mobileer from the located in the trunk of the car, along with circuit when broadcast reception is the transmitter, and only the "Q-5'er" required. A third deck on the switch need be positioned near the driver. may be used to break the battery voltage. 4.11 The Product right up to the switch. Since the input and Detector output leads oftheMOB MEER arebrought to the same switch, this is a likely place for the transfer of broadcast stations past the As mentioned elsewhere in the book, converter. a product detector is a mixer combining the i.f. signals with the b.f.o. to produce HomeThe MOBILEER may be operated a separate i.f.in the audio range.Most Stationfrom a large outdoor antenna a - mixer circuits may be converted to a prod- U se head of a domestic receiver, but uct detector with little alteration. However, it is possible that in areas close to a mixer is used at the front of the i.f. strip strong broadcaststationssome break- where the signal level is very low, and a through may be experienced. If this is the product detector is used at the rear of the case, the crystal frequency may be altered i.f. where the signal level is very high. to shift the 80 -meter band away from that To prevent overload ofthe detector when portion of the dial.Alternatively, a wave - the normal mixer is used, the input signal trap may be inserted in the antenna lead has to be attenuated and the consequent close to the converter. Suitable components loss in recoverable audio is considerable. for a wave trap are shown in Fig. 4.10-B This necessitates additional amplification to A receiver with a built-in antenna should not be used with the converter, for it will bring the signal to par with a.m. detection. A slightlydifferent product detector be almost impossible to prevent broadcast signals from QRM'ing 75 and 80 meter (though it may appear to differ very little from the usual) has large signal handling signals. capabilities and very linear output.The circuit of this detector is shown in simplified form in Fig. 4.11-A.The detector operates with little or no applied collector voltage and will handle as much as one volt of input signal at the base. A large input signal requires that there Figure 4.10-D be a large b.f.o. component applied to the This layout was used in the prototype, emitter circuit.Assume that the required whichleft room for the switch and signal is applied to the emitter from the wave -trap at one end.Any similar b.f.o.Also assume that the bottom end layout may be used. of R2 is grounded.Positive half cycles of Radio Handbook Product Detector 93

An etched circuit version of the product defector shown in Fig. 4.11-B.

b.f.o.voltage will cause the base to be neg- collector voltage, causing it to vary in am- ative with respect to ground, and the base plitude.Negative b.f.o. cycles applied to will be forward biased.The base -emitter the emitter will increase the base emitter path will be a very low impedance, and con- impedance, and the collector current will sequently, the base will assume a positive cease to flow.However the charge on Cl potential with respect to ground. Thebase- will remain. collector path will now be a low impedance, In practice, when the product detector and the collector will also assume apositive is built into a receiver, and where the b.f.o. value.This will cause the capacitor C1 to and signal voltages are not excessive, opti- charge and actually maintain a steady volt- mum output requires that a small d.c. voltage.The transistor has developed its own age be applied to the collector. The tran- collector voltage.This voltage is a prod- sistor is thus biased part way up the knee uct of the b.f.o. voltage and is negative of its characteristic curve. This means that with respect to the base although positive since the transistor is drawing current, the with respect to ground. Thus, a signal at. collector -base impedance is low. The out- the base will be superimposed upon the put impedance of the product detector is consequently very low. 2N1380 AdjustmentFig. 4.11-B shows the circuit of atransistorized product detector which may be added to an existing receiver. The input from the receiver should be adjusted so that overload of the detector and frequency modulation of the b.f.o. does not occur. Figure 4.11-A When the b.f.o. is detuned 10 kc. away THE BASIC PRODUCT DETECTOR from theintermediate frequency, there 94 Receivers The Transistor

I.F. INPUT With the b.f.o. on, the potentiometer is TI TRi 0.251JF adjusted for maximum output consistent AUDIO OUTPUT with linear operation.In many tests carried out by the authors, optimum collector 0 voltage seemed to be around 0.25 volts. I8-9 V. The collector current under these con- REG. ditions will be around 2 ma. with a supply 47 K of 1 2 volts. If a high audio output level is required from the product detector, it may be neces-

.005 5.8 K sary to use a b.f.o. amplifier to increase EA. .001 SILVER MICA the injection voltage.The signal at the 2.711 .005 base of TR1 should never exceed in amplitude the b.f.o. voltage at the emitter.A Figure 4.11-B large signalatthe base may frequency A SIMPLE PRODUCT DETECTOR modulate the b.f.o. unless an isolating This product detector can be added to stage is used. existing receivers. For maximum stability, silver mica capacitors are used in the oscillator circuit. 4.12 A Professional Communications Receiver Parts List Li - 90 turns, #34 enamel scramble wound on 3/8" slug tunedform. This receiver has amplifieda.g.c., a Link 10 turns of the same wire, product detector, and provision for an S wound over primary. meter.The receiver covers the basic fre- T - 455 kc. transistor i.f. transformer quency range of 3.5 to 4 Mc, and can be (J. W. Miller #2041). used in conjunction with a converter for TR] ,- 2N247, 2N274, 2N371, 2N- the other bands.Then the receiver be- TR2 1745, 0C139. comes a tunable i.f. and the stability, even on 10 meters,isexactly the same as on 80meters. To make the project less com- should be no audio output.Any output plicated, the converter was not built into under these conditions indicates that the the receiver.However, there is no reason productdetectorisbeingoverloaded. why this cannot be done, and the only pre- However, when the b.f.o.is turned off, caution to observe is in shielding.This a.m. detection should normally take place. is necessary to prevent receiver oscillator If the circuit is to be used for a.m. recep- harmonics from mixing with other prod- tion, a small amount of bias should be ucts and producing a multitude of "birdies" appliedto the detector base as in Fig. in the converter. 4.11-B to prevent distortion at low signal levels.The bias will not affect the product Howit Because many of the principles detector action. Works used in the receiver arenot widely The value of the collector load resist- known, a fairly complete descrip- ance of TRI is not critical.However, the tion of the design and function ofcompon- operating point may be changed if an in- ents are given in Fig. 4.12-A. correct value is used.A potentiometer The r.f.stage transistor TRI is oper- could be temporarily inserted in the cir- ated grounded base.This is the equiva- cuitto substitute for the detector load. lent of grounded grid in tube circuitry. 1 Radio Handbook Communications Receiver 95

CLOSEUP VIEW OF THE Originally,ther.f.stage was operated COMMUNICATIONS RECEIVER grounded emitter. This was changed when I.F. AMPLIFIER STRIP it was observed that the action of the a.g.c. on the r.f. stage was causing detuning of frequency variations were quite severe. Al- the local oscillator. When the a.g.c. varied, though unnoticeable on a.m., s.s.b. stations the flow of collector current and the out- gave a gargling sound.Because the output impedance of the stage also changed put impedance was very high with ground- and reflected a change in mixer input im- ed -baseconfiguration,animpedance pedance. The input impedance of this change was but a small percentage of the latter stage, as far as the oscillator is con- whole.Thus, the reflected change across cerned, is highly capacitive and is actually the oscillator coil was negligible. Another a capacitor connected across part of the advantage offered by grounded base is free- oscillator coil,L3 .On strong stations the dom from the need for neutralization.

FRONT PANEL VIEW OF TH ETRANSISTOR COMMUNICATIONS RECEIVER The controls,from left to right, are antenna trimmer,r.f. gain, on -off switch, audio gain, and b.f.o. tuning. 96 Receivers The Transistor

.005

lit RF AMPLIFIER, 3.5-4.0 MC. MIXER, 3.4-4.0 MC. TR i La TR 2 IFT1 4rle 1 I 13 OA I T -ii ;

1 ,----- ,a 0 Az ,_ _, r. 0 V. 5 -6.6K 01-)I- L_ _ 2201.c -L01 .005 1 IA T -7.6V .0, 470

ADC OC L

* =SILVER MICA

Figure 4.12-A

SCHEMATIC DIAGRAM FOR THE PROFESSIONAL COMMUNICATIONS RECEIVER Note that voltage data is included, and readings are from the pointsgiven to chassis ground.

IF AMP., 455 EC. /F AMP., 455 MC. AGC AMP.,435 MC. PROD. DET. /AM DET. Nc Rc Nc Rc 390 --v." TR4 TR5 TRe TR7 3.11F r iFrz 7 r 1FT3 7 -3.51/ .11% D I 1 AUDIO d I A '0 I a -05o L _, .01 -7.2 A2,0

120K1

-6.4 V. 100

POINT A (SEE TEXT) C O AGC*1

-4- A=SILVER MICA 1.2 V RF

SIDEBAND SELECTOR 2.7K 2.7K 1 Radio Handbook Communications Receiver 97

Ti AUDIO TRIO T1

8.4V. I ENER DIODE

Figure 4.12-A cont.

Parts List

C10, - Tuning capacitor from BC -455 Ti 10K to 2K interstagetransform- 16,1c receiver. Modified per text. er (Stancor TA35 or Triad TY- IFTI,- 25K to 600 ohms, 455kc. tran- 56X). 2,3 sistor i.f. transformer (J.W. Mil- T2 500 ohms to speaker (Stancor ler 02041). TA35 or Triad TY-45X). Li - 27 turns, 034 enamel, on 3/8" TR1 - 2N247, 2N274, 2N371, OC- diameter slug tuned form (Mil- 3,8 619. ler 04400 or similar), antenna TR2 - 2N412, 0C45. tap at 7 turns, emitter link 2 TR4,5 - 2N247, 2N371, 2N410, turns small hookup wire wound GT139, 0C44. beside or over primary. TR6 - 2N1380, 2N408, GT81R, L2 - 27 turns, 034, form as above, 0072. base link 3 turns as in Li. TR79 - 2N412, 2N1380, 0C44. L3 - 16 turns, 034 enamel, on 3/8" TRio,- 2N188A, 2N408,2N1380, form as in Li. Link, 1 turn at 11,12 0072 (seeMini -Amp data, ground end. Chapter 2). 14,5 - 90 turns, 036 silk covered on Z 8.2volt zener diode (Inter- 3/8" form, tap 30 turns from nationalRectifier1 N151 1or bottom. similar).

No doubt the observant reader has no- The Mixer The mixer is conventional, ticed an unusual connection between the and with the oscillator signal fed base of the r.f. stage and the emitter of the Oscillator into the emitter.The oscil- second i.f. stage in Fig. 4.12-A.This is lator is a very high "C" Col- another way of obtaining amplifieda.g.c. pitts circuit (see Chapter 3). As this is the voltage. Because of the presence of the heart of the receiver's stability, consider- i.f. emitter resistor, the emitter of TR, is able care went into the design. The result negative with respect to ground.Auto- is an oscillator which shifted only 150 matic gain control voltage decreases the cycles in 4 days of continuous operation, i.f.stage emitter current, and the voltage with a maximum excursion noted of400 across the resistor also decreases.The cycles.The operating point of the oscil- stage produces amplified a.g.c. voltage. ator has been very carefully chosen to min- 98 Receivers The Transistor

imize frequency shift due to temperature TR1 0.6 ma. changes and supply voltage variations. Al- TR2 0.2 ma. (200fiamp.) though the oscillator will withstand a cer- TR3 1.0 ma. tain amount of supply voltage change, it TR4 0.8 ma. must be voltage stabilized, nevertheless. This TR5 1.0 ma. is necessary to prevent the very large class TR6 Nil. B current demands of the output stage TR7 2.0 ma. (b.f.o. on) from modulating the oscillator.An 8.2 - 30 amp. (b.f.o. off) volt zener diode regulates the supply to the TR8 2.0 ma. receiver, with the exception of the audio TR9 1.0 ma. stages. TRIo 1.0 ma. 3.5 ma. (approx.) Mechanical considerations will govern TRH 12 the stability of the receiver perhaps more than will electrical.For this reason the Figure 4.12-B excellent variable capacitor from an ARC -5 Chart showing current drawn by PRO- surplus receiver was chosen to tune the FESSIONAL COMMUNICATIONS RE- transistor receiver.This capacitor has a CEIVER transistors.For these meas- built in gear mechanism of unsurpassed urements, the r.f. gainis maximum quality. Unfortunately, its plates are cut and no antenna is connected to the for straight line frequency when straight receiver. line capacity would have been better for this circuit.However, by not using the whole range of the capacitor, and by removing many of the moving plates, excel- transistor, and a voltage drop is developed lent spread and good tracking were ob- across the 1K load resistor in the base cir- tained. cuit.This voltage, being negative in value, causes a collector current to flow and a re- The I.F. The i.f.amplifiers are con - sultant drop in collector voltage.Bias to Amplifiersventional and arebuilt around the i.f. amplifier (and indirectly to the r.f. the compact Miller i.f. trans- amplifier) is reduced.Note that the vary- formers.A duplicate i.f.strip was also ing collector current may also be used to constructed using home -built transform- operate an S meter.This adaptation is ers, and its performance was alsoexcellent. described later. Data for these homebuilt transformers is The ProductThe product detector ( TR7) supplied at the end of this chapter.The and is a mixer circuit, and its i.f. stages are a.g.c. controlled by reducing AM Detectorsimilarity to the mixer stage the negative base bias and thus the emitter at the front of this receiver current.The r.f.-i.f. manual gain system has been described in section 4.11.The controls the same bias line. base of the detector is fed through the 390 /rend. capacitor which was chosen to pre- The AGC The a.g.c. amplifier ( TR6) is sent the correct signal level to the base. Amplifier in reality a detector with ahigh A low collector voltage provides a curve value load resistor. It operates best suited to the purpose.It is signifi- near cutoff, and the standing collector volt- cant that when the b.f.o. is detuned, the age is high.Consequently, the a.g.c. line product detector has no output. The com- from its collector to the i.f. amplifiers is ponent values are somewhat critical, and highly negative.A signal at the base is itis recommended that the transistor in- rectified by the base -emitter portion of the dicated be used. However, information will Radio Handbook Communications Receiver 99

be given later so that builders may adjust sort. The effect was remarkable. With the the components to suit different transis- r.f. gain wide open and the a.g.c. pumping tors.The 0.01 pfd. capacitor from the violently on the strongest of sideband sig- collector to ground is an essential part of nals, not a cycle of shift could be detected. the product detector circuit and should Even c.w. gained a musical quality not pre- not be omitted. viously noticed. When the b.f.o. voltage is removed, the The Audio collector current of TR7 drops to near- Following the detector comes zero. The transistor is forward biased Section aclass A audioamplifier just enough to overcome distortion which (TRIO ). This amplifier drives is the result of the knee at the bottom of a class B push-pull stage (TRII-12) similar the characteristic curve. to the MINI -AMP described in Chapter 2. Because of its low idling current and the The BFO Except for the difference in fre- consequent saving of battery power, class and quency, the b.f.o. ( TR8) is the B operation was chosen. Amplifiersame as the local oscillator described earlier.The large signal at thebase of the detector causes a UNDER CHASSIS VIEW OF THE change inimpedance which, if no b.f.o. COMMUNICATIONS RECEIVER buffer is used, causes a change in the beat Note the construction technique used frequency.If the input signal is reduced to fabricate the i.f. amplifier. The beat in value, the detector output becomes low, frequency oscillator is mounted in a and under certain circumstances transistor can above the chassis, at the lower hiss level can become a problem.The right corner.Components for the re- authors turned to the use of a b.f.o. am- ceiver local oscillator are located be- plifier and buffer ( TR9) only as a last re - low the chassis, just to the left of center.

0411,*

11$41J AO. C.*O cg C &Ms r . `Ns ,---...,,ro 100 Receivers The Transistor

A "gremlin" distorted the signal even TR7 in this, the simplest of circuits. This dis- AUDIO OUT. tortion was traced to an above -audibility audio oscillation and was remedied by placing a 0.05 /dd. capacitor across the primary of the output transformer. The audio output is greater than 250 milliwatts and is of excellent quality when connected to the headphones or to an ex- Figure 4.12-C ternal speaker. The small inboard speaker SCHEMATIC DIAGRAM FOR THE will handle only 100 milliwatts and leaves B.F.O. TRAP USED IN THE something to be desired in the way of COMMUNICATIONS RECEIVER quality. Note that the trap must be grounded at the same point as the collector by- Techni-The zener diode, like the VR pass for TR 7.CoilL consists of 30 calitiestube in a vacuum tube sideband turns, #34 enamel., jumble wound on receiver, is an absolute must.Its a 1/4" diameter form. value is not particularly critical. Any value (J.W. Miller 20A00ORB1) around 8 volts is satisfactory. A great number of different transistors were tried in the various circuits to ascer- of phenolic sheet for the sake of conveni- tain that others than those specified could ence. Components are mounted on top, be used without too much circuit alteration. and the wires are brought through small In most cases it is only necessary to plug holes as in an etched circuit board. In lieu the substitutes into the sockets.The two of a ground strip around the edge, a piece important transistors in the receiver are of #18 tinned wire was run down the two the two oscillators.Of those tried, the long sides of the board and held to the drift and MADT transistors gave superior chassis by the mounting bolts. End wires results.Those tested were the RCA 2N of the components are bent over and sol- 371 and 2N274, the Philco 2N1745, and dered to the closest points. Coupling be- the Amperex (Phillips) 0C139. If the b.f.o. tween sections does not result as one might is tuned to 455 kc., the 8th harmonic is expect. The i.f. cans arelikewisegrounded at 3640 kc. and can be heard.However, to the busbars.Note that the Miller i.f. by placing a small trap in the base lead of transformers shown in the picture havepin the product detector, the harmonic is re- connections slightly different from those duced to negligible proportions. The con- presently made by that company. Thenum- nections for the trap are shown in Fig. bered connections shown on the schematic 4.12-C.To adjust the trap, tune in the are for the newer transformers. b.f.o. harmonic with a VTVM connected The b.f.o. and its amplifier are housed 1 to the a.g.c. line. Adjust the trap for mini- in a surplus i.f. can measuring 2"x 1 3/8" mum negative voltage.This means mini- wide and 4" high. A slightly larger can mum signal from the b.f.o.Repeak the would have allowed the r.f. choke, in the last i.f. transformer. collector of the b.f.o. amplifier, to be included.This is desirable where possible. ConstructionThe prototype receiver was The b.f.o. and amplifier, like the i.f. strip, Hints built upon a standard 5" x are built on phenolic board of a size nec- 11" x2" aluminum chassis. essary to make a neat fit in the can. The A cut-out at the rear houses the i.f. strip can is lined with thick paper to prevent which was built upon a 6 1/2" x 2" piece short circuits, and the ground leads are Radio Handbook Communications Receiver 101

taken to a single lead that is insulated from The b.f.o. may be set to frequency by the can.That lead is brought below the chassis measuringthe frequency separation be- by insulated wire and soldered to a lug on tween harmonics which result when the the chassis alongside the product detector. b.f.o.is coupled to a broadcast receiver. The 0.01 /Ad. disc ceramic, bypassing the The difference between the harmonics is product detector collector, also is soldered the fundamental frequency of the b.f.o. to this point, along with the ground con- The r.f. oscillator section was built onto nection for the b.f. o. trap.If the b.f.o. a small plate and mounted below the chas- harmonic is to be reduced to a minimum, sis because there was insufficient room on these connections must be strictly ob served. the top of the chassis. The empty space at Similarly, the B- lead entering the can is the end of the if. strip is reserved for either also bypassed at this point.The b.f.o. a mechanical or a crystal filter. The oscil- tuning capacitor must also be shielded so lator components must be rigidly mounted. that no harmonic energy may escape to the Only ceramic materials should be used for front of the receiver. the coil form, and the tiepoints must be of good quality wherever they support portions of the tuned circuit. The Tuning The tuning capacitor is re - The r.f. stage is built on a small phenol- Capacitor moved from a surplus BC - ic strip and mounted below the chassis at 455 -B receiver and is modi- one end. The coil (Li) is mounted on the fied in the following manner. Remove all chassis proper, and the adjustment screw the rotor plates except two from the gang is accessible from the top. farthest from the worm drive. Begin at the split plate end and work back. Do this by BatteryWhen the battery is new, the idl- bending each plate back and forth, not by ing current of the entire receiver pulling on the plates.The authors well is around 20 ma. and drops to 17 ma. remember the frantic hunt for the small when the battery is down to 10.5 volts. steel balls which shot out of the end bear- The average current is around 30 to 50 ing when the plates were pulled and the ma., depending on receiver volume. Peaks rotor jumped out of the bearings! With a may reach as high as 100 ma.If the re- small hacksaw blade cut the strap connect- ceiver is used in an automobile and is op- ing the moving plates together.Because erating from the car battery, the stability it is without trimmers, this section of the will be excellent, even though the voltage gang is used in the r.f. stage. Connected may fluctuate as thegenerator output varies. across this section is an auxiliary antenna When the battery voltage falls below the trimmer capacitor which is adjusted from zener diode control level, frequency shift the front panel.The center capacitor sec- will appear on c.w. and s.s.b., and distortion is connected to the mixer and is mod- tion will be present on all signals. ifiedin the same manner.Eight moving plates areleftin the oscillator section. ProtectingIf there is danger of r.f. from the the transmitter entering the CoilsConstructors may wish to use coil Front End receiver and burning out the forms differing from those speci- r.f.stage transistor, a small fied. If this is the case, the coils may be diode may be connected directly across the grid -dipped to frequency.The present first tuned circuit.Because most diodes turns to tap ratios should be retained.Re- (especially the silicon variety) do not con- member that the oscillator coil tunes 455 duct until a certain voltage level is reached, kc. higher than the r.f. and mixer stages. rectification will not occur on the signal. 102 Receivers The Transistor

Even better protection is afforded by two sensitive alignment indicator.To tune to diodes connected in parallel but each facing lower sideband, the b.f.o. capacitor is set in an opposite direction. about 1 kc. lower in frequency than zero beat.Conversely, on upper sideband, the Other If transistors other than the b.f.o. is set about 1 kc. higher. Transistorsdrift type are used in the i.f. To adjust the b.f.o. injection voltage, and stages, neutralization may be tune in a strong a.m. station until it is zero N e utr al - required. The neutralization beat with the b.f.o. Next, detune the b.f.o. ix ation components are shown dot- coil slug off -frequency until the heterodyne ted in the schematic, but the cannot be heard. Output from the receiver resistors and capacitors may not be neces- should drop considerably. Adjust the am- sary. The subject of neutralization has been plifier coil slug (L5) until the station nulls, covered in Chapter 3.If 2N139's or 2N indicating the correct injection voltage and 410's are used, Itc will be 470 ohms and current.The b.f.o. slug is then returned Nc approximately 100 Nifd.If in doubt, to the normal position. make Nc variable and later replace it with Using theBecause the a.g.c. action is ex - a fixed value.The b.f.o. amplifier may be Receiverceptional,a strong pumping any of the lower frequency transistors, and action will take place on strong the a.g.c. amplifier may be any general pur- c.w. or s.s.b. stations.This is caused by pose audio transistor having low IC. and the a.g.c. holding back the receiver gain on drift. strong signals but opening up the receiver sensitivity the moment the transmitter op- AlignmentThe process of alignment is erator pauses for breath.The audio gain identical to that used with tube should be turned up and the r.f. set so receivers. The coil slugs align the low fre- that the a.g.c. just operates on the weakest quency end of the band, and the trimmers station.The tuning may be difficult if the are used for alignment at the high end. r.f. gain is high and every weak signal is The i.f.stages are aligned to 455 kc. A brought up to the same level and allowed v.t.v.m. connected to the a.g.c. line is avery to compete with the stronger stations.

TOP CHASSIS VIEW .4...... - OF THE COMMUNICATIONS niiilli, w-iiiio)----- 7---007) Lis RECEIVER The Mini -Amplifier 114111"1111,111 0 (described in Chap- it 1 I Illifil liit k / laii p, ter 2)is located at I ' ' lafl 0110 I. ,, the left, behind the speaker. The space at the right rear corner of the chassis is reservedforthe crystal i.f. filter Radio Handbook Crystal Filters 103

An I.F.Without an i.f. filter, the selectiv- F ilter ity of the receiver is poor. Three tuned circuits are insufficient to keep out an intruding neighbor. However, the receiver was designed to be used with a filter, and a suitable circuit is shown in the following section.

4.13 Crystal Filters for a Figure 4.13-A SCHEMATIC OF THE 3KC. CRYSTAL Communications Receiver FILTER FOR THE PROFESSIONAL COMMUNICATIONS RECEIVER Three tuned circuits in the i.f. section Crystal Y1 is a surplus channel 327, of any receiver designed to cover the ama- andV2is channel 329. Capacitor CI teur bands are insufficient. Double tuned isa 3-30 pfd. compression padder, circuitswill improve the selectivity to a used to tune the 2.5 mh. r.f. choke marked extent.However, unless ten to (this value iscritical). See text for twelve tuned circuits of high Q and correct transformer and alignment data. coupling factor are used, selectivity, by to- day's standards, will still be poor. Modern receivers,especiallythose designed for connected to the base of the first i.f. tran- singlesidebandreception,use crystal, sistorthrougha 100-1 50Nifd. capacitor. mechanical, or complex tuned filters to Both of these resonating capacitors should obtain a high order of selectivity.Better be adjusted for maximum gain.Use of filters improve passband shape by provid- this filter is described in more detail in sec- ing steep skirts and a flat top passband. tion 4.14. The readily available surplus crystals in the i.f. region offer the chance to construct Filter #1 The filter shown in Fig. 4.13-A a filter costing no more than a few dollars, has a passband of about 3.5 kc. yet comparing favorably with filters costing Insertion loss is very small and no extra ten to twenty times as much. The crystal stages are required to overcome filter loss. filter shown in Fig. 4.13-A uses four 50 The filter may be constructed on the chas- cent surplus crystals in a back-to-back half - sis in the extra space at the input end of the lattice arrangement. Adjustment is merely i.f. strip, or can be built on a separate plate. a matter of aligning transformers T1 and This sub -assembly can be surface, sub - T2 and capacitor Cl for maximum output chassis, or above -chassis mounted. The midway between the two crystal frequencies. i.f. transformers shown in the photograph Skirt selectivity is excellent. are not normally available and are, in fact, Two filters are described in this section. modified broadcast receiver types.How- Each is shown connected into the receiver ever, J. W Miller Co. should have double - just described.These types of filters may tunedi.f.transformers suitable for this be used with other transistor receivers to application.Other transformers may be obtain additional selectivity. used providing they satisfy one or two con- A Collins Mechanical Filter can also be ditions.Choose transformers which are used.The input circuitry is the same as fixed -tuned with a 100 to 200 Nefd. capac- inFig. 4.13-A except that the primary itor. The capacitor in the secondary wind- transducer coil is resonated with a 100-150 ing of T1 and the capacitor in the primary capacitor.The output transducer is of T2 are removed and replaced by two 104 Receivers The Transistor

between these two frequencies.Align all transformer slugs for maximum output. Connect a VTVM to the receiver a.g.c. line. Tune the generator across the passband. If there is a large peak at one side of the passband,itindicates that the generator was not set at mid -frequency, or that the crystals were not matched for amplitude (or Q).It is possibletominimizethis condition by realigning to the lower frequency side until the two peaks are equal in height. Next, adjust C1 for more capacitance, (approximately one-half turn) until the skirtsare attheir maximum steepness. This point should coincide with a dip approximately 7% of total passband amplitude.If the trimmer is screwed in too far, the dip will become excessive. The selectivity of the filter is such that the carrier of an a.m. station may be dropped right off the THE COMMUNICATIONS RECEIVER passband and the signal actually turned in- CRYSTAL FILTER FOR to s.s.b. On a.m., high frequency response TWO KILOCYCLE BANDWIDTH will be considerably down. To obtain maximum fidelity, the receiver should be de 250 pfd. capacitors series -connected as tuned a little to favor one sideband. shown in the schematic.Transformers Care must be used to prevent signal with different L/C ratios may give consid- from leaking around the filter. You have erably different results. a shielding problem if the signal is still ChoosingCrystals should be fairly well heard when the crystals are removed. the matched before being used in Crystals the filter.Connect one pin of F filter #2 The passband ofthe filter shown the crystal to a frequency meter in Fig. 4.13-B is only 2.0 kc. or signal generator, and with the r.f. probe wide.Thus, the recovered audio in the of a VTVM on the other pin, tune the os- communications receiver will tend toward cillator through the crystal frequency.At bass.If the ultimate quality in selectivity the series resonant frequency, the meter is desired, this is the filter to use. How- will show a large peak and a deep null a- ever, the high audio frequencies attenuated longside it.Choose two crystals of the to this degree may be objectionable. same channel numbers with peaks of the The alignment procedure is the same same height and similar frequencies.All as for the filter just described, except that four crystals should show the same peak the trimmer C1 is tuned for maximum out- amplitude. Crystals are readily available put and left at that setting. from various sources ready -matched and paired. InstallingThereareseveralways of the mounting the filterin the re - AlignmentAdjust the frequency meter or F ilter ceiver.If the receiver is already signal generator to midway be- built, the filter assembly may tween the two peaks as read on the volt- be placed on end in the space saved near meter.The generator must be midpoint ther.f. end of the i.f.strip. The filter Radio Handbook Mechanical Filter 105

sary to use series tuning on the secondary side of the filter in order to obtain an impedance match to the base of thetransistor. Becausethis means inserting a capacitor in series with the filter output winding, it is necessary to add an r.f. choke to complete the d.c. circuit. The small 22Nifd. capaci- Figure 4.13-B tor across the secondary brings the winding WIRING DIAGRAM FOR THE 2KC. to resonance.It is possible that othertran- PASSBAND RECEIVER FILTER sistors with different input capacities may Crystals V1 andV2are war surplus require adifferent value, in which case types for channel 327 and 328 re- the mfd. capacitor may be replaced with a spectively.All other component val- 3-30 mfd. trimmer. A number of transis- ues are the same as in Fig. 4.13-A. tors have been tried with little variation in gain. No other changes need be made in the base circuit of the i.f. amplifier. should be connected to the receiver through a short length of small coaxial cable. Note that the filter is installed between Transformer T1in the receiver should the first and second i.f. amplifiers, not be- be disconnected or removed.No other tween the mixer and the first i.f. amplifier wiring in the receiver is altered. An altern- as is customary.It was found that a better ative position for the filteris on the i.f. impedance match was possible when the strip itself, with the mixer moved to make mechanical filter was driven by an i.f. am- space for it. plifier stage (in preference to the mixer), and a considerable increase in gain result- Which Those who have neither used nor ed.Ground the case and make sure that Filter? constructed filters will no doubt no signal is getting around the filter instead of through it. be in a quandary as to which filter to build.Filter #1is best for those who Both the 3.1kc. and 2.1 kc. plug-in prefer a.m., yet it will allow good s.s.b. or type Collins Filters gave excellent results in c.w. copy.Filter #2 is essentially for the the Communications Receiver. The 2.1 kc. c.w. or sideband man, and a.m. may be (type 455 J 21) is now a permanent part copied only on one sideband. Of course, of the device. QRM with the narrower filter is considerably reduced.

151- IF AMP. MECH. FILTER 1u00 2ND IF AMP. 4.14 A Mechanical Filter

for the 14 Communications Receiver

72 K

Those fortunate enough to possess a TO AGC mechanical filter may wish to install it in the receiver just described.The method Figure 4.14-A of connection is applicable to other receiv- SCHEMATIC DIAGRAM FOR COLLINS ers such as in Fig. 4.14-A.The second MECHANICAL FILTER i.f. transformer is removed and replaced I.F. AMPLIFIER MODIFICATION by the mechanical filter.The primary cir- (Project 4.12) cuit is otherwise undisturbed.It is neces- 106 Receivers The Transistor

There is ample gain in the receiver in er( TR2) also operates in the grounded - spite of the relatively high filter attenuation, emitter configuration.Both the oscillator and no further amplicication stages are voltage and the signal are fed into the base necessary. through a capacitive impedance transfor- The b..fo. capacitor setting is quite crit- mation arrangement. The oscillator ( TR3) ical with this type of filter. isa form of Colpitts, and is a very active circuit.This circuit gave no difficulty in 4.15 A Transistor the several converters which were built. All -Band Converter However, it is important to watch that the The converter described in this section oscillatorisactually operating at the fre- is crystal controlled.This means that the quency of the crystal.Oscillation at other stability on any band is determined only frequencies may take place when the slug by the receiver to which it is connected.If coil in the collector circuit is adjusted in- the receiverisstable on 80 meters, then correctly. 10 -meter signals will be equally stable. The output circuit on 3.5 to 4.0 Mc. is The converter can be used ahead of the re- loaded with resistance to reduce the gain ceiver described in section 4.12, or with of the converter.The gain was originally any receiver that covers 3.5 to 4.0 Mc. so high it caused cross modulation in the The circuit is shown in Fig. 4.15-A. receiver r.f. stage. Note that on 80 meters the antenna is How it The r.f. amplifier (TRI ) is acon- connected through coil L to the base of the Works ventional grounded -emitter mixer ( TR2 ) which, in the absence of an stage. A capacitive divider across oscillator signal, acts as an r.f. amplifier. the tuned circuit establishes a correct im- There are several advantages in using the pedance match to the transistor. The mix- 80 -meter connection shown in the sche-

L -12V. TR 2 L4 220 L2A L2A

LIA 10.! LIB 1,4, 5 3

.005

CURRENTS 39K of TR 1-1.6 MA. Xie TR2- 1.0 MA. TR3- 1.4 MA. 411

.005 2 1C.7- quo0 M. 1010

XIF 1111 L3 001 D Figure 5.15-A

SCHEMATIC DIAGRAM OF THE ALL -BAND TRANSISTOR CONVERTER Transistors can be 2N384, 00619, 2N371 or 2N1745. All capacitors marked M should be mica, others may be disc ceramics (d) or similar. Radio Handbook All -Band Converter 107

L 65 turns, #36 scc jumblewound on 1/4" form, tap at 10 turns.

11 40 meters, 32 turns, #36 scc jumble or pi wound, tap at 5 turns. 120 Same as Libut tap at 8 1/2 turns. 13. 15 turns, #22 enam., closewound.

11b 20 meters, 20 turns, #26 enam. closewound, tap at 3 1/2 turns. 126 Same as Lib, tap at 7 1/2 turns. 13b 15 turns, #26 enam. closewound. LI, 15 meters, 14 turns, #26 enam. closewound, tap at 2 turns. 12, Same as Li tap at 7 turns. 13, 8 turns, #26 enam. closewound. 11d 10 meters, 9 turns, #26 enam. closewound, tap at 11/2 turns. 12d 10 turns, #26 enam. closewound, tap at 4 1/2 turns. 13d 4 turns, #26 enam. closewound. L4 40 turns, #36 scc jumble or pi wound, 1/4" wide, link 8 turns. Xtals International Crystals, type FA9. XI. 40 meters - 11.0 Mc. Xlb 20 meters - 10.5 Mc. X1, 15 meters - 17.5 Mc. Xld 10 meters (28.0 to 28.5 Mc.) 24.5 Mc. Xi e 10 meters (28.5 to 29.0 Mc.) 25.0 Mc. X11 10 meters (29.0 to 29.5 Mc.) 25.5 Mc.

Figure 4.15-B COIL AND CRYSTAL CHART FOR THE CONVERTER SHOWN IN FIG. 4.15-A

matic.(1)No extra wafer is required on Construction The construction shown the bandswitch. (2) The gain on80meters Hints in the photograph is prob- is approximately equal to that obtained on ably the ultimate in sim- other bands.(3) Wiring is extremely plicity.At the same time itis effective, simple. small, yet easy to work on.A piece of Note that the 10 -meter crystals each aluminum measuring 5 1/2" x 5" is the make use of the same oscillator coil. Also, base plate upon which everything is mount- only one 10 -meter coil is used in the r.f. ed. The various components are mounted and mixer stages. This system has proven upon a terminal strip measuring 5" x 2". adequate. The extra coils which were orig- The method of mounting components is inally in the circuit were removed and the shown in Fig. 4.15-C. This allows a clean switch sections were rewired. board layout which slips easily beneath the It is emphasized that in numerous tests wavechange switch.The ground connec- the transistor converter was as good as sev- tions are made by drilling through the lug eral tube counterparts.In addition heater into the base plate, tapping the hole, and voltage is not required.The only power installing a suitable bolt. These bolts serve requirement isa supply of 12 volts at 4 also to hold the terminal strip in position. Mc.This may be a simple power supply If a terminal strip is used with rivets which as shown in Fig. 4.15-D. go through the board, insulating material 108 Receivers The Transistor

CHASSIS VIEW OF THE CONVERTER BEFORE THE BAND - CHANGE SWITCH WAS INSTALLED

must be placed between the board and the Other The observant reader will have aluminum. Bands noticed the provision for another It is suggested that the transistors first band.There is no reason why be temporarily mounted as shown in the coils for the 10 Mc. section of the short- wave bands could not be installed to permit photograph.This leaves the openings between the switch sections clear and allows reception of WWV. Even 6 meters may be tuned, although some experimentation may easy access with a soldering iron.If the coil forms are different from those specified andit becomes necessary to cut and 13- TO I-2 a- prune to get the coils right, the switch may be left out and the coils temporarily connected in the circuit. They may then be adjusted, sealed, and later connected to the switch.Several converters have been built with the coil data provided.Each functioned without coil pruning of any sort. Transistor sockets are not necessary IP and were used in the prototype converter II merely to allow experimentation with different types of transistors.If the converter is to be a unit separate from a receiver, it Figure 4.15-C may be encased by a U-shaped cover. The COMPONENT LAYOUT ON THE converter shown was built into a receiver. TERMINAL STRIP NI The convenient size allows either under- The strip stock is similar to J.W. Miller chassis or above -chassis mounting. #460. Radio Handbook All -Band Converter 109

TOP VIEW OF THE COMPLETED CONVERTER be required with the capacitive matching to stages are best aligned to the center of the achieve a good noise figure. band.The r.f. trimmer will take care of any peaking thatis necessary at the two AdjustmentSwitch to the higher10 - ends of the band. meter crystal, and adjust the 10 -meter coilLad until stable oscillation Alignment of the other bands is accomplished by adjusting the respective oscil- isobtained.Signals should be apparent immediately at any setting of the r.f. and lator coils to obtain oscillation, and then adjusting the r.f. stage and mixer coils for The signals should not mixer coil slugs. maximum converter output. shift more than a few cycles when the oscillator slug is tuned.If the slug is too far The output coil should be adjusted to into the coil, spurious oscillation may re- the center of the band covered by the result.There is no doubt about the correct ceiver, with the 1K resistor temporarily position once it has been obtained. It disconnected.After the resistor is con- should only be necessary to align the r.f. nected, the tuning is so broad that it is dif- and mixer stage for maximum output on ficult to tell when the coil slug is at the the other 10 -meter position.These two correct setting. 110 Receivers The Transistor

The prototype converter was designed if 10 -meter operation is desired. Although to work into a receiver covering the range the difference in gain is slight, there is a 3.5 to 4.0 Mc.The top part of the 10 - worthwhile improvement in the noise fig- meter band was omitted in order to make ure at this frequency.AVOW 0C169's available the spare switch section required also functioned very well in the converter for the 20 Mc. band mentioned earlier.If and were those in use when the photo- the remainder of the 10 -meter band is re- graphs were taken. No doubt other high - quired, it is necessary to provide one more frequency transistors will also performwell crystal socket. in the circuit without alteration. If the receiver covers a greater range than 500 kc., some of the 10 -meter crystals The Placement of the output coil was may be eliminated. Outputa problem. Finally, it was placed Coil under the switch between the Transistors RCA2N371's may be used in the oscillator and mixer mixer and the oscillator switch wafer. This proved to be an ideal location, stages. However, theRCA2N3 84 or Phiko 2N1745 is recommended for the r.f. stage asit permitted very short leads without cramming. Itis very important that shielded wire be used between the output coil and the SIDE VIEW SHOWING THE RELATION- receiver.Unshielded wire will cause pick- SHIP OF THE COILS TO THE BANDSWITCH Radio Handbook Autodyne Converters 111

12 V. where they are again amplified, creating a D 100 LIF CATH, 12 WVDC feedback loop.Thus, the stage oscillates 0 +I at a frequency determined by L2 and its 6.3 V.A.C.25JJF 12 WVDC 02 associated components. The oscillator CAT H. energy combines with the incoming signal to produce an intermediate frequency Figure 4.15-D in the manner of all superheterodynes. Thiscircuitwill be extremely The remainder of the circuit is straight- useful when operating the con- forward with the exception of the output verter in conjunction with a vac- transformer T1 . Pin 4 was used rather uum tube receiver. The simple than the customary pin 5 (the tap) for the voltage doublersupplies 12 collector connection. This permits capaci- volts d.c. by rectifying the 6.3 tor Cl to act as both a bypass for the end volt filament potential. of the collector feedback winding and as a tuning capacitor for the primary of T1. up of numerous 80 -meter signals which Capacitor C1 is built into the transformer. will ride over those coming through the The Autodyne Converter is quite use- converter. ful in the VHF region where compact, low- cost equipment is often required. Several German portable radios are being market- 4.16 Autodyne ed in this country and employ novel front Converters ends for the 88 - 108 Mc. f.m. band. En- terprising experimenters could easily adapt The autodyne converterisa "some- these circuits for other VHF applications, thing -for -nothing" circuit. That is, a single even though no coil winding data is avail- transistor is used for both the mixer and able. oscillator.This is relatively easy to accomplish in vacuum tube circuitry because the 23MC. INPUT tube exhibits nearly constant gain, and fairly high impedances are involved. However, the transistor has a decreasing gain with frequency. Its terminal impedances vary considerably, and there is much less input- output isolation than the vacuum tube e- quivalent.Thus, the adjustment of transistor autodyne tends to be critical, and

occasionally the device refuses to oscillate +10V. -10 V.

for no apparent reason. Even with its faults, L1=12 T. *24 INSULATED, SPACED TO OCCUPY 5/16'. TAPPED AT 3 T. it is useful in certain applications and should LS -6 FORM 1/4DIA. L2= 86W MIN/DUCTOR 03004 11T. FEEDBACK COIL 3T. *24 INSU- LATED, SPACED DIA. OF WIRE. WOUND OVER TOP END 0E413004. not be neglected. EMITTER TAPPED AT 3/4 TURN FROM COLD END. One example of a transistor autodyne T 1= 0.455 MC. ME/SSNER *16-9014 OR N. MILLER 2041. converter for 27 - 30 Mc. is shown in Fig. RLF 4700. 4.16-A.Signals from the antenna appear across LI and are coupled to the transistor base. Energy in the collector circuitis Figure 4.16-A coupled to L2 through the feedback wind- SCHEMATIC DIAGRAM FOR A ing.The emitter, which is connected to TUNABLE 27-30 Mc. L2,reinjectssignalsinto the transistor AUTODYNE CONVERTER 112 Receivers The Transistor

WHIP ANTENNA used in the device. The circuit differs from 00615 6,75 MC. TRAP 21,4384 those just discussed in that feedback occurs L4 2N1745 L3 between collector and base, while the signal is applied to the emitter.The output coil L3 , although tuned, represents a fairly high impedance at the oscillator frequency VHF TRAP and permits the stage to oscillate at the 1/4 A STUB 50 500 crystal frequency.

500 -9V A third overtone autodyne circuit developed by W6TNS is shown in Fig. 4.16- D. The circuit is similar to 4.10-A. How- Figure 4.16-B ever, a resonant circuit tuned to the third A ONE -TRANSISTOR overtone of the fundamental crystal fre- V.H.F. CONVERTER quency is inserted in series with the col- This converter could be adapted to lector signal path. This raises thecollector either two or six meters. The circuit impedance at the third overtone, permitting is neutralized to prevent self -oscilla- the stage to oscillate at this frequency. tion at the signal frequency. Thecircuithas one unusual feature Fig. 4.16-B is the circuit for an extremely which is the subject of a patent application. ingenious single transistor converter with Although not immediately obvious, the os- an output i.f. of 6.75 Mc. Signals are fed cillator coil is in series with the i.f. signal into the input tuned circuit through a series and the image.If the crystal is on the high 6.75 Mc.i.f.trap and a shunt stub trap frequency side of the signal, coil L2 can be tuned to the image frequency (or interfer- used as an image trap, in addition to its ing television signals).The autodyne cir- normal function.It is possible to obtain cuit is very similar to the unit is Fig. 4.16- an image rejection approaching 40 db. at A. However, the output i.f. transformer 30 Mc., even with a 455 kc. i.f. system. In is resonated by circuit stray capacitance and production equipment, rejection ratios ex- acts as an r.f. choke at the signal and oscil- ceeding 30 db. can easily be obtained. lator frequency. Feedback occurs between the collector and emitter.Notice that the 00615 00615 2 0384 2N384 201742 2N1745 oscillatorcoil containsaneutralization 16 0 T1 winding.The input (base) circuit is neu- 276 tralized through a 14 mfd. capacitor to 3-12 10 prevent the converter from oscillating at

the signal frequency. 2.56 500 4.7K An autodyne converter and r.f. ampli- qT Hit fier are shown in Fig. 4.16-C. Both tran- 40 470 470 sistors operate in a common -base configu- 7211. ration.The autodyne stage does not require neutralization because L2isvery lightly coupled to the emitter. The autodyne principle can also be ap- Figure 4.16-C plied to crystal -controlled converters. The AUTODYNE F.M. CONVERTER oscillator -mixer shown in Fig. 4.10-A was This converter employs an r.f. ampli- developed by ZL1AAX to simplify circuitry fier and is designed to operate from and minimize the number of transistors a 6 -volt source. Radio Handbook Six -Meter Converter 113

Li M current is increased, the voltage between 21 2N1745 the collector and emitter drops due to the Ti series resistors R4 and R5.Hence, the .005 gain of the stage drops. The Micro Alloy Diffused Transistor is well suited for this 27NI100 L type of gain control and provides much .1-12 V. better overload performance than the conventional reversegaincontrolmethod. R.f. gain control R2 provides maximum Figure 4.16-D stage gain when set fully clockwise.By A THIRD OVERTONE varying the gain control counterclockwise, AUTODYNE CONVERTER the value of collector current increases, The values shown are correct for the causing the gain to drop. Emitter resistor 27-30 megacycle band. Both coils are R4 and the bias dividing resistor network 13 turns, #26 on a 5/16" form. The consisting of RI, R2 and R3, provide the base tap on Li is one turn up from necessary d.c. stabilization for ther.f. stage. the ground end. A 2N1743 is used as a mixer with the signal being applied to the base through 4.17 A Transistorized a tap on the coil L3.The output transformer T1tunes to about 8.5 Mc. and Six -Meter Converter couples the output from the collector to This converter employs three Philco the output connector. A loading resistor R9 is placed across the primary winding MADT v.h.f. transistors and operates at a L4of transformer T1 to flatten the LE re- supply voltage of 12 volts.A communi- sponse of the output circuit with some cations receiver capable of tuning the 7 to sacrifice of converter gain. Emitter resistor 11 Mc. frequency range can be used as the i.f. system. R8 and bias dividing resistor R6 and R7 provides the d.c. stabilization for the mixer stage. How it A Philco 2N1742 transistor is Works employed in the neutralized r.f. Emitter injection is obtained by tapping amplifier stage (see Fig. 4.17- the emitter capacitor Cio on the oscillator

A).It operates as a common -emitter stage tank coil L6. An injection voltage of 0.15 with the incoming signal being applied to to 0.25 volts r.m.s. should be measured the base through the input tuning network at the emitter terminals of the mixer. If an consisting of coil L1 and shunt capacitor r.f. voltmeter is not available, a test of the C2.It works with either a 50 or 70 -ohm localoscillator injection may be accom- antenna system. The output utilizes a plished in the following manner.Place a double tuned circuit made up of coils 3 ma. d.c. meter in series with emitter re- tuned to the desired frequency.Neutrali- sistor R8 and positive 12 volts.This is zation is provided by capacitors C7 and C4. done by unsoldering the lead from R8 to The tuned circuits are sufficiently broad- the positive 12 volts, and inserting the band so that they can be fixed tuned. A meter between these two points. The pos- standby/receive switch is inserted in the itive side of the meter is attached to the emitter lead of the r.f. amplifier. positive 12 -volt point.Now, increase the Manual forward gain control is used to loading on the oscillator coil L6 by moving reduce the gain on strong signals.This the point where Cie taps onto the coil to- method of r.f. gain control is accomplished wards the collector end and readjust capac- in the following manner. As the collector itor C12 .A point will be found where the 114 Receivers The Transistor

R.F. AMP. PH/LCO Li 2N1742 SHIELD ANT. je C4 3 INPUT rd 2 MIXER PH I LCO e 2N1743 Ti TO RECEIVER

LOC. OSC. Cis L6 PHIL CO 2N1744 R.F. GAIN 10.91\1ON -OFF MA. SWITCH CONTROL 1.71 - MA. 112 113

-O OF -CASES GROUNDED TO CHASSIS 12 V. BAT CI4 Rio

Figure 4.17-A A TRANSISTORIZED SIX METER CRYSTAL CONTROLLED CONVERTER The difference between total current and individual transistor current is drawn by the resistive voltage dividers in each stage.

Parts List C12 - 1-18 /gad. piston capacitor, VC - C1- 0.0054d. 70V disc ceramic 4G JFD C2 - 1-18 ,ufd. piston capacitor VC - C13 - 5.0 Awfd. ± 10% silver mica 40 JFD C14 - 470 Amfd. disc ceramic C3- 0.0015 /Ad. stand off ceramic C15 - 0.005 #fd. 70V disc ceramic capacitor, Sprague 508C RI - 1200 ohm carbon 1/2 W C4- 1.2 Aufd axial ceramic R2 - 10K ohm potentiometer C5- 0.005 pfd stand off ceramic ca- R3 - 2700 ohm carbon 1/2 W pacitor, Sprague 508C R4- 1000 ohm carbon 1/2 W C6-1-18 ivfd. piston capacitor VC - R5 - 1000 ohm carbon 1/2 W 4G JFD R6 - 12K ohm carbon ± 5% 1/2 W C7- 40 Nifd. ± 5% R7 - 2700 ohm carbon ± 5% 1/2 W C8- 1-18 nufd. piston capacitor, VC - R8- 1200 ohm carbon + 5% 1/2 W 40 JFD R9 - 3900 ohm carbon 1/2 W C9- 0.005 pfd. 70V disc ceramic R10 - 10K ohm carbon ± 5% 1/2 W C10 - 0.005 pfd. 70V disc ceramic R11- 3300 ohm carbon ± 5%1/2 W C11 - 3 nufd. ± 5% silver mica R12- 1500 ohm carbon ± 5%1/2 W

emitter current begins to rise due to the crystal is used to control the frequency of increase in the injection voltage. Any fur- the local oscillator.A Philco 2N1744 is ther increase will cause the emitter current used for the local oscillator. to rise higher. A tap point to use onL6is PerformanceWith the gain control set the one just below the point that causes the for maximum, an overall emitter current to rise rapidly. Emitter re- power gain of 46.0 db was obtained at sistor R10 and bias dividing resistors R11 51.5 Mc. The gain drops off to about 43 andR12provide d.c. stabilization in the db at 50 and 53 Mc. The noise figure of local oscillator stage. A 43 Mc. overtone the converter is about 4.0 db with a 0.4 Radio Handbook Two -Meter Converter 115

ANTENNA COIL by capacitor C5.Capacitance dividers C1 Li = 8 TURNS OF BOW M/N/DUCTOR #3003. BASE TAP 3 TURNS FROM COLD END. ANTENNA TAP 2 TURNS FROM COLD END. and C2 provide a 50 -ohm match to the input circuit. Coil Li and capacitor C3 form INTERSTAGE COIL MADE FROM A SINGLE SECTION OF BOW M/N/DUCTOR #3003. the input tuning. The base of the amplifier THE COIL IS BROKEN AT 8 1/2 TURNS. THE LEADS ARE THEN UN- WOUND A HALF TURN IN BOTH DIRECTIONS AND ARE USED TO is tapped on Li to match 75 ohms. Coil CONNECT THE COLD ENDS OF L2 AND L3 TO THE PROPER POINTS. L2 and capacitors C7 and Cg tune the output of the amplifier. A portion of L2 to- Bd W 43003-imuumumui gether with neutralizing capacitor C5 form MUM Minn the neutralizing network. The base of the TO L2 L3 TO COLLECTOR BASE Philco 2N1743 mixer is tapped down on TO TO TO TO TO L2. The output of the mixer is coupled CR G7 GND G9 Cs L2= &TURNS OF BOW AIIIVIDUCrOP #3003 from the collector by capacitor C10 and out- L3= 8 TURNS OF BOW M/ANDUCTOR #3003. MIXER BASE TAP AT 2 TURNS FROM GROUND. put coil L3 at 7 Mc. Winding L4 provides OUTPUT TRANSFORMERTI an output at 50 ohms to permit coupling L4 =CLOSEWOUND#30 NYCLAD COPPER WIRE ON A 1/2" DIAM. FORM TO OCCUPY A WINDING LENGTH OF 15/18. to the input of a communications receiver. L5= IS TURNS 0E4028 NYCLAD COPPER WIRE CLOSEWOUND OVER COLD END OF L4. A Philco 2N1744 is employed as a local oscillator and operates 7 Mc. higher than TO GND. TO GND. the signal frequency.Coil L5 and capaci- TO OUTPUT CONNECTOR tors C12 and C13 form the tank circuit. TO COLLECTOR injected Of MIXER The local oscillator signal is FORM =CAMBION L57 into the mixer emitter through capacitor IRON CORE=CAM5/ON X2018D RED DOT. C1 iby tapping the oscillator coil L5.

OSCILLATOR COIL OperationThe r.f.bandpass is about L6 = 9 TURNS 880/ M/N1DUCTOR *3003. MIXER EMITTER CAPA- 4 Mc. at the 3 db point. A CITOR TAP 1/3 TURN FROM GROUND END. communications receiver capable of tuning Figure 4.17-B the 7 Mc. band should be used as the i.f. SIX METER CONVERTER COIL DATA

L I. 4 TURNS Isle BARE COPPER WIRE 1/4.I.D., WINDING LENGTH 1/4.. BASE TAP I TURN FROM GROUND END OF LI. L2 = 8 TURNS *16 BARE COPPER WIRE 1/4" WINDING LENGTH 4/104. ,v, sensitivity for 10 db signal to noise GROUND TAP 4 TuRN5 FROM COLLECTOR END. ratio. OUTPUT TAP 3/4 TURN FROM GROUND TAP. L3= 8.30 NYCLAD CLOSEWOUND TO OCCUPY I/2" OF WINDING Thisconverterwas designed by J. SPACE ON A 3/8" COIL FORM (CAME/ON LS -5), SEE BELOW FOR CONSTRUCTION DETAILS. (RED Dor cox, ) Specialny, Jr. and described in Philco Ap- TO GND. TO GND. plication Lab Report #667. TO OUTPUT

4.18 A Transistorized SEC. TO COLLECTOR PRI, Two -Meter Converter

This 144 to 7 Mc. converter provides L4= TURNS1430 NYCLAD OVER COLD END OF L3. excellent results in the 2 -meter band. Tran- L5. 41/2 TURNS 4418 BARE COPPER WIRE 1/4" I.D., SPACED TO OCCUPY 5/8". sistors are used throughout, and the only EMITTER TAP 1/8 TO 1/4 TURN FROM GROUND END. NOTE IIN TUNING THE 7 MC. OUTPUT CO/L, THE POWDERED /RON supply voltage necessary is a 12 -volt battery. IS VAR/ED 50 THAT IT MESHES ONLY THE COLLECTOR END OP L3. How It The circuit (Fig. 4.18-A) is con - Works ventional and no difficulty should be experienced in duplicating it. Figure 4.18-B A Philco 2N1742 is employed in the r.f. TWO METER CONVERTER COIL DATA amplifier stage which is fixed neutralized (Courtesy of Philco Corp.) 116 Receivers The Transistor

PH/LCO 2N1742 PN/LCO LI La 2N1743 50I1 OUT

1=6 Ral.6 6.8K3.9K 1.5K 5000

9k -CASE GROUNDED R9 NOTE -CAPACITY VALUES /N LLUF. 1.8K

12 VOLTS Figure 4.18-A SCHEMATIC FOR THE TWO METER TRANSISTORIZED CONVERTER The variable frequency local oscillator permits use of virtually any i.f. (Courtesy of Philco Corp.)

Paris List C14 - 1.2##fd. =601 ceramic C1 - 6.8 ##fd. mica ± 5% C17 -1.0 Mid. mica C2 - 22 Aufd disc ceramic R2 - 3.9K 1/2 waft carbon C3,8 -1.0 - 8.0 Hifd. Tublar trimmer R1 - 6.8K 1/2 watt carbon Erie #532-8 R3 - 1.5K 1/2 watt carbon C4, 6,9, 11, 15, 16 R4 - 15K 1/2 watt carbon - .0054d. disc ceramic 70V R5 - 4.7K 1/2 watt carbon C5 - 5.0 ##fd mica + 5% R6,9- 1.8K 1/2 watt carbon C7 - 5.0 #fifd. mica + 5% R7 - 18K 1/2 waft carbon Cio- 30 Mufd. mica ± 5% for 7 mc. i.f. R8 - 4.7K 1/2 watt carbon output TR1 - 2N1742 C12- 6.0 #fifd. silver mica ± 5% TR2- 2N1743 C13- 1.5-3.0 ##fd. air variable TR3- 2N1744

system. If a fixed -tuned converter oper- cations receiver in this case is a fixed tuned ation is desired, the tuning range will be i.f. system operating at 7 Mc. limited to about 2 Mc. with the mixer out- A standby/receive switch should be lo- put coil used. The frequency range of 144 cated in the positive leg of the 12 -volt sup- to 146 Mc. can be tuned without touching ply.The coaxial antenna switching relay the converter once the local oscillator fre- should be located as near as practical to quency has been set.The i.f. system then the input terminals of the converter. tunes from 6 through 8 Mc. PerformanceThe power gain at 146 Mc. If continuous tuning of the converter is is about 30 db and falls off desired, a vernier dial and a panel can be to 27 db at 144 and 148 Mc. The noise added to the converter.The communi- figure of the particular 2N1742 used was Radio Handbook 220 Mc. Converter 117

5.0 db at 200 Mc. and the overall noise characteristics.The term forward comes figure of the converter should beno greater from the fact that the collector current is than 5.0 db at 144 Mc.It is believed that increased to reduce the stage gain, rather the newer Phiko MADT type T2028 r.f. than decreasing the collector current as is amplifier will provide a noise figure sub. done in the reverse method. A resistor R5 stantially below 4.5 db at this frequency. is inserted in series with the output circuit Thisconverter was designedby J. and the negative terminal of the power Specialny, Jr. and was originally described supply. As the current increases by adjust- in Philco Application Lab Report #651. ment of the gain control potentiometer R3, the voltage available between the collector and emitter of the r.f. stage decreases. This 4.19 A 220 Mc. causes the gain to drop.This drop in Transistorized Converter power gain is nearly linear, as the collector - to -emitter voltage is varied from 8 volts to 0.5 volts.Resistor R4 provides emitter This converter operates at a supply volt- stabilization and resistors RI, R2 and R3 age of 12 volts and works into acommuni- determine the biasing level.The value of cations receiver capable of tuning the 10 collector current varies from 2.5 to 6 ma. to 15 Mc. frequency range. depending on the setting of R3. The normal operating value is 2.5 ma. for maxi- How it The unit (Fig. 4.19-A) employs mum gain. A standby/receive switch is in- Works five transistors, all operating in corporated in the emitter lead. Transistor TR1 , operating as a neutralized r.f.amplifier,iscoupled to the mixer through a double -tuned circuit. This meth- The outputof ther.f. amplifieris od of interstage coupling is preferred be- coupled to the mixer transistor TR2 by cause of its ability to reject signals outside loosely coupling mixer coil L3 to amplifier coil L2 (see coil data, Fig. 4.19-B). Capac- ther.f. bandpass and to minimize feed- through at the if. frequency. The antenna itor C7 tunes coil L3 and the value of capac- is coupled to the amplifier through a tap itor C8 is selected to match the input re- on the input coil L1.Shunt capacitor CI sistance of the mixer. The local oscillator tunes the input circuit to the proper fre- power is injected into the emitter terminal quency. A series matching capacitor C2 ap- by returning bypass capacitor C13 to ground plies the incoming signal to thelow imped- through a tap on coil L8. An i.f. frequency ance base which typically is about 60 ohms. of 10-15 Mc. was selected.Coil L4 and Neutralization is provided for by a capaci- capacitor C9 tune the collector output to tor network consisting of C3 and C6. Neu- this frequency range. The output is coupled tralization provides an increase in r.f. pow- to the load through coil L5, which is wound er gain of approximately 3 db as well as over the cold end of coil L3 .The 3 db i.f. good circuit stability, although the ampli- response of the converter is about 3 Mc. fier would be stable if neutralization were Since most of the activity is centered around not used. 221 Mc., the i.f. response was peaked to The r.f. amplifier output circuit is tuned 11 Mc. The r.f. response at the mixer base by inductor L2 and capacitors C4 and C5. is quite flat from 219.5 to 225.5 Mc. Manual r.f. gain control is incorporated to reduce the gain on strong signals.The Emitter resistor R8 provides d.c. stabili- method used here, forward gain control, zation and resistors R6 and R7 determine is used because of its excellent overload the operating point. 118 Receivers The Transistor

R. F. AMPLIFI ER PH ILCO T1832 2N1742 MIXER 2 TR I!SHIELD PHI LCO T1833 -2N1743 F 1 F =10-15 MC. INPUT 220-225 MC. L3 TR a " RECEIVERTC 1 1 1- 114 T5 -I -,- = t_r - LI= R4'---1 2.5MA.ti .15 Rs C13 R.F. GAIN 0..". CONTROL STANDBY- IB - RECEIVE --- = SWITCH TC14 SHIELD 011

ON/OFF S 1.12 112.0MA. TR5 TR4 2N1744 F = 210 MC. 12 V. BATTERY 2N1744 T1859 OVERTONE XTAL, 52.5 MC. T1859 T1695 -101- F= 52.5 MC.T1695 F=105 MC. C21 TR C2C

HARMONIC GENERATOR C 6 SECTION 'R171115 MA. R12 R R14 IC17 iR151R16 R9 'Rio

Figure 4.19-A

A TRANSISTORIZED CONVERTER FOR 220 Mc. The local oscillator energy is furnished by a threetransistor harmonic generator.

The harmonic generator section pro- capacitor C11.The 105 Mc. output from vides at least 180 millivolts r.m.s. of in- frequency doubler TR4 is used to drive jection voltage to the emitter terminal of another frequency doubler TR5 through the mixer TR2 .The local oscillator fre- coupling capacitor C21. The output frequency is on the low side and the output quency of TR, is tuned to 210 Mc. by coil

frequency is 210 Mc. This high frequency L8 and capacitor C12 output is obtained through the use of two stages of frequency doubling and a single Emitter resistors R11 , Rig and R17 pro- stage overtone oscillator operating on a vide the necessary d.c. stabilization and frequency of 52.5 Mc. biasing resistors R9, R10, R12, R15 and R16 determine the biasing current of their respective stages. Transistor TR3 is used in the crystal controlled oscillator circuit.Coil L6 and The actual collector current flowing in capacitor C10 are tuned to 52.5 Mc., the transistors TR4 and TR5 is influenced to overtone frequency of the crystal. The os- some extent by the level of r.f. excitation cillator output drives TR4 through cou- from the oscillator TR3 ,since a combination of fixed and self biasing is employed pling capacitor C20The output is tuned to a frequency of 105 Mc. by coil L7 and in these stages. Radio Handbook 220 Mc. Converter 119

SEC L5 it up on a carrier from the transmitter or 10 TO GND. TO GND. TO OUTPUT from an r.f. signal generator. TO pql M. CONNECTOR MIXER 4 TO COLLECTOR I TO COLLECTOR PLS5 If a variable capacitor is used for C2, FORM alternately adjusting C1 and C2 for maximum output should peak the input properly. The point -of -best -noise figure should LI-B TURNS 420 TINNED COPPER WIRE 3/16" I.D. 1/2. WINDING LENGTH. ANTENNA TAP 1 TURN FROM GROUND END. coincide very nearly to the point of maxi- L2-3 1/2 TURNS*26 TINNED COPPER 5/32.I.D., 1/4. W,L. mum power gain. The noisefigure should L3m4 1/2 TURNS*24 TINNED COPPER 5/32"1.0., 1/4" W.L. L2 AND L3 FORM A DOUBLE TUNED CIRCUIT. AIR WOUND IN THE be in the vicinity of 5.5 to 6.5 db. SAME DIRECTION. SPACING BETWEEN COILS AS NOTED ABOVE. L4.72 TURN5*34 NYCLAD COPPER WIRE CLOSEWOUND. W.L. ABOUT 5/B" ON 3/B" CERAMIC FORM (CAAIB/ON TYPE PCS-5 2C42) POWDERED IRON SLUG. L5=.15 TURNS ik.34 NYCLAD COPPER WIRE CLOSEWOUND OVER GROUND OF L4 (SEE SKETCH ABOVE) PerformanceThe overall power gain is L6=.9 TURNS 03005 MIN/DOCTOR ( Raw) OR A/R DUX #476r. L7=3 TURNS OF*3003M/N/DUCTOR (B&W)OR A/R OUX #416T. about 22.0 db. An addi- LB -U TURNS * 18 TINNED COPPER WIRE 1/4" I. D.,1/2" W.L. TAPPED ABOUT 1/4 TURN FROM GROUND. tional 1.5 to 2.0 db can be realized by inserting a series -tuned 11-12 Mc. trap between the mixer base and ground because Figure 4.19-B the input circuit does not completely short 220 Mc. CONVERTER COIL DATA the 12 Mc. input admittance of the mixer. The additional tuning procedure involved did not warrant the addition of the trap. AlignmentThe sweep generator method of alignment is suggested in Thisconverterwasdesignedby J. up the converter.However, the Specialny, Jr. and was originally described unit can be tuned up fairly well by peaking in Phi/co Application Lab Report 079. CHAPTER FIVE

Transmitters

When this book was started, high pow- 5.1 R.F. Power Amplifiers er r.f. transistors were still laboratory cur- iosities. Shortly thereafter companies such as Pacific Semiconductors, Texas Instru- An r.f. power transistor, whether oscil- ments, Fairchild, RCA, and many others lator or amplifier, is rarely matched to the had developed devices capable of generating load.The load value is a function of the many watts of r.f. at very high frequencies. desired power output and is found from The work of these companies has opened the formula: the way for developing new designs which RL= 0.5 Ke2 will not only render obsolete present tube equipment, but will provide considerable Po savings in bulk, weight, power consump- where: tion, and, in many cases, cost. RL= the load resistance To properly utilize the r.f. power tran- Vice = d.c. collector voltage sistor,itisnecessary to readjust one's Po = required power output thinking to new concepts in transistor The transistor may be considered a theory. It is the purpose of this chapter switch which turns the d.c. on and off at to introduce these new concepts. an r.f.rate.Assume that 5 watts is required from an amplifier and that the power supply is 12 volts. Fromtheaboveformula,

RL - 0.5 x122 14.4 ohms 5 In order to develop the 5 watts of power, the transistor must "look into" a load of 14.4 ohms.If, however, the load has an impedance of other than 14.4 ohms, an impedance transformation must be made using transformers, pi networks, or other matching devices. This photograph shows the waveform The output from a single transistor op- at the collector of a class B erated in Class B or Class Cwillbe a series r.f. amplifier. of half -waves, as illustrated in figure 5.1-A. RF Power Amplifiers 121

One purpose of a tuned circuit is to, by The preceding discussion has conveni- -virtue of its flywheel action, restore the ently ignored the unloaded Q of tuned cir- missing half -cycle. A waveform other than cuit components. A complete explanation a sine wave contains a great number ofhar- of unloaded and loaded Q is beyond the monics.These harmonics are eliminated scope of this book.However, in most when the waveform is converted to a sine practical cases,if the unloaded Q of the wave by means of resonant circuits.To components is at least 10 times the loaded restore the waveform to a sine wave effi- Q, the preceding formulas may be con- ciently, the output circuit must have a rea- sidered sufficiently accurate.It is a simple sonable Q. From the formula: matter to obtain unloaded Q's of 60 or more. Coils should be largeand airwound Q = RL or, as an alternative, wound on some of X, the newer very high Q ferrite toroids.

Bias The primary difference it will be seen that to maintain Q at a given Considerationsbetween the various clas- figure, a reduction in RL necessitates a de- ses of transistor opera- crease in X, .The term X, ,in this in- tion is that of bias. A Class B amplifier stance, represents the reactance of either is operated at the bottom of itsVbe -I, the coil or the capacitor. When the load curve.Because the transistor is not con- resistance is very low, "C" becomes very ducting during one half of the input cycle, high and is often impractical in value. Rep- heating is reduced and efficiency, compared resentative Q figures range from 3 to 15. with Class A, is increased. Efficiency may By transposing the formula for Q, we be further increased by biasing the transis- find that: tor even beyond the cutoff point so that conduction takes place only for a portion XC = RL of one-half cycle. Q Bias may be supplied from a fixed source or be self -developed.The circuit Assuming a Q of 5 and a load resistance shown in figure 5.1-B obtains its bias from of 14.4 ohms, X, will have a value of 2.8 self -rectification in which a voltage is de- ohms.At a frequency of 5 Mc. this would veloped across the 100 -ohm resistor in the be represented by a tuning capacitor of ap- base circuit. proximately 0.01 pfd.This is impractical and would result in a coil so small that it would be made up almost entirely of the capacitor lead inductance.

A low value of load resistance may be effectively transformed to a higher value by tapping the collector down the coil.If the collector is tapped midway down a coil, the 14.4 ohm load resistance will be transformed to 57.4 ohms (turns ratio2). Any value of load resistance may be transformed Figure 5.1-B to almost any higher value, and the L/C Large signal NPN r.f. amplifier with ratio of the tuned circuit may be chosen collector impedance accordingly. matching to the tank coil. 122 Transmitters The Transistor

ing could be greatly in error. This is because the half -wave type of VTVM measures OUT only one half of the waveform. Readings would differ according to whether the positive or the negative half were being read. To convert the pulse -type waveform of Figure 5.1-C figure 5.1-A to a sinewave, it is often nec- A bandpass coupler will greatly essary to incorporate an additional tuned attenuate harmonics with little or no circuit between the transistor and the an- insertion loss. tenna in the form of a band-pass coupler as shown in figure 5.1-C.As an alternative, the pi coupler may be used to remove The Input The input impedance of an the harmonics.However, when the im- Circuit r.f. power transistor is usu- pedance of the load is similar to that re- allyquite low, and suitable quired by the transistor, it may be impos- impedance matching must be employed for sible to construct a single pi network of maximum power transfer from the driver. sufficient Q.In this case a double pi, in Matching may be accomplished by any one which the input impedance is stepped up of the usual methods, as discussed in Chap- to a higher value (about 10 x RL) and then ter Three.Some germanium r.f. power down to the antenna impedance, may be transistors have a fairly low base -to -emitter used. The waveform at the antenna may be breakdown voltage. In this case, care must viewed conveniently on an oscilloscope for be exercised to avoid high values of resis- distortion, a direct indication of harmonic tance in the base circuit, across which ex- content. Preferably the oscilloscope should cessive self -bias may be developed. This is have provision for direct connection to the quite important.Several transistor break- deflection plates unless it is capable of am- downs in some of the authors' experi- plification of frequencies at least six times mental breadboard units occurred before the amplifier output frequency. this fact was realized. Neutral- The subject of neutralization The OutputAs mentioned earlier, the ization and unilateralization was dis- Circuit output of the Class B and a nd cussed in Chapter Three. Im- C transistor stage is not a Unilateral-proper neutralization of an sine wave and the output circuit should be ization r.f. power amplifier may cause of high Q to prevent the transfer of har- a severe loss of transistor monics. This is particularly important gain.At radio frequencies, feedback may when the tank circuit feeds an antenna. Not be degenerativeand yet become regenerative only will the harmonic radiation cause in- terferance with other stations and services, but it will also subtract from the available fundamental power.Figure 5.1-A isa photograph of the waveform at the collect- or of a 10 -watt r.f. power amplifier. A watt- meter connected to the output of this transmitter would give an entirely false reading. The harmonic output would be very high. In addition, where the power is read by Figure 5.1-D measuring the r.f. voltage across a known A neutralized version of the value of resistance, the actual voltage read- r.f. amplifier shown in Fig. 5.1-B. Radio Handbook RF Oscillators 123 when the output circuit is tuned. In addi- the Hartley circuit of figure 5.2-B, the im- tion to the neutralization methods discussed pedance is adjusted by varying theBfeed - in Chapter Three, neutralization may be point.This point, as far as r.f.is con- accomplished as shown in figure 5.1-D. cerned, is connected to the emitter via the In this instance, a tuned circuit consisting bypass capacitor and the common ground. of the inductance L and the base -collector Thus, the coil is divided each side of the capacitance is resonated at the operating emitter.The impedance of the winding frequency. At resonance, Xc = XL.It will feeding the base will be reduced as the tap be seen that the internal capacitance of the is moved toward the bottom of the coil. transistor (between base and collector) is The Colpitts circuit is matched in a simi- effectively cancelled.The arrangement is lar manner by selecting the ratio of the two similar to that employed with tube cascode capacitors across the tuned circuit.This amplifiers. is shown in Fig. 5.2-C.

5.2 R.F. Oscillators CrystalTo convert a self-excited oscil- Controllator to crystal control, it is nec- Almost all the preceding remarks con- essary to insert the crystal in cerning amplifiers apply to the oscillator. series with the feedback path. The crystal, This is true whether the latter is crystal - to all frequencies except that at which it is controlled or self-excited.An oscillator series resonant, will represent a high im- may be considered asan amplifier, in pedance.Oscillation at these frequencies which energy is extracted from the output normally will not be possible.At reso- and returned to the input.The fact that nance, the impedance of the crystal may be the input is supplied from the output cir- in the range of 50 to2,000ohms. In the cuit is of little consequence to the transis- three preceding oscillator circuits, the crys- tor.The input of the transistor should be tal would be operating in the series reso- matched to the driver.The driver here is nant mode. To use the crystal in the paral- the output circuit and, accordingly, an at- lel mode, it would be connected in place of tempt should be made to effect a match be- the inductance in Fig. 5.2-C. In this case, tween the base and the output circuit.In feedback would still be controlled by the figure 5.2-A, this is accomplished by ad- ratio of C1 to C2. justing the ratio of the feedback winding In these circuits, crystals can also be turns to that of the collector winding. In operated on overtones, or odd multiples of the fundamental frequency.For ex -

Figure 5.2-C TheColpittscircuit Figure 5.2-A uses a capacitive A tickler coil provides Figure 5.2-B voltage dividerto feedback in this oscil- The Hartley oscillator establish the level of lator circuit. employs a tapped coil. feedback. 124 Transmitters The Transistor

may take place in the transistor, necessi- tatingincreasedfeedback. A point is reached, however, where all the available a output energy is being fed back to the input, leaving little power to be delivered to the load.In circumstances such as these, itis usual to correct the phase of the feed- Figure 5.2-D back with an L/C or other network so that A grounded -collector r.f. am- the feedback arrives in phase w ith the signal. plifier which is oft en confused withthe common -collector Circuit When analyzing crystal os- configuration. Configurationcillator circuit diagrams, itis helpful to break the ample,a 9 Mc. crystal can be made to os- oscillator down into its basic type.Gen- cillate at 27 Mc. by resonating the tuned erally,it may be stated that all the well- circuit at approximately the third harmonic known oscillator configurations, such as of the crystal frequency.Crystals which Colpitts, Hartley, or Ultra Audion, may have a fundamental frequency above 25 Mc. be found in transistor circuitry. However, are extremely difficult to make, for they be- it should be remembered that the transis- come quite fragile.At frequencies above tor circuits may not, at first evaluation, ap- 15 Mc., overtone operation is desirable, pear to be any known configuration. This because it avoids the use of expensive crys- is brought about by the fact that the capac- tals and eliminates the components of a itance which exists between the elements of frequency multiplying stage.Since un- the transistor may eliminate the need for wanted frequencies can feedback through an external capacitor.In the circuit of Fig. the holder capacitance, crystals used in- 5.2-C, for example, C2 may be made up overtone service may require neutralization. entirely ofbase-emitter junction capacitance. The effect can be eliminated by placing an When changing a transistor for another inductance across the crystal and resonat- type in an existing circuit, it is well to re- ing the holder capacitance slightly above member this point. The replacement tran- the overtone frequency. sistor may have considerably different in- If the feedback voltage is not exactly in ternal capacitances, and may result in im- phase with the input, it will be necessary proper operation.This is true in any r.f. to increase the amplitude of the feedback circuit which employs transistors. In high- in order to maintain oscillation.At the er power r.f. oscillator and amplifier cir- higher frequencies, undesirable phase shift cuits, there isa notable tendency to operate transistorsin the common -emitter mode and thus take advantage of the higher power gain. However, near the limiting frequency of the transistor, greater power a gain willusually be obtained from the common -base connection. Thecommon - collectorconnection should not be confused with the grounded - Figure 5.2-E collector common -emitter stage.This is The grounded collectorr.f. shown in Fig. 5.2-D.The circuit is re- amplifier is redrawn here to drawn in Fig. 5.2-E, and inspection will show the actual circuit more show that the circuits are identical and are clearly. of the same configuration as in 5.1-B. The Radio Handbook Linear Amplifiers 125 fact that the collector is grounded does not When the transistor operating level is affect the transistor configuration.Often increased, non -linearity of transistor char- it is convenient to connect the collector of acteristics becomes a problem. Fig. 5.3-A the transistor directly to the chassis in ord- shows a curve for collector current plotted er to obtain better heat dissipation. against collector voltage. The base current is the running parameter on which a load 5.3 Linear Amplifiers line has been drawn. The subject was discussed in Chapter Two. The base current Transistor linear power amplifiers pose must swing from 0 to 10 ma. to obtain the problems not always encountered with oth- greatest output. Note the shape of the outer stages.As the name implies, the stage put waveform over this range of base cur- must amplify the signal presented to the in- rent.The effect is due to the crowding of put of the transistor in a linear manner. the base lines as the base current increases. Linear amplifiers may be operated Class A It is pointed out that this is an exaggerated or B but never Class C. The single -ended case but none -the -less valuable as a lesson. Class B stage may be confusing to those The set of characteristic curves, in this case, used to audio circuitry.In this case the belongs to a silicon transistor but is plot- missing half cycle is replaced by the tuned ted at low voltage levels. For comparison, circuit.It is essential, therefore, that the the high voltage characteristics are shown tank circuit of a Class B linear amplifier in Fig. 5.3-B using the same transistor. have a high Q. Note the evenly spaced base lines and linear Low level Class A stages are readily de- output.In selecting a transistor for use signed to amplify a signal in a linear man- in high -power linear amplifier service, one ner.An example is the LE stage in a tran- must make a close study of the transistor's sistor receiver wherein the transistor is op- characteristic curves. erated well within its capabilites and at cur- The linearity of the amplifier may be rents very close to 1 ma. considerably imp roved by applying negative

, BASE CURRENT LOW VOLTAGE

500

450

400EMMEN'MA5 MA Min. 4-4Mm 350MIEMEN WA IW/ 1WA CC CC 300 U 250 MA O OUTPUT - 200 COLLECTOR U CURRENT 150rzsmoommilm MA O 1001/11111MENIMME

U 50 IMMEMENE I B0 0 I 2 3 4 5 6 7 6 9 10 VCE -COLLECTOR VOLTAGE -VOLTS

Figure 5.3-A BASE CURRENT CURVES FOR LOW COLLECTOR VOLTAGES Note the severe non -linearity. 126 Transmitters The Transistor

feedback around the stage. Negative feedback has the effect of spacing the base lines evenly.However, in some amplifiers, un- F0UT wanted phaseshifts in associated components may make the application of negative feedback difficult, especially at the higher frequencies. Figure 5.3-C The Emitter Considerable output may be In linear amplifier service, a stable Follower obtained from a transistor bias point can be obtained by employ- connected as an emitter fol- ing a zener or silicon diode, as shown lower if due attention is paid to correct im- here. pedance matching. The one -hundred per- cent inherent feedback makes it necessary to apply considerably more drive to the in addition to the fixed bias. On large sig- stage than is required in the common -emit- nals, the self -bias may cause the stage to ter connection. enter the class C region, thus creating severe distortion of the waveform. Bias If the linear amplifier is op - In the interests of temperature stabili- Consid er- erated Class A, the stage must zation, an emitter resistor is mandatory. ations be biased so that the collector Unfortunately, an emitter resistor will cause current will swing equally in a bias which is subject to variation with each direction.This corresponds to the changing signal levels.This may be over- 15 ma. base current in Fig. 5.3-B. Class come by shunting a zener diode across the B operation demands that the stage be resistor so that the bias level is held con- biased close to cutoff. As in class B audio stant. stages, a small fixed bias should beapplied However, a zener diode connected at to the transistor to lift the operating point this point will subtract from the usefulness above the curved portion of the transistor of the resistor as a temperature stabili- characteristics. zation control. When a low emitter bias is When the transistor stage is operated required, the barrier potential of a german- classB,itmust be provided with a stiff ium or silicon power diode may be used source of bias.Any resistance in the base to hold the voltage constant. This arrange- circuit will allow a self -bias to be developed ment is shown in Fig. 5.3-C. Thepotential

61 IOU VOLTAGE Figure 5.3-B A 60 The same transistor, as in Fig. 5.3-A, 55MNIIMMENNEMINNI when operated at high collector volt- 4 5 ages. Notice the improvement in I._ 45 z linearity. LLI 40 CC CC 35 7 30IPERMINIENSEINII cr 10111.102:31.21M 0 2511111111EMIles- 2° MIA 0 15 MA 141

_J 15 0 A05 MA 10I.1MAiimiNUINIMIEN111 IB= 0

0 5 10 15 2025 30 3540 45SO 55 60 VC E - COLLECTOR VOLTAGE- VOLTS Radio Handbook Modulation 127 drop across the diode will be approxi- MODULATOR FINALDRIVER mately 0.6 volts for silicon. Two or more diodes may be connected in series. When difficulty is experienced in finding a germanium diode capable of carrying the current, the base -emitter or base -collector junction of a germanium power transistor may be utilized. Figure 5.4-A The transistor linear amplifier should When high-level modulat- be driven from a higher impedance source ing transistor transmitters, so that base -emitter impedance variation is itis usually necessary to but a small percentage of the total imped- also modulate the driver ance in the circuit.This will prevent dis- and even the oscillator. tortion of the signal due to thenonlinearity of the base -emitter junction. an exact replica of the modulating wave form, and distortion will occur.In Fig. 5.3-A it was shown that decreased spacing 5.4 Modulation between the base lines, especially at the higher current levels, created harmonic distortion. A low level modulated stage, dur- Transistor r.f. power amplifiers may be ing the higher peaks of modulation, will either high or low level (or efficiency) mod- also have a non-linear output. Accordingly, ulated.The low level system, though in- the stage should be operated withinthe efficient, requires only a small modulating limits of the transistor.The transistor power.High level modulation is effieient characteristics illustrated in Fig. 5.3-B show and generally is more easily put into op- excellent linearity over a wide range of col- eration.However, modulator power out- lector currents. put requirements are high.More than 50% of the r.f. power amplifier d.c. input is required for 100% modulation. The High High level modulation isa Level process in which an external The Low Anr.f. power amplifier may System voltage supplied by the mod- Level be low-level modulated by ap- ulatoris added to and sub- System plying the modulating signal tracted from the r.f. amplifier collector volt- to the base of the stage.An age.Because extra power is supplied by audio signal at the base has the effect of in- the modulator, the r.f. amplifier is operated creasing and decreasing the base bias with at greater efficiency than is possible with audio signal.It will be obvious that the the low level system. However, on positive collector current must be able to follow the cycles of modulation (NPN case), the col- base excursions if modulation is to take lector voltageisdoubled during 100% place without distortion.The transistor modulation peaks, and the collector -to -base collector current must have a mean value breakdown voltage is equal to four times about which the current swing may take the supply voltage.Linearity of the stage place.Thus, the transistor is not able to will be affected by crowding of the base operate at maximum level and its efficiency lines on the Ic / Vcecharacteristic curves. is low. Positive modulation peaks should not be If the current gain of the transistor is allowed to run the transistor into the non- not constant over the entire range of col- linear area. Similarly, on negative peaks of lector current, the r.f. envelope will not be modulation, the transistor should not be 128Transmitters The Transistor

MODULATOR FINAL AMP. draw heavy currents, and the transformer core size must be adequate for this. This is an important but often overlooked point. AUDIO CHOKE To prevent saturation of the core, caused by the flow of d.c. current through the transformer, the secondary may be isolated from the supply as shown in Fig. 5.4-B. Figure 5.4-B The choke must offer a high impedance to An audio choke will improve quality the audio frequencies. by preventing saturation of the modulation transformer core. 5.5 A Transistor Phasing Exciter

This exciter has a performance equal in allowed to saturate, for distortion will oc- every respect to its tube counterpart. The cur.The transistor contains some resis- output is sufficient to drive a pair of 68Q5's tance between the emitter and the collector, and therefore the collector voltage cannot to 10 watts on either 80 meters or 20 meters. The exciter has no tuning controls be reduced to zero.The above requirements make it difficult, if not impossible, and has only one switch to change when to obtain 100% modulation when the tran- going from one band to the other.The entire exciter, including batteries, measures sistor is operated at or near maximum dis- only 12" long, 2 3/4" high, and 5" deep. sipation. In practical cases, 70%-85% The total weight is less than 4 1/2 lbs. The modulation is normal. unit uses only ten low-cost transistors and Modulation percentage maybe increased draws only 12 ma. at 12 volts.Standard by simultaneously applying power from the modulator to a driver stage. See Fig. 5.4- components were used throughout, and no attempt was made at super miniaturization. A. Thisis accomplished by providing either a tapped or a separate secondary wind- "Barefoot," the rig has worked several ing on the modulation transformer. countries on 20 meters with a cubicle quad No hard and fast rule can be stated thirty-five feet high.Admittedly, the con- about the division of modulator power be- ditions were excellent and free from QRM tween the final and the driver. This is dependent upon a number of factors.It is better to apply as much modulation aspos- sible to the final.This is because modulation applied to the driver causes the final to be partly efficiency -modulated. Thelimit is set by the factors outlined above.

The The modulatortransistors Modulatormust be capable of supplying an output power of at least 50% of the r.f. power amplifier d.c. input. Measuring only 12" long, 2 3/4" high In practice, due to transformer losses, con- and 5" deep, this little s.s.b. exciter siderably more power than this is usually has worked several countries running required for 100% modulation. The mod- "barefoot". Controls are, from left to ulation transformer must be capable of sup- right, 2 carrier balance potentiome- plying the power to the transmitter without ters, sideband selector, band change core saturation.Transistor r.f. amplifiers and on/off switch. Radio Handbook Phasing Exciter 129

TRANSISTOR PHASING EXCITER Liberal use of tag boards has allowed a compact construction. Large space at the left is the 12 -volt battery compartment. The small space is for the v.f.o. at the time, but it does indicate that the ex- The audio phaseshift network was con- citer has creditable performance. structed with 1% high stability resistors and 2% silver mica capacitors.The compon- How It Apart from the first stage, the ents were taped together and placed in an Works audio amplifier preceding the aluminum can which may be seen beneath audio phaseshift network is con- the 9 Mc. crystal in the center of the photo- ventional transistor circuitry and was de- graph.The values shown in Fig. 5.5-A scribed in Chapter Two. The first stage are also used in the Central Electronics was designed to be driven by a crystal mi- PSN, which can be substituted for the crophone, and it was necessary to raise the homemade network. Other networks have amplifier input impedance. This was done the wrong impedance for use in conjunc- by applying feedback via C1 to the base of tion with transistors. a common collector stage. The100Kre- The 0.01 //id. capacitor across the pri- sistor in series with the microphone also mary of transformerT1is necessary to fur- increases the input impedance. In addition, ther limit the high frequency response. it acts as an r.f. filter in conjunction with This value was found to be most effective the 5K bypass capacitor. The capacitor al- with the particular transformers and micro- so restricts the high frequency response. phones used.It is possible that a different This is necessary to meet present day band- microphone or other makes of transform- width requirements. ers may require a slight change if identical 0075Di 26109 C12 26109 Q3 6/T Q4/Q5OC 75 'S 06/07 100K 6/1T2 MIC. 100K 100 0 4- 1 5M - 5 K 100K 47K 10K 100K 8 + -I.5M B-12 V. 8-12 V. 100 K B-12 V. 6/T3 SW I 500.UHRFC 1KP3 D1,2,3,4, L3 2N 371 Q9 A r-T-1K qau--1 470 PF 500 PF L620 R.F. OUT. B -1-1, RFC50001J'H 470 PF 2N 274Qa La 2,175 PF B Q10ON 274 30011. B 12 V. - 12 V. 7 4 MC. = IN OUT 100K 100 K l_ AUDIO PHASE-SH I FT NETWORK 133.3607.5100K K ,218 This two band phasing type s.s.b. exciter will drive two EL 84/61305 tubes to 10 watts input using only 10 low-cost transistors. Figure 5.5-A Phasing Exciter 131

Parts List - Figure 5.5-A T, B -

D1,D2, 03, D4 - Germanium diodes, 1N34, 1N35, 1N198, 0A85 or similar. Li- 0.8 microhenry.9 turns #18 enamel wound on the threaded end of a 1/4" bolt. Remove bolt and screw in a powdered iron coil slug which has an external thread. Figure 5.5-B The audio preamplifier compon- 12 - 15 turns #22 enamel 3/4" long ents are mounted on the termi- on1/2"diameter slug tuned nal board as shown. The strip form.Link 5 turns #22 enamel following the PSN isconstructed close wound over cold end. in a similar manner. 13 Total 6 turns.Each winding 3 turns.#22 enamel to a winding length of 3/4" on a 1/2" slug results are to be obtained. A tone fed into tuned form. the microphone jack should produce ap- L4 3/4" length of #30 on 3/8" form, proximately 1.5 volts of audio at the secon- slug tuned.Secondary, 7 turns dary of Tiwell before distortion occurs. #30, close wound, cold end. The total d.c. current drain of this section is 2.7 ma. at 12 volts. Production spread 15 Pi or jumble wound coil #26 ny- in the transistors and components may lon covered wire.1/4" wide winding in the center of a 3/8" modify these figures somewhat, but large differences in the readings would indicate slug tuned form. (Wire length 6') something wrong. Potentiometer P1 is the Link 8 turns of the same wire as close as possible to primary. ratio control, pertinent to most phasing rigs whether they are tube or transistor 16- 16 turns #22 enamel wire spaced types.The control may be either carbon tooccupy 3/4" on3/8" slug or wirewound.Its purpose is to divide tuned form. Link 4 turns over the audio into two sections of unequal am- cold end. plitude.This allows the audio phaseshift network, which has more attenuation in P1 , P2, P3, P4- Carbon or wirewound. one leg than the other, to deliver two out- 01,02,03,04,05,06,07 - May be puts of equal amplitude. replaced with 2N1274, 2N408 Following the PSN are two emitter fol- or similar. lowers,Q4andQ5.Emitter followers were T1,T2,T3 - 10,000 ohm primary.6/1 used here because of their high input im- turns ratio. pedance. They are followed byQ6andQ7, which are amplifiers operating with a high R1- 100 ohms, 1watt, carbon. Select negative feedback.This has thedualpur- value on ohmmeter as close to pose of stabilizing the transistor gains and this as possible. raising the input impedance.If the input C2 - Made up of 150 pfd. and 25 impedance ofQ6andQ7is raised, this sifd. capacitors in parallel. En- naturally increases the value of the load im- deavor to match these compon- pedance onQ4andQ5.The input imped- ents within 5%. ance ofQ4andQ5is beta times the load 132 Transmitters The Transistor impedance.Thus, the impedance change Figure 5.5-C is reflected through to the audio PSN. It THE BIFILAR-WOUND is necessary to avoid shuntingthe PSN with BALANCED MODULATOR COIL a load, for this will depreciate the sideband suppression considerably. The potentiometer P2 corresponds to TO DIODES the audio balance control in a tubecounter- Note how th e start 1K1 RFC L part and insures that the outputs from the of one winding is two channels are equal. The output of T2 -1- connected to the 1. and T3is more than ample to drive the di- finish of theother. ode -balanced modulators. 4111 The link is wound 1.---F-1 Transistor Q8 is a 9 Mc. crystal oscil- L3 over the center. lator. The associated components have been very carefully adjusted to provide optimum output.This oscillator feeds the be selected by placing them acrossan os- r.f. phaseshift network, which is made up cillator and noting the frequency shift that occurs. of C2,Li , and RI. When a capacitor and resistorin series are shunted across a The link winding from the balanced source of r.f., a 90° phaseshift occurs only modulators provides a good impedance if the circuit is unloaded. When the cir- match to the base of the 2N371 mixer. The 2N2745 Mc. crystal oscillator has been cuitis loaded (in this case by balanced designed to provide optimum mixer in- modulators),itis necessary to insert rejection. actance of the opposite sine in the network The r.f. signal at the collector of the mixer should be approximately 4 volts to correct the phaseshift and maintain equal with full carrier insertion. amplitudes.The slug in Liis a vernier This is more than sufficient to drive the tube amplifier and its adjustment is not critical. shown in Fig. 5.5-E.In the initial stages Select the diodes by measuring their of developement, a 2N371 r.f. amplifier forward resistance on an ohmmeter. The was added to the mixer. This proved not resistances should be within 10%.The only unnecessary but undesirable.It com- combining coilin the output isbifilar plicated the switching and made swamping wound, and the two capacitors across it of the mixer output necessary to prevent should be matched on a bridge.If the flat topping.The increase in output was bridge is not available, the capacitors may small.

The second audio board. This board follows theaudio phaseshiftnetwork. The input isat the right Radio Handbook Phasing Exciter 133

Construction The mechanical construc- beneath P2 .The audio phaseshift network tion of the exciter is un- is in the aluminum can beneath the crystal. orthodox but has many advantages over The wires are of sufficient length so that normal construction.With this system, when the strips are removed the wires may miniaturizationisaccomplished without be left connected and the circuit operated creating a "rat's nest", and sections are in this manner. accessible for testing or alteration.If a Transistors Q4 , Q, , Q6, and Q7 are on larger physical size can be tolerated, there the 3 1/2" long strip second from the rear. is no reason why conventional construction The wires connecting T2 and T3 may be cannot be employed. seen passing through the rubber grommet Where possible, sections are built upon at the left hand end of the partition.It is terminal strips which are in turn bolted to necessary to undo these three wires when aluminum shield partitions 2 1/2" high. removing the strip. The wires at the right The terminal strips are held away from the hand end are of sufficient length to allow metal by small spacers slipped over the the strip to hinge upwards. T2 and T3 are bolts.This leaves sufficient space to clear mounted on a small bracket attached to the the components, which are mounted on shield partition, allowing them to be re- both sides. A layer of thick paper is glued moved as a unit. to the partitions wherever there is a possi- The small terminal strip at the right of bility of shorts occuring. The shield par- T2 and T3 contains the r.f. phaseshift net- titions are held to the base plates by two work and the 9 Mc. crystal oscillator components.The coil is mounted in the com- self -tapping screws.A section may be removed easily by undoing these two screws partment beneath the 2N274 transistor and is attached to the bracket which holds the and then lifting up the strip and aluminum. Whenever practical, the wires are connect- crystal.The bracket is fixed to the shield ed at one end of the board and are long partition. This whole unit may belifted out enough to allow the sections to hinge. by removing the two self -tapping screws at the bottom. Audio The audio amplifier, com- The bifilar-wound balanced modulator Amplifiersprising Ql, Q2, and Q3 is on a strip at the left rear of output coil L3 (immediately behind the microphone socket) is in the front compart- the chassis.The layout is shown in Fig. Small diameter coaxial cable con- ment along with the germanium diode -bal- 5.5-B. anced modulators.These diodes are on nects the crystal microphone jack(left front panel) to the base of Qi through the the terminal strip behind the carrier balanced modulators.These diodes are on 8 #fd. capacitor and the 100K resistor, the terminal strip behind the carrier bal- which are mounted on the terminal strip. ance potentiometers. The control near the The coaxial cable may be seen forming a center is the sideband selector switch. half circle near the 9 Mc. crystal. The cable enters the audio section at the right Figure 5.5-D hand end. Transistor Qi is at theleft hand THE OSCILLATOR COIL,12 end of the strip.The strip material on which the components are mounted measures 2" wide, with lugs along the two TO PSN The link is wound edges spaced 3/8" apart.Grounded ter- over the cold end. minals are connected to the mounting bolts. Note that the hot The potentiometer at the end of the endisclosest to strip and close to the battery compartment 01111111 7 the chassis. is the audio balance control, P2. The ratio Lz 8 - control, P1 , and the transformer, T1,are 4*0 134 Transmitters The Transistor

A coaxial cable is connected from L3 to Coils Itis recommended that coils Li, the mixer Q9 and is taped to the micro- L2, and L3 conform with the sup- phone cable.Transistor Q9 may be seen plied data.A small shield, not visible in behind the band switch, which is the knob the photograph, was placed between the to the left of the crystal socket. Transistor 20 -meter coil L6 and the 5 Mc. coil L4, mid- Qio, the 5 Mc. crystal oscillator, is behind way between the 5 Mc. crystal socket and the crystal socket.The slide switch next the wavechange switch. to the crystal removes the B minus from the unit. Balanced The particular diodes used The mixer and 5 Mc. oscillator compart- Modulatorswere of English manufac- ment were the most difficult to assemble. ture.Other diodes may be Eventually,a new split front panel was substituted without circuit alteration. The made. Now, by undoing two self -tapping two disc ceramics are mounted on the ter- screws and removing the nuts and bolts minal strip containing the diodes and are holding the controls to the panel, it can be connected across the carrier balance po- removed and the compartment completely tentiometers. exposed. AdjustmentsAdjust P2 to the center of The space at the right front of the chas- its range. Use an r.f. probe sis was left for the v.f.o.Initially, each por- or a receiver and adjust L2 for oscillation. tion of the rig was breadboarded while the Turning the carrier balance potentiometers "bugs" were ironed out.This practice should not stop the oscillator, although a was used with the v.f.o. also.However, slight change in frequency when thepotenti- the lack of a suitable dial and capacitor ometers are at an extreme is permissible. gearing mechanism that would fit the space If the frequency change is too great or the available has made the v.f.o. a project for oscillator stops altogether, the link is too the future.The space available is 2 1/2" large or the coupling too tight. No trouble long, 2 1/2" high, and 2" deep. Experi- was experienced in three duplications of ments were conducted with variable capac- this portion of the circuit.With the car- itance diodes in order to use a potentiom- rier inserted, resonate L3. Next, adjust the eter as the tuning component. The prob- slug in L4 until the crystal oscillates. There lem is a mechanical one and not electrical, should be 0.25 to 0.5 volts of r.f. applied for an excellent v.f.o. was built separately to the emitter of Qg. Resonate the 20 and and is described in section 5.7. 80 -meter coils for maximum output. The

The microphonepre- amplifier audio board isfitted to a small aluminum par - titian. The input isat the left. Also shown aretheratio and audio balance potentiometers. Radio Handbook Phasing Exciter 135

6BQS/EL84 Adjust the slug of Li to half way in the coil. Adjust P1 for minimum ripple on the scope.Switch sidebands and readjust Pt .If a different setting is obtained, set it half way between. Go through the same procedure with P2. Repeat the adjustments

with Pi. When no further improvement can be obtained, adjust Li and switch side - bands as before. Repeat Pi and P2 adjust- Figure 5.5-E ments if necessary.There should be no This amplifier, if used after the exciter, noticeable ripple on a two inch pattern, will deliver a large signal to the load. and 40 db of suppression is readily ob- Coils may be bandswitched or plugged tainable. Itis important, when making in as desired. Make certain that the these adjustments, that no sidebandis input and outputcircuits are well favored. shielded from each other. Do not be concerned if P2 is not in the center.This might be caused by a differ- tuning of these stages is fairly broad dueto ence in the gain of the transistors. How- the loading of the transistor.Nothing is ever, too much unbalance is undesirable, accomplished by tappingthe collector and an attempt should be made to match down the coil, for it then becomes neces- the transistors more closely. sary to load the coils with resistance to broadband the tuned circuit. There should BatteriesThe battery consists of eight be no difficulty in obtaining 4 volts of r.f. flashlight cells connected toget- at the grid of a tube amplifier if a step-up her and then suitably insulated from the matching device, such as a link coupling, metal compartment. This compartment is is used. located at the right rear corner of the cab- Balance out the carrier using first one inet. The space around the batteries is control and then the other, returning again filled with corrugated cardboard to prevent to the first, and so on. The carrier should them from moving about. balance out completely, and there should be a negligible 5 Mc. component in the AmplifierAnamplifier has been built in output. No trouble with self -oscillation of breadboard fashion.It per- any stage was experienced, and it is logical forms very well and develops more than 10 to suppose that with such low impedance watts input with a B supply of only 250 circuits, none should occur. volts. Thought has been given to building the amplifier into the battery compartment Next, short out the microphone input along with a transistor power supply. This socket and feed a tone through a large cou- would be ideal for mobile work where an pling capacitor to the collector of Q3. If the external battery is already available.It does tone output is very low it may be fed into present some difficulties concerning con- an earlier stage.It is of the utmost im- trols on the front panel. No doubt each portance that no overloading occurs. Over- constructor will have his own ideas on this. loading creates a distortion which, when viewed on the scope, is indistinguishable Gain In order to keep the number of from that due to poor sideband suppres- Control controls to a minimum, no gain sion. Adjust the tone gain so that approx- control was incorporated. This imately 2 volts of r.f.is developed at the has disadvantages, and a miniature potenti- collector of the mixer. ometer could be built in. The 10K resistor 136 Transmitters The Transistor

in the collector ofQ2may be a potentiometer, with the moving arm connected to the base ofQ3through the 8/Ad. capacitor. In lieu of this, gain can be adjusted by de - tuning the grid circuit of the amplifier.

5.6 A Transistor Filter Exciter

The exciter shown in Fig. 5.6-A is presented for those who prefer the filter system of s.s.b. generation. Many of the comments made on the phasing rig just described also apply to this unit. The exciter develops approximately the same power The carrier oscillator, balanced mod- as the phasing rig and can be used to drive ulator and grounded -base amplifier the 6BQ 5 final shown in Fig. 5.5-E. board (left to right). The potentiometer is the carrier balance control. The How It The modulation section is simi- trimmer capacitor at the top of the Works lar to the phasing exciter but board also aids in balancing out the uses only two transistors.The carrier. audio amplifier consists of a common collector stage with additional feedback to raise the input impedance to a high figure. one side of the balanced modulator circuit The stage is designed to be driven by a and ground. With other transformers the crystal or ceramic microphone. The sec- value may have to be changed or even con- ond stage is a conventional common -emit- nected to the other side of the bridge. ter amplifier feeding a step-down trans- The amplifier following the balanced former. modulator is connected as a common -base stage in order to raise the output impe- Transistor TR1 is a conventional crystal - dance.A high output impedance is re- controlled oscillator which supplies the quired to prevent severe loading of the carrier. The r.f. voltage across the second- i.f.transformer, which would result in a ary winding of T1should be at least one poorly loaded Q.The filter circuit has volt. Different transistors or matching been described in Chapter Four in con- transformers may require readjustment of junction with the transistor communica- the value of capacitance connected between tions receiver.It should be pointed out the base of TR1 and ground. that as the surplus crystals have aged, many of them have changed frequency over the The balanced modulator circuit is a con- years.It may be necessary to select carrier ventional half -lattice.As in the phasing crystals to match the filter. Carrier crystals circuit, the diodes should be matched for should be chosen to operate about20db forward resistance. The 1 K potentiometer down the filter slope. balances out the carrier by electrically balancing the bridge. In actuality only a resis- The output of the filter feeds an ordinary tive balance is obtained, and some carrier i.f. amplifier which, in turn, drives a mixer will remain unless an attempt is also made stage.Note that the mixer, without v.f.o. to obtain a capacitive balance. This is the injection, has no bias. Bias is obtained by purpose of the capacitor connected between rectification of the v.f.o. signal.Correct SCHEMATIC DIAGRAM FOR THE FILTER EXCITER Figure 5.6-A 2 N410 TR1 - 2 N371 TR z The v.f.o. is shown in section 5.8. 2N371 TR 3 2N371 TR a 2N1381 0,...-LIII-- MILLER 101 Ti ...6,T j._ 330 .r17/1 0C169R.F. AMP. FILTER. SEE SECT/ON 4.13 0C169 LLER 2041 Tz 0C169 Cr o- gt!LO.C17-1- -I TDI 4 2 I .2.05 VV. R F S 111.001 111 IDS 1.05 D2 2.5 MHRFC I 2N1274 2N1274 3 M1C. 10011 0075 IooK 100 K N- 2N1380 OK -soon 10K GAIN 9-12V. TR i = .6 MA. 9-12 V. 1005 Parts List - Figure 5.6-A TRaTR 3z = 1.71.6 ma.MA. MA. CARR/ERVOLTAGES CONDITIONS, ARE UNDER FULL Di Li, D2 - 1N34, 0A85 ,or T2 similar. - Miller30 turns 2041 #30 455 enamel 20A000RB1.kc. i.f. transformer. on 1/4" form. 10,000 ohms to 600 ohms. Link 4 turns hook up wire. Miller 138 Transmitters The Transistor collector current, as shown in the chart, indicates proper v.f.o. drive. The output of the mixer also contains the v.f.o.signal, and the tuned circuits following this stage should be of high Q.

Construction Theconstruction techni- H ints que is similar to that used in thePhasingExciter. This subject has been covered in the preceding section. The accompanying photographs show the layout of the terminal boards. The photograph of the completed exciter was not satisfactory for reproduc- tion. Unfortunately, this was not discovered until after the unit had been dismantled. THE I.F. AMPLIFIER In constructing the exciter, none of the AND MIXER BOARD circuits were found to be critical. The layout, using the terminal strip technique, can A complete carrier null should beobtained. be fabricated to suit the builder. Unbalance the carrier slightly and adjust the AlignmentConnect a VTVM to one side filter circuit as described in section of the carrier balance poten- 4.13, using the carrier rather than a re- tiometer and adjust Ti for an indication of ceived signal as the signal generator. Peak r.f. output.The slug in T1 should be set the secondary of transformerT2and the so that the drive is approximately the same output coil Li to the center ofthe operating for both crystals.The oscillator should frequencies. start immediately upon the application of The circuitry of the v.f.o. is not shown but is described in section 5.8. The v.f.o. power.If the activity of the crystals is dis- similar,it may not be possible to obtain can be mounted on the same chassis as the equal drive.Next, balance out the carrier exciter or fed to the exciter through a link by alternately adjusting the 1K potentio- as shown. meter and associated 3-30 Nifd. capacitor. To test the exciter, connect a microphone to the input or apply a suitable audio tone. If the diodes are not properly balanced, the carrier may be reinserted with changes in audio or drivelevels. Imbalance is caused by the differing currents flowing through the diodes with changes in operating conditions.Since the unequal diode resistance causes a different voltage drop across each, it unbalances the bridge.

5.7 40 Meter SSB Transceiver The authors are indebted to Mr. Jo Emmet Jennings, W6EI, for the design The audio boardtakes up very little and construction information presented room under the chassis. below. Radio Handbook 40 Meter SSB Transceiver 139

The Audio The microphone pre -amplifier contains an emitter follower driving a common -emitter stage. The emitter follower is necessary to create a high input impedance, thus allowing the use of a ceramic microphone. A variety of transistors, other than those shown, will perform well in this circuit. On TRANSMIT, the output of the microphone pre -amplifier is connected to the input of an unmodified International TRA- This neat little 40 -meter transceiver 2 sub -assembly.This assembly acts as a package was designed and built by modulator in the TRANSMIT condition Jo Emmet Jennings, W6EI. and as an amplifier to,the speaker in the RECEIVE condition.Audio is more than The transistorized transceiver is basic- ample to drive the balanced modulators. ally made from components which are The balanced modulators are conventional readily available from the International Cry- and differ little from those used with tube stal Company.Some modifications are circuitry.Output from the balanced mod- necessary, and these are covered in the fol- ulator combining coil is fed to the input lowing paragraphs and photographs. of a modified TRB-1 board.

VIEW OF THE V.F.O. Note how the components are made firm by keeping the wires short and making use of tie points. FINALEXTERNAL T2 MC LINEARMODIFIED TRT- 2 T2 MC A T.2MC 2N109 SPEECH AMP. 2N109 T250/W 'ROB TR sw 15V. REC. I .1 01 CONTROL SWA A01 TI 2N248 Ta 2N1143 T3 10 10 JJ T12 OF AUDIO 470 200 1 270 L J XMIT 9AUDIO = 7. 2MC 1ST R.F. AMP.MODIFIED CRC- I 7.2 MC 2ND R.F. AMP. -15V. MODIFIED TRB- t A - 2N140PRODUCT DET - REC. MIX -XMIT T4 A01 100 PF "MECHANICAL b FILTER I rl 1-4:s TIOI 01 TR sw 1.'61 01 ISS KC 150 PF firc 001 7 2 MC 00 It 4 L 2200 .0027 g 4.71 39 T2- V. TR -15V. 330 2N248 I 15 sw REC. AUDIO .005 JJF0.1 TR sw .G TAPERAUDIO2010 Im= 003 7141 -11 B.F. O. 2.5 MH = zR " II TRO-1HMODIFIED 170 _ _J 0-11, 1 N35 CARRIER BAL. 210'150 150I rtt 11 I- TII TI A01 A01 1 V. F. O. AMP. FROM OUTPUT OF TRA- 25-75 PF II. .003RI' BALANCED T I (1 I !TRIM. 50 PF I /0 75 SPEECH AMP. X TRANSISTOREXISTING T TRANSISTORS T T 4: M. 50011MODULATOR 1500 1800 PF I TR sw TR swy TIN AUDIOL 05008 I 1711-17I TURNS AWG 15 TAPPED' 2111 I 735 TURNS FROM BOTTOM,TOP, .01 Ile° It UNUSED 20 IJF --- NOTES: rRsw SHOWN IN RECElla Id\ L. TOROID FORM. 3900 ON -OFF SO if -DEPENDS ON FILTERPOSIT USED. ION. SCHEMATIC DIAGRAM FOR THE COMPLETE 40 METER S.S.B. TRANSCEIVER 320-ASARNES-rARZIAN Figure 5.7-A ,IS v = 40 Meter Transceiver 141

Parts List - Figure 5.7-A The TRB-1 Modifications to the board are as follows. Remove the Coil #1- 30 turns #26 wire.Tap 5 squelch amplifier from the board.In its turns from bottom.Coil is place build the product detector. On wound onoriginalform TRANSMIT, the product detector func- which is taken from the In- tions as a mixer to 40 meters. Next, re- ternational unit. move the first and last i.f. transistors and Coil #2- 29 turns #26.Tap 4 turns replace them with those indicated on the from bottom. schematic. The original transistors have the base internally connected to the tran- Coil #3- 29turns#20. Center sistor case, which, in the higher gain unit, tapped. Wound on origi- is undesirable.The second r.f. collector nal coil form.The capaci- coil is now rewound to resonate at 40 me- tor used to tunetheantenna ters, and the second i.f. transistor is re- on the International board moved. is now used to tune this coil. The transistor socket is used to connect the leads of a small sub -assembly As seen, there is a two turn containing the transistors and the Collins link. Mechanical Filter into the circuit. The os- Coil #4- 35 turns #30 wire.Tap 5 cillator portion of theboard is now returns from bottom.Link 3 moved, and the unused section oftheboard turns. Use originalcoil cut off. form. The TRC-1 This board is modified by Coil #5- 22 turns #28 wire. Center removing allbut one r.f. tapped.Link 4 turns. Use stage.If desired, a new board maybe built original coil form. containing just the one amplifier shown on the schematic.The latter is perhaps the Coil #6- 25 turns #28 wire. Tapped better course, because the modifications to 6 turns from top and 4 turns the TRC-1 are somewhat drastic. The from bottom.Use original TRC-1 board is used on RECEIVE only. coil form. Coil #10 - 25 turns #28 wire.Tap 4 The TRO- The b.f.o. board is not 1 H, BFO turns from bottom. Center altered electrically except tapped. Use original oscil- for the addition of some decoupling com- lator coil form. ponents and a change of value of the output capacitor.The additions and changes Coil #11- See schematic. are clearly shown on the schematic.The Coil #12 - 25 microhenry r.f. choke. exact frequency of the cyrstal will be dependent upon the type of mechanical filter Coil #13 - 25 turns #28 wire.Center used.Note that in the TRANSMIT con- tapped.Link 4 turns. 3/8" dition, the b.f.o. is the carrier generator. slug tuned form mounted Its output is connected to the diode bal- in i.f. can. anced modulator.In the RECEIVE con- Coil #14 - Miniature transistor i.f. dition, the output of the b.f.o. is connected transformer.15,000 ohms to the emitter of the product detector. to 600 ohms. The capacitor inside the can is removed. The V.F.O. Thev.f.o.isthe familiar Colpitts oscillator followed TRANSMIT/RECEIVE switch- Minia- by an isolation stage and an amplifier. The ture ceramic wafer. DP8T. oscillator tank coil is wound on a toroid 142 Transmitters The Transistor

The TRA-2 board is shownattheleft. The otherboards may be seen at the left of the chassis.

form.This results in a reduced physical parts due to over -tight wires. size and a smaller electric field, allowing Duringthe TRANSMIT condition, the the v.f.o. unit to be housed in a small con- output from the v.f.o. amplifier is connect- tainer. The v.f.o. is operated from a 6 -volt ed to the emitter of the product detector tap on the battery. A silicon diode is in- which now functions as a mixer. Output serted in the battery lead to prevent theflow from the mixer is connected to the input of current when the transceiveris turned of a modified TRT-2 board. off. A separate switch, ganged to the main on/off switch, may replace the diode. V.f.o. construction is clearly shown in The TRT-2 Several modifications are re- the photograph.Other layouts and con- Linear quired on this board. The tainers may be used if due regard is paid to Amplifier modifications may be made stability requirements.Mount all compo- Board in the following order: nents firmly by short stiff wires. However, a) Remove the crystal socket and cut be careful to avoid stress on thecomponent off that end of the board.

Another view showing the placement of the various boards. Radio Handbook V.F.O. 143

b) Change thefirsttransistorto an L1 0-50 PRESET VAR BLE A01 (Philco) and the second to a 5.6 I( 2N248.

c) Remove the coils and replace with 5,

40 meter coils wound according to GA -51469 NTC the data provided. OPTIONAL d) Add forward bias resistors to the three stages to operate the transis- LOW Z tors class A. The resistor values are OUT shown on the schematic. Note that the last stage is neutralized to C3 prevent oscillation. The output coil should 220 M be wound exactly as specified so that neu- 2.5 PAM RFC

tralization is accomplished without the need COLLECTOR CURRENT _L-3'2:1 for coil or component alteration. TR,.1.5 MA. = TR2=4mA. Alignment Alignment is most readily accomplished if a signal gen- Figure 5.8-A erator is available.In lieu of this, the v.f.o. DIAGRAM OF THE V.F.O. and b.f.o. may be set on frequency by lis- Note that several volts of r.f. can be tening to their respective outputs in a re- obtained by using the high impedance ceiver tuned to the required frequencies. output. In any case, a receiver should be available and tuned to the output of the transceiver. Parts List - Figure 5.8 Coils, i.f.'s., etc., may then be peaked for C1 - 385 iridd. variable.Semi -circu- maximum S meter reading.The length lar plates and ceramic insulation. of the antenna on the external receiver End plates should be slotted for should be adjusted to prevent overloading alignment adjustments.Broad- and false meter readings. cast type capacitor. Output from the transceiver, when the C2 - National APC-50. stages are aligned for maximum signal, is 11 - 9 turns #22 enamel on3/8" di- approximately 1/2 watt - sufficient to drive ameter ceramic form.Miller, a 6BQ5 or similar tube linear. catalog #4400.Wind link of 1 turn of hook up wire over cold end of Li . 12 28 turns #30 enamel on 1/4" 5.8 A VFO for AM, CW,orSSB form. Miller 20 A000RB1 or similar. Link 4 turns hook up The v.f.o. to be described was engineer- wire over cold end. ed to have better than average stability and TR1 - 2N384, 2N371,0C169,2N1745 linear dial reading.As it so happens, the TR2- 2N371.Other transistors may requirement for a linear reading dial is that be usedif the neutralizing ca- there be very high fixed C in the oscillator pacitor C3is suitably adjusted. tuned circuit, preferably as much as 4 times R1 Negative temperature Resistor. fixed to variable.This is also a condition Approximately 4,000 ohms. of high electrical stability in a v.f.o. High (Thermistor) GA -51489 or simi- C circuits are necessary to swamp out var- lar. iations of capacitance in the transistor it- Resistors marked 1 watt may be carbon self. or wire wound. 144 Transmitters The Transistor

The v.f.o.is designed to cover either ed.Remove any mica trimmers which are 3.5 Mc. to 4 Mc. or 3950 kc. to 4450 kc. part of the capacitor. The latter range is suitable for use with All fixed capacitors in the tuned circuit 455 kc. crystal or mechanical filter SSB must be silver mica. The trimmer capacitor exciters. C2is a ceramic unit such as the National The r.f. voltage at the collector of the APC-50. output stage is in excess of 2.5 volts when Construction The construction of the loaded, which issufficient to drive most Hints v.f.o. must be mechanically small pentode tubes.The output is more sound.The best electri- than sufficient to drive the average s.s.b. cally designed v.f.o. in the world is only as exciter. good as, the mechanical construction which goes into it. Avoid stresses and strains on How It The oscillator is aparallel-tuned components, particularly the coil, thevaria- Works Colpitts.This is followed by a ble capacitors, and the dial mechanism. If lightly coupled tuned amplifier. the dial mechanism does not line up with The amplifier is neutralized to prevent vari- the capacitor shaft,it will be necessary to ations in output loading from being re- keep one hand on the tuning knob at all flected back to the oscillator.The values times in order to stay on frequency. Con- ofC3andC4should not be changed from struction may take almost any form as long those shown on the schematic diagram. as it is sturdy.If the v.f.o. can be dropped from a height of three inches without the The The tuning capacitor must be frequency of oscillation moving more than Capacitorsof the straight line capacitance a few cycles, the v.f.o. is mechanically stable. variety (semicircular plates) Naturally, the unit is mounted in its cabinet if a linear dial reading is to be obtained. for this test. Tie points, when they support The capacitor may be a standard broadcast components "hot" to r.f., should be ofcer- unit and should have ceramic insulation, amic or similar material. if possible, and slotted end plates.In the At first, no attempt was madeto stabilize prototype model, a two gang capacitor was the oscillator against thermal variations, used only because it happened to be in the but it was found that thefrequency shift was junk box. One section was left unconnect- considerable when a change in room tern -

UNDERSIDE VIEW OF THE V.F.O. The oscillatorisat the left and the am- plifierat the right. Radio Handbook V.F.O. 145

The tiny transmitter in the foreground is intended for satellite service and uses the mercury batteries shown at the right.An equivalent tube transmitter would use the batteries at the left. (Courtesy of DuKane) perature took place. Most ofthe movement supply should be fed to the oscillator and was traced to the coil.Frequency move- the amplifier.For normal fixed station ment due to the transistor itself was very work, no stabilization is necessary. small.After a little experimentation, a negative temperatureco -efficient resistor AlignmentTo obtain a linear dial read- (thermistor) was added to the base circuit ing, the slug of L1 should be of TR1.This component reduces the fre- adjusted for the low end of the band and quency movement to negligible propor- the preset trimmer set for the high end. tions for all but excessive variations.It is One adjustment will affect the other, and recommended that the oscillator be totally the builder must work back and forth be- enclosed so that the temperature changes tween the two. If a 100 kc. crystal calibra- take some time to soak through to thecom- tor is available, align the low end to the 3.6 ponents.In this circuit, the NT C resistor Mc. check point and the high end to the 3.9 actually overcorrects the transistor, and by Mc. check point.If there are variations at doing so takes into account the movement the intermediate check points, these maybe of the coil and other components. corrected by bending the slotted vanes of C1. On the prototype model seen in the photograph, v.f.o. dial readings are accur- Voltage A reduction in supply vol- ate to within 1 kc. except at the very high Stabilizationtage from12 to8 volts end of the range, where the minimum capa- moves the frequency of the citance of the capacitor is too high. Only oscillator by 800 cycles. Ifthev.f.o. is to be the last 30 kc. are affected in this manner. used in mobile work the supply should be All other readings, right down to 3.5 Mc., stabilized with a zener diode. The stabilized are exact. 146 Transmitters The Transistor

2N706 PT880 X00 PT879's P7716

- 12V. -1 - Figure 5.9-A A completely transistorized 6 -watt 10 -meter transmitter uses silicon devices manufactured by Pacific Semiconductors, Inc. Tomodulate the circuit, break the 12 -volt line at the points shown.

Parts List - Figure 5.9-A 14- 5 turns #20 onCambridge Therm - ionic form 227-14. Li - 12 turns #221/4" form. Secon- 15- 4 turns #20 on CambridgeTherm- dary 4 turns #26. ionic form SPC2-J-21. L2- 9 turns #22 1/4" form. Secon- C1- 8-60 ,u/rfd. ARCO 404. dary 3 turns #26. C2 - 90-400 mufd. ARCO 429.

L3 - 7 turns #20 on CambridgeTh erm- C3 - 90-400 ,u/dd. ARCO 429. ionic form 227-14. Xtal - 3rd overtone.

5.9 A 3 -Watt C.B.or 10 -Meter Similarly, the two paralleled PT879's de- Transmitter pend upon the developed bias to establish the operating point.The use of separate bias resistors tends to compensate for un- For some time, Pacific Semiconductors, equal transistor gains and prevents either Inc. have had available a line of silicon one of the transistors from doing most of transistors which have made possible an in- the work. expensive transmitter capable of delivering 3 watts of r.f.to the antenna.With the The Output The output circuit consists values shown in the schematic ofFig. 5.9A, Network of a tuned r.f. choke (low the transmitter is capable of operating over Q resonant circuit), follow- both the Citizen and the 10 -meter ham ed by a double L network. The network bands. has been designed to present a specific load to the transistors and at the same time pre- The The oscillator is a PT716 operat- vent harmonics from reaching the antenna. Circuit ing as a Colpitts. The overtone Due to the low impedances involved, this crystalisin the feedback path. portion of the circuitis perhaps more Following the oscillator is a PT880 buffer complex than would be required of tube amplifier.The stage is not provided with transmitters.However, the circuits tune fixed bias but depends instead upon recti- broadly and merely need peaking for max- fication of the signal to develop a self bias. imum output. Operation isin the region between class Input to the finalis approximately 6 B and class C and is a function of drive. watts and efficiency is about 50%. Radio Handbook Two Meter Transmitter 147

TO BUFFER TO MODULATOR AND FINAL OSCILLATOR lead lengths.Each component and circuit must operate at top efficiency to prevent circuit losses from destroying the small power generated by the transistors. Selecting transistors of low cost with a TRANSMITTER high alpha cutoff and low collector capacitance also proved to be a problem. The I RCA 2N384 type transistor proved to be an excellent oscillator and multiplier, but Figure 5.9-B did not provide sufficient power output All three stages must be modulated to as a 144 Mc. amplifier.The final ampli- reach the 100% level, as shown here. fier transistor which provided the greatest power output in this application was select- If the transmitter is to be modulated, ed from the available types. The modulation transformersecondary Three transistors are used in a 72 Mc. should be center tapped and modulation oscillator, doubler, and power amplifier applied simultaneously to the oscillator configuration as shown in Fig. 5.10-A. from the tapped winding. Both the buffer The oscillator, a 2N384, is connected in and the final are modulated from the top of the common -base circuit and employs a the winding.The connections are shown 5th overtone crystal, which oscillates on in Fig. 5.9-B. 72.02 Mc.Feedback for the emitter circuit is taken from a low impedance tap on the coil in series with the crystal. A neu- 5.10 A Two Meter Transmitter tralizing coil Li insures oscillation at the crystal fifth overtone.The oscillator out- Itis hard to believe that low cost re- put coil L2 feeds 72 Mc. energy to the ceiving type transistors can develop suffi- doubler through a modified pi -network. cient power on two meters to light a pilot The highest doubler power output is lamp, but this is exactly the case with the obtained when the stage is operated be- circuit shown in Fig. 5.10-A.This can tween class B and C. The 2N384 doubler only be accomplished by careful construct- stage operates without bias, and positive - ion. Thought must be given to layout and going r.f. cycles initiate conduction.

UNDERSIDE VIEW OF THE TWO METER TRANSMITTER Notethespacing and coupling of coil L4. The adjustment is rather critical. 148 Transmitters The Transistor

Qt Q2 Q3 2N384 L, 2N384 2N1407

ANT.

SF= SELF -RESONANT FREQUENCY(SEE TEXT) -12 V. MOD.

Figure 5.10-A SCHEMATIC FOR THE TWO METER TRANSMRTER Note that only the final stage is modulated. Some improvement may be obtained by also modulating the doubler. Parts List C1 , C2 -15 sifd. variable 13- 5 turns, #14, 3/8" diameter (Hammarlund MAPC 15) L4- 4 turns, #16, 3/8" diame- C3 - 5 mufd. variable (E. F. John- ter.Adjust coupling and son 5M11) spacing for max. drive. C.4, C5 - 7-50it#fd.rotary ce- L5- 5 turns, #14, 3/8" diameter ramic (Centralab) L6- 4 turns #24 enamel, inter - Li- 21 turns, #28enamel, wound in L5. Cement to L5 closewound on 3/16" form when correct coupling is (1 meg., 1 watt resistor) obtained. 12-10 turns, #22, 5/8" diam- 01,02 - 2N384 (RCA) eter, 8 turns per inch, xtal Q3- 2N1407(Texas Instruments) tap 7 turns from bottom, Xtal- 72.02 Mc., 5th overtone B- tap 5 turns from bottom (International Crystal Mfg. (AirDux #508) Company)

The 144 Mc. r.f. drive from the doubler Construction There are no particular is applied to the power amplifier (a Texas H ints problems in constructing Instruments type 2N1407) through a link the two meter transmitter, and impedance matching capacitor.The but the buider is cautioned to keep all circuit is series tuned and resonated at 144 leads extremely short.The photographs Mc. This stage is not forward biased. show one of several layouts used by the Therefore, positive r.f.cycles cause con- authors. Although not especially attractive duction, producing near class C operation. the component crowding permits short The output of this stage appears acrossL5 lead lengths. The 2N384 transistors were which is also resonated at 144 Mc. A res- mounted in clips drilled from an old fuse onated link coil L6 couples r.f. energy to holder.The crystal, oscillator transistor, the antenna. The series capacitor tunes out and bias components are mounted on the the reactance of the link and serves as a rear apron. The 15/i/efd. tuning capacitors, loading control. R.f. chokes, self resonant the doubler transistor, and coils are mount- at the operating frequency, are used for ed on the top of the chassis box. The isolation in all circuits. power amplifier transistor (mounted in a Radio Handbook Two Meter Transmitter 149

heat sink clip), capacitor C3, antenna jack, and matching components are on the remaining apron of the box. The only critical component is the neutralizing coil L1 connected across the crystal.This coil must resonate with the crystal capacitance at 78 - 80 Mc. After winding the coil, clip the leads short, solder it across the crystal pins and check the resonant frequency. Add or subtract turns as required to tune 78 - 80 Mc. The r.f. chokes are made by winding 20-30 turns of very fine wire on a one megohm, one watt carbon resistor. Check and adjust the resonant frequency to either 72 or 144 Mc. by adding or subtracting turns.The exact frequency is not critical.

Testing A grid dip meter is mandatory when working with high fre- A COMPLETELY TRANSISTORIZED TWO -METER TRANSMITTER quency circuits.First, apply 12 volts to the This transmitter will supply sufficient oscillator only, and check for operation power to illuminate the #49 pilot lamp. with the dipper switched to the absorption wavemeter mode.Check the second harmonic on a receiver to insure that the sig- series with the final supply lead. Peak the nal is crystal controlled.Next, connect a doubler tuning for maximum final current. milliameter in series with the doubler sup- Finally, observe the output with a grid ply lead and peak the oscillator capacitor dip meter, field strength meter or receiver Ci for maximum collector current. Con- S meter. Repeak all adjustments for maxi- nect the supply lead for Q2 directly to the imum r.f. output.The circuit values have 12 volt line, and connect the milliameter in been selected so that excessive dissipation in any stage cannot occur. The final stage will draw about 10 ma. when fully loaded. It should be possible to see a noticeable glow in the #49pilotlamp used as a dummy load (see photo).

5.11 A Potpourri of Circuits

Although the authors feel that amateurs should know how to design their own circuits, this is not always feasible. Engineers working in plants manufacturingtransistor equipment are no different from amateurs. Everyone borrows circuits forprojects when it will save time and get the job done. VIEW SHOWING THE OSCILLATOR This section presents a group of inter- COIL 12 esting and useful circuits which have been 150 Transmitters The Transistor

(Ti) (Ti) (Ti) (Ti) 2N716 2N716 soo 2N716 Soo 2N716 L3 .5_0eL4 00000 000001.--t-1

RFC g. 2h. 1.2 Ce RFC 1.2 C4 1.5 211H 1- 001 .1.) 1.1H 2 H

RFC 1. 2JJH

RFC, 1.211H RFC, I. 2 LH 4- 300. 500 - 500 C

Figure 5.11-A

CIRCUIT FOR THE QUARTER WATT TRANSMITTER DESIGNED FOR 140 Mc. OPERATION Parts List C1, C2, C3, C4, C6 - 1.8-13 Aufd. trimmer C5 - 7-45 rotary ceramic trimmer C7 - 3.2-50 NM. tuning capacitor Li - 8 turns of #408 AirDux, tapped 2 turns from ground end.

12 - 4 turns of #12 enamel, 1/2" diameter, 3/4" long. 13 - 3 turns of #12 enamel, 1/2" diameter, 7/16" long. 1.4 - 4 turns of #12 enamel, 1/2" diameter, 3/8" long. Xtal - 72 Mc., 5th overtone type (International Crystals Mfg. Co.) extracted from company application notes its output network consists of a pi section and instruction books. With minor adapta- feeding a 2N716 common -base amplifier. tions (in some cases) they are all useful in The final stage, a 2N716 common -emitter amateur applications. Although the circuits power amplifier, drives the antenna through have not been tried by the authors, they do a pi network.Note the liberal use of r.f. not appear to present any construction or chokes to prevent unwanted coupling be- alignment problems. tween stages. This appears to be the secret in building high efficiency v.h.f. transmitt- VHF Circuits Fig. 5.11-A shows the cir- ing equipment.R.f. chokes are used as cuit for a 140 Mc. trans- d.c. returns in all circuits to avoid EI losses mitter which can be returned to 144 Mc. in decoupling resistors.The power out- with no coil or tuning capacitor modifica- put of this transmitter is1/4 watt (250 tions.The circuit was designed by Texas milliwatts) at 140 Mc. Instruments. Fig. 5.11-B isa common -base power The oscillator, a T1 type 2N716, is a amplifier for the 100 - 200 Mc. range. Un- 5th overtone circuit on 70 Mc.In -phase like the preceding circuit whiich uses all feedback occurs between a tap on the out- silicon devices, this circuit employs a ger- put coil and the emitter through thecrystal. manium diffused v.h.f. transistor.Three It is interesting to note that 70 Mc. drive different types are given for a variety of for the doubler is taken from the low im- power outputs. R.f. is applied to a suitable pedance emitter in the oscillator circuit. tap on the input circuit (depending on the The doubler, another T1 2N716, operates drive impedance) and is coupled to Q1, in the grounded -base configuration, and through a capacitive voltage divider. This Radio Handbook V I-1 F Circuits 151

1.6-133111F Q1 0.25 LH 50 CLOUT T1657 5.0ILUF e.3RD OVERTONE 5031_ MC.XTAL IN RL 50 n

5 50 LAUF 30 V.

2N1141 =1110 MW. OUT. 2N1142= 170 MW. OUT. 2N1143=160 MW. OUT. +12V. 1C= 2.5 MA. P.O. = 7.5 MW. Figure 5.11-B CRYSTAL OVERTONE OSCILLATOR CIRCUIT POWER AMPLIFIER This power amplifier uses low cost Figure 5.11-D germanium transistors. A THIRD OVERTONE OSCILLATOR Can be used in conjunction with war serves to impedance match the base of the surplus 8 Mc. crystals. transistor to the tuned circuit. The operating point of the transistor can be adjusted overall efficiency of 45%. The power gain with the1 K potentiometer, which varies is 10 db at 160 Mc., indicating that a drive the base bias.The output circuit, which level of 25 milliwatts would be required, matches the collector to the antenna, is a neglecting circuit losses.The input cir- pi network.It is particularly important to cuit consists of a capacitive divider similar note that the d.c. return for the amplifier toFig. 5.11-B.However, base bias is is through the load.The circuit should provided by self rectification of the r.f. not be operated without the load connect- current flowing through R h. The trimmer ed, as excessive base bias, due to self rec- capacitor in the emitter circuit series re- tification, may develop. sonates the emitter inductance, providing The new Motorola epitaxial mesa family, a true ground. The r.f. choke simply pro- types 2N1141 through 1143 and 2N1195, vides a d.c. return.The output circuit is make excellent power amplifiers in the two the familiar pi network to match a variety meter band.The circuit shown in Fig. of antenna impedances.The collector is 5.11-C will deliver 1/4 watt to a load at 160 shunt fed for d.c. through an r.f. choke. Mc. and should develop slightly more at Amateurs who prefer not to use 72 144 Mc.At this level, the power input is Mc. fifth overtone crystals will be interest- 560 milliwatts (14 volts at 40 ma.) for an ed in a circuit described in Philco Applica- I+ 14 V. - tion Lab Report 646. This report describes 2N1142 a circuit for operating 8 Mc. crystals on 0.11 LH their third overtone. The circuit( Fig. 5.11- D) employs a Philco type T1657 transistor, which is believed to have been replaced by the JEDEC type 2N1866. The output tank, consisting of L1 and C1, is tuned to the third overtone of thecrystal, or approxi- 160 MC. POWER AMPLIFIER mately 24 Mc. One and one-half turns of coil L1 provides a degree of regenerative Figure 5.11-C feedback for the crystal, since it is not cut QUARTER WATTOUTPUT forovertoneoperation.CapacitorC2 POWER AMPLIFIER matches the transistor collector to the load Amplifier uses new epitaxial mesa impedance.Coil L1 consists of 12 turns device. of #3003 B & W "Miniductor" tapped at 152 Transmitters The Transistor

Nol lulu num mar

SILICON GALLIUM ARSENIDE

Figure 5.11-E

Tunnel diode characteristic curves for silicon and gallium arsenide. (Courtesy of General Electric)

11/2 turns for feedback.Since the value The tunnel diode exploits thefirst meth- specified for C2 is not readily available, a od of conduction. The electrons move so 3-30 trimmer could be used.Capacitors fast that they seem to actually "tunnel" C1 and C2 would be adjusted alternately for maximum power output, which is given TUNNEL DIODES as 7.5 milliwatts. The overall efficiency of These tunneldiodes are currently the oscillator is approximately 28%. The available to experimenters. The Gen- drive from this circuit could be applied to eral Electric 1N2939 (front) and the a suitable tripler-doubler circuit for output RCA TD100 (rear) are compared to a on 144 Mc. common straight pin to illustrate their The When a semiconductor diode is compact size. Tunnel forward biased, it does not be - Diode have in the same manner as a good conductor (such as a piece of copper wire).In a wire, one electron will "bump" the next one, which in turn "bumps" the next, and so on. This movement isconducted down the wire even though the individual electrons may have moved only a short distance.In a semiconductor, current flow takes place due to an exchange of electrons from one orbit to another, which slows down the action ap- preciably. Radio Handbook The Tunnel Diode 153

TUNNEL 0100E

Figure 5.11-F TUNNEL DIODE TRANSMITTER This transmitter can be used on 80 and 40 meters. through the potential barrier of the diode. See Chapter One. The effect is conduction similar to that which takes place in a wire. The quantum mechanical tunneling was first discovered by aJapanesescientist named L. Esaki. The tunnel diode is often A solar operated 80 meter c.w. trans- referred to as the Esaki diode. mitter. The solar cell,in the fore- The important feature of the tunnel diode ground, is an International Rectifier isits negative resistance quality. Unlike a SD -1020-A. pure resistance, current in the tunnel diode can decrease with increasing applied voltages.Characteristic curves of voltage ver- The transmitter can be powered by either sus current for two different types of diodes a 1.5 -volt penlite cell or a single solar cell are shown in Fig. 5.11-E. The breakup of as shown in the accompanying photograph. the negative resistance slope is probably The tunnel diode requires less than one- caused by parasitic oscillation in the test half volt and draws less than 5 ma. of cur- jig. This is one of the tunnel diode's rent. Obviously, it is not capable of gener- characteristics and itis difficult to prevent ating much power, at least in itspresent them from oscillating! form. However, Lester A. Earnshaw, when A simple tunnel diode transmitter for operating from ZL1AAX in Warkworth, the 80 or 40 meter bandsis shown in New Zealand, was ableto contact Jack King Fig. 5.11-F and the accompanying photo- ZL1AOF, in Whakatane, N.Z., a distance graphs.The only parts required are the of 160 miles.His RST 339 signals were tunnel diode, a crystal for either band, an later confirmed with a QSL and tape re- r.f. choke (2.5 mh.), and a potentiometer. cording.

TUNNEL DIODE TRANSMITTER This transmitter was used by ZL1AAX to contact stations 160 miles distant. 154 Transmitters The Transistor

tion types in the audio section. The trans- mitteris suitable for short range civilian defense and amateur activities on the10- meter band.

A crystal controlled oscillator drives an unneutralized r.f. amplifier, which delivers power to the 50 -ohm load.Transistor TR3 provides a high impedance input fora crystal microphone, while TR4 serves as an audio driver for the modulator transistor TR5.The modulator is a series type which does not use a matching transformer.

The transmitter should be tuned for c.w. conditions. Point A of the emitter resistor in the final amplifier is first connected to the battery.Adjust C1 in the oscillator for maximum drive to the final, using the current meter as an indicator. Next, tune C2 for minimum final collector current, while adjusting C3 for desired loading. The col- THE HEATH GW30 lector current should be 5.5 to 6.0 ma. May be easily converted to 10 -meter This tuning procedure does not necessarily operation by replacing the crystal and deliver the maximum output to the load but retuning the receiver and transmitter has been found to be satisfactory. A better coils. (Courtesy of Heath) method would be to use a sensitive field strength meter located near the antenna. The power input for c.w. is approximately A Ten MeterThe current crop of Citi- 72 milliwatts, and the measured power out- Phone izen's Band equipment has put is 52 milliwatts. Transmitter resulted in numerous walkie-talkie and handie-talkie For phone operation, point A ofthefinal devices. These units may be easily con- is tied to A , the collector ofthemodulator. verted to 10 meters by replacing the trans- Bias resistor R is adjusted to a value which mitter crystal with a third overtone type for causes the collector current to drop to one- the desired operating frequency.If the re- half the c.w. value at about 3 ma. This receiver iscrystal controlled,thiscrystal sults in about 32 milliwatts input to the final should be replaced with a third overtone r.f. stage. The modulation envelope appears type which is displaced from the operating clean at full modulation (about 85%), and frequency by the i.f. value. The r.f. tuned on -the -air reports indicate a good quality circuits will have to be repeaked for best phone signal.Care must be exercised not performance. to remove the antenna from the final with A 30 milliwatt 10 -meter phone transmit- the transmitter on! ter, which uses inexpensive transistors, was Although the transistor specified is an described in Philco Lab Report #610. The early type, the T1657, it is believed that the circuit is given in Fig. 5.11-G.Philco JEDEC type 2N1866 makes an excellent MADT (Micro Alloy Diffused) transistors replacement and may provide even more are used in the r.f. section and alloy junc- power output. Radio Handbook Ten Meter Transmitter 155

TR TR2 11657 11657 50.U.UF PH/ LCO PHILCO 0HIC L L 2 10-75 5011 470 1JUF OUT L 11.1JF

4715 0.5-20 1J1J F

330 I.00I .0OtI 1270 1.00101C=3 MA.

12V. A -VCC 1111141H_ Z150 KR TRa ° TR5 HI Z 1.01JF 2N 207 2N 223 MIC. 7- 211F t 4.7H IS fL4.7 GAIN CONTROL 2.515 21JF _L NOTES: TT = DAW 3003 12 T. COLLECTOR TAP 8 T. FROM COLD END. COUPLING TAP 2.5 T. FROM COLD END. L2= t3aW 3004 10T . L3 = 4 T. *24 NYCLAO WOUND OVER COLD END OF L2. X = 27-30 MC. 3RD OVERTONE CRYSTAL Figure 5.1 1-G THIRTY MILLIWATT 10 -METER PHONE TRANSMITTER The transmitter uses low cost germanium transistors. CHAPTER SIX

Power Supplies

6.1 A.C. to D.C. Conversion

If transistorized equipment is used in a location where alternating current is available, there is no advantage in using batteries when an inexpensive power supply may be easily constructed. A simple transistor power supply is shown in fig. 6.1-A and is suitable for experimenter applications.The regulation of the supply is ample for most needs and only high power audio amplifiers and modulators will require a larger power supply. When connected to thetransistor communi- A SIMPLE POWER SUPPLY FOR cations receiver described in Chapter 4, the TRANSISTOR EXPERIMENTS output voltage does not change more than Even without regulation the stability 0.4 volts, even when the receiver is operated at maximum audio output. of the output voltage is excellent.

The transformer, T1 , used in the supply CH is a discarded 12 -volt vibrator transformer. However, a commercial unit such as the taV DC Triad F -92A which is capable of supplying approximately 16 volts at more than 500 ma. will be suitable. Figure 6.1-A A SIMPLE POWER SUPPLY FOR How It The circuit is a full -wave bridge TRANSISTOR WORK Works rectifier which utilized both halves The transformer and choke are de- of the a.c. cycle. The rectified d.c., scribed in the text. The diodes maybe which is pulsating at 120 cycles per second, RCA 1N1764 or any similar 500 ma. requires filtering to provide pure d.c. The units. filter, which consists of the input and out- A.C. to D.C. Conversion 157

UNREGULATED REGULATED INPUT OUTPUT

Figure 6.1-B THE ZEN ER OR AVALANCHE DIODE This diode can be used to regulate specific voltages.The diodes are manufactured in different voltage groups. put capacitors and the filter choke, forms a single pi -network. Increasing the size of the input filter capacitor will increase the output voltage and improve regulation. In- creasing the capacitance of the output filter capacitor will decrease the ripple and also improve the regulation. Construction Any form of construction MINIATURE SILICON and layout is practical and ZEN ER VOLTAGE no special precautions need be observed. REFERENCE ELEMENTS The choke may be constructed by winding Courtesy of International Rectifier asmall3 -watt audio -output transformer Corporation) core full of #24 enamel wire.The wire may be layer or jumble wound on a homemade bobbin.Once the core is stripped When wiring the supply, do not connect it should take only 10 minutes or so to either the positive or negative lead to the wind the wire and re-insert the core. chassis.The two circuits should be inde- pendent of the chassis, as shown in fig. BURGESS BATTERIES 6.1-A.This prevents transformer leakage The battery is still a convenient power from damaging transistor equipment.In supply for transistor experimenters. addition, a short from the power supply These Burgess units are sealed re- chassis to the equipment being tested could chargeable nickel cadmium type damage the supply if this precaution is not observed. It is advisable to insert a 1/2 ampere fuse in series with the secondary winding to protect the power supply in the event of an external short circuit.

Voltage One of the simplest devices Regulationfor regulatinga varying source of d.c.is the zener diode.A property of the zener diode is that when the voltage across its terminals exceedsacertain level, the zener diode conducts. The result is a regulated output voltage much the same as that accomplished 158 Power Supplies The Transistor with a gaseous voltage regulator tube, al- o 0 though the mechanism for accomplishing UNREGULATED REGULATED INPUT i OUTPUT regulation is quite different. Note that the diode is connected in the reverse direction o T o (Fig. 6.1-B).At the breakdown potential Figure 6.1-C (analagous to ionization in the voltage re- A battery can also be used to gulator tube) the current carriers avalanche establish a specific bias voltage and the diode assumes a relatively low re- for a transistor regulator. sistance. The circuit for a simple zener diode re- tion battery voltage, which acts as a refer- gulator is shown in fig. 6.1-B. The resis- ence for the transistor regulators. The a.c. tor,R1, prevents excessive current flow ripple on the output d.c. will vary between through the diode when the load is re- 4 and 28 millivolts as a function of load moved. voltage and current. A transistor can also be used to regulate This type of regulator is commonly re- a power supply as shown infig. 6.1-C. As ferred to as an emitter -follower wherein Q3 the output voltage is decreased with in- and Q4 serve as variable resistances in creasing loads, thetransistor bias increases. series with the load.The terminal impe- This causes the collector -emitter resistance dance of the regulator is less than one ohm. to decrease. More voltage is thenpermitted A regulation curve for the supply is also to develop across the load. Thus the output shown in Fig. 6.1-D. voltage will be regulated at approximately The series -regulator transistors should the battery potential. on a large aluminum heat A heavy duty supply using a small battery sink as they dissipate up to 60 watts with for stabilization is shown in fig. 6.1-D. the control set for low voltages. The con- This circuit, described in a Motorola appli- trol transistors, Qi and Q2, may also re- cation bulletin, uses inexpensive Motorola quire heat sinking as they each dissipate 2N554 transistors.The supply delivers approximately 1 watt. continuously variable voltage at currents Note that when regulation is employed up to 4 ameres. As in fig. 6.1-C, the out- less elaborate filtering circuits are required. put voltage is determined by the stabiliza- A simple explanation is that the ripple ap-

UNDERSIDE VIEW OF THE POWER SUPPLY Radio Handbook Regulated Power Supply 159

2N554 (4) (EMITTERS NOT USED) 2N554 C a, No:..ur (4) Qi Q2 Q3 Q4

0-12 V., 0-4 A. (RIPPLE 50 MV. AT 2 A.)

10 MAX. VOUT <

? 6

6. 4 Rz SET AT ÷ ROTATION O 2

0 2 3 4 OUTPUT CURRENT -(A.) Figure 6.1-D A HEAVY DUTY POWER SUPPLY A heavy duty power supply using a battery stabilized emitter follower which drives two parallel connected series control transistors.

pears as a varying load voltage which is current, and its peak inversevoltage should counteracted to a large extent by the regu- be at least 75 volts.Due to the capacitive lator. load, the voltage is higher than that nor- A zener diode may replace the battery mally encountered in this type of circuit. in the circuits described above. The action Resistor R1is used to limit the peak of the zener diode is the same as in the currents to a safe figure. When the supply simple regulator in Fig. 6.1-B. That is, it isfirst switched on the sudden surge of establishes a fixed potential across its ter- current into the 2,000 pfd. capacitor may minals. Thus, it functions as a battery, but reach many amperes without R1. The cur- does not require periodic replacement. rent is limited by the resistance in the circuit, which is composed of the d.c. resis- A RegulatedWhen the load on a recti- tance of the transformer and the resistor Power fier-type12 -volt supply is R1.Without knowing all the circuit com- Supply varied, a change in the out- ponent values it is difficult to statea definite put voltage is to be expected. value for R1. However, the specified value If the load variationissever, as when a of 1/2 ohm will give adequate protection class -B amplifier is connected to the supply, for most applications.Resistor R1 will the voltage may fall as much as 50%. also provide protection against accidental The schematic of fig. 6.1-E shows an in- short circuits at the output of the supply. expensive regulated supply which will provide excellent regulation over a wide range A regulated output is available at point of currents to 1 amp. Even greater currents A in fig. 6.1-E.In a great number of ap- may be supplied by cascading regulator plications the current available at this point stages, as outlined later. may be sufficient.This is especially true The transformerfurnishesapproxi- if the zener diode DI is capable of pass- mately 24 volts to the rectifier. The rectifier ing large currents.however, large zener must be capable of carrying the peak load diodes are somewhat expensive, particular- 160 Power Supplies The Transistor lyfor experimental or amateur applica- 2N301's tions.Less expensive diodes may be em- TRIAD IA. A F -40X ployed by using the amplification ability 73 zoo of the power transistor connected in the ITIMPI II alPra 1000OF common -collector or emitter -follower con- 27R 1. figuration. A regulated voltage applied to trik the base of the transistor will be amplified N91 by the transistor action.If the transistor has a current gain of 50 and a small zener diode rated at 50 ma. is used to stabilize Figure 6.1-F the base, the transistor will be capable of AN UNUSUAL REGULATED stabilizing the voltage at currents up to POWER SUPPLY 2.5 amperes.The transistor must be ca- WITH VARIABLE OUTPUT pable of carrying the current and must be The circuit was designed by C.T. Gage able to dissipate the power dropped across of Sigma Instruments. the transistor. A 1,000 ohm load resistor is used to maintain a small but constant load at the rent gain of the second transistor.Also output terminals. shown in fig. 6.1-E is a two -stage regula- If further control is required the out- tor made by adding a 2N1 381 transistor put of the transistor may be connected to to the circuit.This simple modification the base of a second emitter follower. The results in a constant output voltage from control will then be amplified by the cur - no load to a full load of 2 amperes. An unusual circuit for an adjustable regulated supply is illustrated in fig. 6.1-F. TT 1N1525 2N234A It may be constructed by using two inexpensive RCA 2N301 power transistors and a 1N91 diode in addition to the semiconductor rectifiers.The supply features 0-25 volts output at up to 500 ma. The output impedance varies between 0.5 and Ti -110 V. -20V. AT I A. 2N1381 2N234A (TRIAD F-92.41 -2 ohms as a function of outputvoltageand load.The output ripple is less than 100 millivolts peak -to -peak over the entire voltage and current range. The circuitis conventional except for (AMPLIFIED REGULATOR MODIFICATION) the use of negative current feedback in the positive lead.The output current passes Figure 6.1-E through the 1N91 regulator.Thus, in- A SIMPLE REGULATED creasing load currents lowers the bias of POWER SUPPLY Q2due to increased voltage drop across The series current -passing transistor is the diode.This action increases the for- stabilized by a zener diode. Thediode ward bias applied to Qi , lowering its re- ismade byInternationalRectifier sistance and maintaining the load voltage Corp. The transistor is a Bendix 2N - constant. The regulation linearityisa 234A.Also shown is a simple modi- function of diode characteristics and a ficationto increase regulation with number of units must be tried to obtain high current loads. best regulation. Radio Handbook D.C. to D.C. Conversion 161

6.2 D.C. to D.C. Power transistor, unlike the usual vacuum tube, will not conduct, and therefore not amplify Conversion until a small amount of negative bias is applied to the base.In a transistor power The power transistor is finding increas- converter thisis called the starting bias. ed use as a replacement for the vibrator in d.c. to d.c. power conversion applications. It can be seen that if two transistors are The high efficiency, lack of moving parts substituted for the vibrator, the only prob- and freedom from interference due to arcing lem is to turn them on and off by control- of the vibrator points make the transistor ling the base bias.The circuit shown in power converter a useful device. Fig. 6.2-B accomplishes this job and is called a power multivibrator.As in Fig. Transistor The discussion will be con - 6.2-A the battery is connected to the center - fined to the PNP power tran- Power tap of the primary winding. The ends of Cony erterssistor.Except for reversed voltage polarities, the oper- this winding are connected to the collectors of the two transistors. The circuit is com- ation of the PNP and the NPN transistors pleted by returning the emitters to the pos- are identical. itive battery lead. A few turns are added to In power -converter service operation the transformer to provide a source offeed- the transistor is nothing more than an elec- back voltage to sustain oscillation. In ad- tronic switch.The collector -to -emitter resistance is controlled by the influence of dition, a voltage divider consisting of two resistorsis connected across the battery. When the collector is negative the base. This provides the starting bias so that the with respect to the emitter, and the base is zero or positive, the junction resistance transistors will conduct initially. These re- will be high.The collector -emitter junc- sistors play an important part in the op- tion will remain a high resistance until the eration of the supply and are explained in base is ma de more negative than the emit- detail under the heading, "How to Set the ter.With sufficient negative bias applied Starting Bias". to the base, the collector -emitter junction When the battery is connected to the will saturate and exhibit a very low resis- circuit the transistor with the lower junc- tance. It should be stressed that a PNP tion resistance will conduct first.For the

A MINIATURE TRANSISTOR POWER CONVERTER This converter is suit- ableforreceivers and QRP transmitter applications. (Cour- tesy of Mini-Verters, Los Altos, Calif.) 162 Power Supplies The Transistor

Notice that during most of the heavy TO RECTIFIER FILTER SYSTEM collector current condition of Qi, the transistor was saturated and that over the same period Q2 was cut off. The cycle that has just been described BATTERY will continue to repeat until the power Figure 6.2-A source is turned off. Although many steps TYPICAL VIBRATOR are involved, the whole process happens ALTERNATOR CIRCUIT very quickly. The entire cycle usually takes Note that electromagnetic ac- place in about 1/400th to 1/ 2000th of a tion switches the contact back second. and forth connecting the ends In addition to inducing a voltage in the of the primary alternately to base winding, the lines of force generated the battery. by the collector current induce a voltage in the secondary that is a function of the primary -to -secondary turns ratio.It should purpose of explanation, assume that the be pointed out that when one transistor is upper transistor, Qi , is the first to con- conducting and the other is cut off the col- duct.Current flows from the negative bat- lector current of the conducting transistor tery terminal through the upper half of the will induce an equal voltage in the other winding, then through the transistor col- half of the primary. Thus, the end -to -end lector -emitter winding and back to the pos- voltage across the primary will betwo times itive battery terminal.The lines of force the supply voltage. This explains the term emanating from this half of the primary 2E -( E = supply voltage) that is found in induce a voltage in all the other windings, transformer formulas.For example, if a including the other half of the primary. supply is operating from 12 volts, and 144 The base impedance of the transistor is volts are desired from the secondary, it is much lower than the collector impedance. necessary to have six (not twelve) times the Thus, the base winding requires fewer number of primary turns onthesecondary. turns than the collector winding to provide The number of secondary turns may be the necessary step-down ratio.The base adjusted to give the desired output voltage winding leads must be connected so that from the rectifier -filter system.The wave- the polarity of the voltage to the base of Qi form applied to the rectifiers will be a near - will be negative -going when the collector voltage is positive -going.It will be seen that as the collector current increases, the base voltage also increases. This forces the collector to draw even more current. When TORECTIFIER the current reaches a predetermined ampli- FILTER SYSTEM tude the transformer core will begin to saturate. This means the corewill not mag- netize further and the current fed back to the base from the winding will begin to BATTERY fall. Thus the condition is reversed; the base voltage will decrease and as a conse- Figure 6.2-B quence the collector current will decrease. Transistor powerconverter At this point Qi is no longer in a heavy equivalent of the vibrator cir- current condition and Q2 begins to con- cuit. The transistors replace the duct. moving contact. Radio Handbook D.C. to D.C. Conversion 163

is when the transistor is switching from the on state to the off state, or vice versa.It should be obvious that the faster the switching action occurs, the more efficient is the supply. Since the efficiency figure is directly related to transistor heat,it can be as- sumed that the faster the switching time, the greater the power which can be handled by a given pair of transistors. It can be shown that toroid tape wound cores using Deltamax or similar alloys produce the fastest switching times, and are used in the most efficient supplies.Cer- tain powdered iron cores are only slightly less efficient than the tape wound style, but are far less expensive.Keep in mind that poor efficiency always means heating (usually in the transistors) which necessitates a reduction in the maximum power handling capabilities.The transistors always produce heat, and it is the junction temper- A TYPICAL MOBILE ature that determines the power handling POWER CONVERTER ability of the transistors. Made by Universal Transistor Products In effect, the transistor only works dur- Corp. ing the instant of switching and rests between these periods.It would be correct ly perfect square wave because of the rapid to assume that the transistor will handle switching action of the transistors. A sim- more power thanit would under condi- ple full -wave or bridge rectifier will convert tions of continuous operation such as class this signal to almost pure d.c. and very A. In actual practice, it has been found small filter capacitors may be used to re- that any transistor can safely switch from move the remaining "spikes". five to eight times its class A power rating. If the system for transferring heat is very EfficiencyThe efficiency of a transistor efficient,thisrule -of -thumb may be in- power converter may be ex- creased to ten times the class A power pressed as the ratio of power output to rating.Thus, a transistor that is rated at power input.Thus, for a given input, the five watts may switch as much as 50 watts. higher the power output the more efficient Naturally, two transistors would be re- the power supply. The losses always show quired for the power converter and they up in the form of heat, and are referred to would be able to handle 100 watts of pri- as IR losses. mary power. When one of the switch transistors is cut off, it will have only a small IR loss be- RectifiersVacuum tube rectifiers arevery cause the junction resistance is so high inefficient devices and are very that only a small current can flow. By the rarely used in transistor power converters. same token, when the transistor is saturated Silicon rectifiers, by virtue of the fact that there will be little IR loss because there is they have a fixed 0.6 volt internal drop and only the small resistance of the junction. no filament are ideal for this application. Thus the only time majorIRlosses occur Silicon rectifiers may be connected in series 164 Power Supplies The Transistor

SWITCH INC RECTIFIED FILTERED If the automobile power source has a

OTYPICAL A.G. POWER negative ground, byall means use the SUPPLY WAVE FORMS common -collector configuration and bolt the transistors directly to the chassis.If the automobile has a positive ground, then either configuration may be used.The 0 TYPICAL VIBRATOR 0_ M WAVE FORMS transistors will have to be insulated from the chassis in either case. l- A typical circuit for the popular Triad toroid transformers is shown in Fig. 6.2-E. 0 vT EY,PrI °GRA LWAPOWER -j_ m WAVEFORMS J Note that the emitter -to -base impedance step-up is accomplished by connecting the individual base windings in series with the Figure 6.2-C emitter winding.This provides an auto - TYPICAL WAVEFORMS transformer action.The starting bias re- Typical waveformsencounteredin sistors must be installed in series with each various power supplies. base winding. TheearlierTriadferrite -core trans- to obtain a higher breakdown voltage, and formers were designed for common -emitter balancing resistors are not needed.This applications exclusively. However, these application is explained more fully in the transformers, except the TY-69S, may be section devoted to rectifiers and filter sys- modified for use in common -collector cir- tems. cuits.Notice that the base winding consists of enameled leads with two paralleled Circuit The circuit shown inFig. Variations 6.2-Biscalled a common - emitter configuration. Avari- MINIATURE SILICON RECTIFIERS ation of this circuit is the common -collector These miniature silicon rectifiers are shown in Fig. 6.2-D.In this circuit the rated at 400 volts, 500 ma. and are primary is connected between the emitters, ideal for power converter applications. and the collectors are connected together. (Courtesy of International Rectifier For PNP transistors the common -collector Corporation) connection must be returned to the negative battery lead. The theory of operation is the same as the common -emitter circuit discussed earlier.However, this circuit has the advantage that the transistor case (which is the collector) may be bolted directly to the grounded chassis when the supply is used with a negative -ground automobile ignition system. This eliminates the need for insulation of the collectors, as in Fig. 6.2-B, and allows the most effieient transfer of heat to take place.In this configuration, the emitter impedance is lower than the base impedance, and the transformer must provide a step-up of approximately 1:1.25. Radio Handbook Setting the Bias 165 conductors. Although not shown on sche- is applied, the efficiency will be reduced. matics, the base winding is actually con- Excessive bias can also damage the transis- nected as shown in Fig. 6.2-F.Separate tors due to additional heat. More impor- the two outside leads ( B1 and B2 ). With tant, with excessive bias, the supply may an ohmmeter establish the two windings not be self -protecting. When the supply is and determine which one is center -tapped. operating properly, a short or overload in Tape up the center tap as this lead will not the output circuit will absorb the feedback be used.Connect an a.c. source (between voltage and the supply will stop oscillating. 6.3 and 30 volts) to one half of the secon- The battery current will drop to a low val- dary, and experimentally connect the three ue, thereby protecting the transistors from windings as shown in Fig. 6.2-E. Connect burn out.The bias should be set so that an a.c. voltmeter across the full primary none of the transistor ratings can be ex- and alternately reverse the two base leads ceeded. on each base winding until a maximum You may want to substitute transistors reading is obtained on the voltmeter. This or optimize designs.The bias may be set procedure will phase correctly allwindings in this manner.First, short the output so and the transformer may be connected as that the supply will not oscillate. Then in- shown in Fig. 6.2-E. sert typical values, such as 2,000 ohms and How to Set Recall that the transistors 20 ohms, in the R1 and R2 positions. To the will not conduct initially a great extent, the larger resistor controls Starting Biasunless a small amount of the static current (current drawn when the forward bias is applied to supply is not oscillating). The smaller of the bases.In the common -emitter circuit the two resistors controls the base current the bias is connected to the center tap of when the supply is oscillating. As a rough the base or feedback winding. The voltage rule -of -thumb, the resistors should be se- is obtained from a resistor voltage divider lected so that the transistors draw their connected across the battery. These resis- normal class A current when the supply is tors are shown as R1 and R2 in Fig. 6.2-D. not oscillating. To show atypical example, If the transistors do not have enough consider the Sylvania 2N307. It is rated at bias, the supply will not start when con- two watts and a pair will handle four watts. nected to a heavy load.If too much bias To comply with our rule, the bias resistors must be set so that the transistors consume four watts of d.c. from the power source. Thus, for a 12 -volt system, the transistors should draw approximately 333 ma. static TORECTIFIER current.Typical values for the two resis- FILTER SYSTEM tors, with these Sylvania transistors, might be 820 ohms and 10 ohms. The next step in setting the bias is to remove the short and replace it with a resistor that represents the normal full load. BATTERY Uponapplyingpower,theconverter Figure 6.2-D should oscillate instantly and produce full COMMON -COLLECTOR power in the load.If it does not, try re- CONFIGURATION ducing the value of both resistors by half. Common -collector configuration for Keep the value of these two resistors as grounded negative automotiveig- high as possible, consistent with the con- nition systems. ditions outlined earlier.If the larger value 166 Power Supplies The Transistor

10 LW a pilot lamp, or series of lamps, that measure about the same resistance.Substitute the lamp, or series of lamps, for the large value resistor as shown in Fig. 6.2-G. The 22011 TO RECTIFIER idea behind this scheme is that the cold re- FILTER SYSTEM 2200. sistance of the bulb(s) will be low enough to start the supply. But when it starts, the current through the bulb(s) will increase the filament resistance, thereby decreasing IOLF TRIADTOROIO TRANSFORMER the base current.

Figure 6.2-E RectifierAlthough most transformers COMMON -COLLECTOR CIRCUIT and are designed to work into a Common -collectorcircuit with auto - Filter bridge rectifier system, you need transformer stepup. Systems not confine yourself to this circuit. Most power -converter resistor is reduced too far, excessive static transformers will work equally well with current will flow.If the lower half of the half -wave,full -wave, or voltage doubler divider is too smallitis unable to limit rectifiers. base current during signal peaks.Either As a practical example, let us assume excess can damage the transistors. that you have purchased a Triad TY-79 If it is still not possible to obtain full transformer.In a bridge rectifier config- power, or easy starting, this indicates that uration, this transformer will develop 300 the transistors have very low Betas when volts at 200 ma.Following are some of saturated.If this is the case, it will be nec- the things that can be done with this trans- essary to parallel additional transistors. former and a discussion of the advantages If the supply does not start easily, but and disadvantages of the various rectifier can be started by removing the smaller re- systems shown in Fig. 6.2-H. sistor, a silicon power diode may be substituted for the smaller resistor. The cath- Half WaveWith a half -wave rectifier sys- ode of the diode will connect to the tap on tem (a), you will obtain the the base winding.In the common -collec- same voltage and current as with thebridge tor circuit, the diode can be added in series configuration.However, only half cycles with, or in place of, the lower value base are rectified and the output is more difficult resistor.See R2 in Fig. 6.2-D.In oper- to filter.You will save on rectifiers, for ation, the diode will not conduct until the only half as many are required (two in supply has started. The forward resistance series, with the Ty -79). should be about the same as the resistor it replaced.If R2 is normally many times the Full WaveThefull -wave rectifieris forward resistance of the diode, it should shown in (b).In compari- be connected in series with the diode. son to (a) this configuration will produce There is another trick that can be used one-half the d.c. output (150 volts), but to improve starting under load.If the sup- twice as much current (400 ma.).The ply refuses to start until the larger resistor amount of filtering required is the same as is reduced in value, try this remedy. Cal- for the bridge circuit.Only two rectifiers culate the value of the large resistor needed arerequired,oneineach leg.Small to start the supply (even though it may amounts of negative bias (10 to 20 volts) draw excessive static current). Then locate can be obtained by connecting a resistor in Radio Handbook Filter Systems 167

series with the grounded secondary center - Filter Normally only a minimum of tap connection.This resistor must be by- Systemsfilteringis required when com- passed with an electrolytic capacitor, 50 pared with 60 -cycle systems (10 /dd., 50 w.v.d.c., positive end grounded. /dd., or so) because of the high operating frequency and fast switching time of the Voltage The voltage doubler circuit (c) transistor power converters.If high peak Doubler isvery popular asitreduces currents are drawn from the supply, it is the total number of rectifiers advisable to install a large capacitor (80 needed for a given output voltage.If you /Ad.) across the output as an energy stor- use the voltage doubler with the TY-79, it age device. will be possible to obtain twice the voltage As a safety factor, the voltage rating of (600 v.) at one-half the current (100 ma.). the filter capacitors should be at least 25% The filtering will not be as good as the higher than the no-load output voltage. bridge system, and it will be necessary to Higher -than -normal battery voltage could use capacitors that are four times as large damage a capacitor with a marginal voltage asin the bridge and full -wave circuits. rating. For output voltages in excess of For short duty cycle applications, such as 400 volts (no-load), two 450 -volt electro- power for a s.s.b. or agated final amplifier, lytic capacitors should be connected in you can use very large filter capacitors (80 series. The voltage across these capacitors /Ad.) and obtain peak currents equal to the can be equalized by shunting them with bridge system (200 ma.). A power con- 100K ohm, 2 -watt resistors. When using verter suitable for use with a s.s.b. linear the bridge configuration, another method amplifier is shown later in the chapter. of equalization is to connect the secondary center -tap to the junction of the two filter The voltage doubler may also be put to capacitors.One-half voltage exists at this good use when 12 -volt power converter point and it will automatically equalize the transformers are operated from a 6 -volt capacitors. power source.Approximately 90% of the In some applications it may be necessary normal out -put voltage will be obtained be- to include r.f. chokes and disc ceramic ca- cause the transistor losses will be higher pacitorsin theB+circuit.Harmonics, when used on 6 volts. contained in the square wave, can reach the

AN INEXPENSIVE POWER CONVERTER TRANSFORMER USING FERRITE CORES (CourtesyofTriad Transformer Corp.) 168 Power Supplies The Transistor

Figure 6.2-F ducts and the other is cut off, a voltage ACTUAL CONNECTIONS equal to the supply will be induced in the FOR EARLY non -conducting half of the transformer TRIAD TRANSFORMERS primary winding. The conducting transis- 2 The modifications for com- tor is a virtual short. Therefore, two times mon -collector autotransfor- the supply voltage will appear across the COLLECTOR WINDING mer connection are de- cut-off transistor and the full primary. The scribed in the text. collector -emitter breakdown rating ( ) must be high enough to withstand this peak potential.Occasionally the break- r.f. circuits and cause beat notes or "bird- down rating listed in the transistor litera- ies" on received stations. A 2.5 mh. choke ture is only an average. As a safety factor, in series with theB+lead and bypassed to it is an excellent idea to allow a larger mar- ground at each end with 0.001 /it'd. disc gin. For example, assume a 12 -volt mobile ceramic capacitors will eliminate this pos- supply is to be constructed.As just ex- sible source of trouble. plained, a minimum potential of 24 volts will develop across the transistors.Con- Selecting There are many combina- sidering that the battery terminal potential Componentstions of components that might reach 14 volts, a transistor with a may be used in conjunction 30 volt collector -emitter rating would be with commercial power -converter trans- marginal.It is interesting to note that you formers. The transistors and rectifiers can rectify the full primary voltage to make usedin this chapter are not necessarily a 6 -to -12, or 12 -to -24 volt converter.In recommended. They only demonstrate such a unit the secondary would be unused. that these, or similar parts, may be used. Inan improperly designed supply, it is You may want to use your favorite type of possible to have "spikes" or switching transistor, or those in your parts stock. transients on the leading edge ofthe square Here are some of the things you should wave. These transients can create peak po- know when substituting transistors or rec- tentials that are well in excess of twice the tifiers. supply voltage.If in doubt, when checking a new supply, operate it at reduced in- Transistors There are three important put voltage and observe thetransistorwave- thingstoconsider when selecting the switch transistors fora supply. 4- 441 PILOT LAMPS These are power handling ability, break- SERIES -CONNECTED down voltage ( Vce ), and saturation resis- 1 N91 tance. The primary power flows through the transistor junction and it must be capable of handling the current and dissipating the heat generated in the process. As explained earlier, transistors can be used that were Figure 6.2-G originally intended for class A or class B Two tricks to prov ide for easier starting audio applications. The transistors usually under heavy load.Four pilot lamps have the ability to switch eight times their and a rectifier replace the resistors class A audio rating, or four times the dis- shown in Fig. 6.2-D as explained inthe sipation rating. text.These components would be sat- You must considerthe breakdown isfactory for a high power supply using rating as well.When one transistor con- Delco 2N278's. Radio Handbook Rectifiers 169

ard current of one ampere. In either case, the lower the figure the better the transistor. When paralleling transistors, the emitters and the collectors may be connected together.However, the base of each transistor should have a small (4.7 ohm) series Figure 6.2-H resistor to equalize the feedback voltage. Typicalrectifiersystems used with transistor power converters. RectifiersLike the transistor, the silicon rectifiershas certain specific form with an oscilloscope. Small "knobs" ratings such as breakdown voltageand cur- at the beginning of the square wave flattop rent handling ability which must be con- may be seen, but they should be less than sidered when constructing a power supply. 2.5% of the total amplitude. However, Most silicon rectifiers will handle 500 ma. spikes must be eliminated.A 1600 -volt with ease, and this rating usually is not of buffer capacitor across the secondary will importance in amateur equipment.Recti- remove the transients, but will reduce the fiers which handle less current may bepur- switching time. chased at a slightly lower cost. However, be certain that they are adequate for thejob. The saturation voltage mentioned earlier Whenever there are two separate current may be somewhat confusing.If a transis- paths, such as the full -wave and bridge cir- tor were a perfect switch it would havezero cuits, the rectifier system will be able to ohms resistance when conducting.Un- handle twice the current rating of one recti- fortunately, this is not the case. Even when fier. saturated, the junction will always have a The peak inverse voltage rating must be small resistance. The larger the resistance, closely followed, for the rectifier can easily the greater the voltage developed across be ruined by exceeding this voltage.A the junction. Since the junction is in series common value rectifier with a PIV of 400 with the primary, the voltage dropped volts can withstand between 150 and 200 across the junction cannot appear across volts of square -wave energy. For example, the primary and must be considered lost. in a 300v., 200 ma. transformer, end -to - For example, assume that a transistor has end secondary voltage is 300 volts, peak - 0.1 ohm saturation resistance and the 12 - to -peak.If this transformer were used in volt converter in which it is used draws 10 the half -wave circuit, it would be necessary amperes.Thus, by Ohm's Law, we find to connect two rectifiers in series. In a full that one volt will be lost across the transis- wave circuit it would be necessary to use a tor.Only 11 volts will appear across the rectifier on each side of the center -tap.In primary, and the secondary voltage will be the bridge configuration, the diodes are low. If a commercial power -converter connected across the secondary and two transformer is used and the supply has low rectifiersmust be connected in series. output voltage, this is probably the reason. Since two current paths exist, itis neces- Low output can be minimized by selecting sary to use a total of four rectifiers. The transistors that have a low saturation resis- same conditions are present in the voltage - doubler circuit, and a total of four recti- tance or by paralleling transistors. A good fiers are required. transistor will have a saturation resistance less than 0.1 ohms. Some manufacturers The rectifier junctions are self -equaliz- rate the saturated junction by resistance, ing and any number of silicon rectifiers while others list the voltage drop at a stand- may be connected in series without external 170 Power Supplies The Transistor

equalization.The current rating remains tor with a BKrating of more than 30 the same no matter how many rectifiers are volts should be adequate. connected in series.The PIV rating will Consider, however, what occurs in a increase with each rectifier that is added. mobile installation when the starter is ener- gized with atransistor power converter Transient Transient protectionis one connected across the battery.The starter Protection other power converter con- winding represents a large inductive load sideration which should be and the back e.m.f. can generate transients discussed. An improperly designed power across the battery. Any induction load on converter can provide satisfactory opera- the supply can be a source of transients. tion for a long period oftime and then sud- Although the average energy contained in denly, for no apparent reason, blow a tran- the transient may be low, the peak voltage sistor.Examination will show there is no can reach many times the supply voltage. front -to -back ratio between the collector The transient, if allowed to reach the tran- and emitter.This indicates the junction sistors, can add to the 2E collector po- has punched through or broken down. tential and destroy the transistor. Even What causes this to happen?As ex- when high -voltage transistors( BV of plained earlier, the autotransformer action 60 volts or more) are used, external tran- results in a collector -to -emitter potential of sients which occur during the switching approximately two times the supply. In 12 - period can add to the leading edge over- volt mobile applications this amounts to shoot and break down the junction. approximately 24 volts.Even with the In the circuit of Fig. 6.2-I, note that slight overshoot normally encountered in two double -anode breakdown diodes are switching, the peak -collector voltage usually connected between the collector and emitter will not exceed 30 volts.Thus, a transis- of each transistor. The diodes are designed to conduct whenever the diodeterminal potential exceeds 60 volts. As can be seen in Fig. 6.2-I, any transient which could de- (4EACH) stroy the transistor is effectively clipped by 1N539 2011,1W 4- the diode.Transient suppressors should 250 DC 75 MA. be used in any high reliability power converter application to minimize the possibility of destruction due to transients. The units are made by General Electric under the trade name Thyrector and by Internation- alRectifier Corporation as Klip-Sets, and ao-V. are available with a clipping level as low as

60 it Ajik 7/. _ VCB MAX. 25 volts.As a minimum precaution, any 'rkZENER VOLTAGE 40 40- OF DIODES power converter should have a large elec- 28 V CI C 20 20- trolytic across the supply terminals to ab-

0 sorb as much transient energy as possible. A capacitor connected across the secondary Figure 6.2-I will absorb transient energy, particularly Transients in power converters can be that produced by transformer leakage re- destructive. A double anode clipper, actance, but it also reduces the switching suchastheInternationalRectifier speed.Generally it is best to avoid thi s "Klip-Sel" will short circuit transients transient suppression technique, since the before they reach destructive slower switching time increases transistor potentials. dissipation. Radio Handbook D.C. to A.C. Conversion 171

6.3D.C. to A.C. Conversion TRANSISTOR RI =RI,R2=R2,C1zC.1, C3 DELCO 2N278 2000. 211. 2 .UF 111F 05506 10W. 5 W 50 W.V. 200 W.V.

Direct current power may be converted 2000. 511. 2 lif 1.1./F BENDIX 2N678 10W. 2 W. 50 W.V. to alternating current by the technique de- 111F CLEVITE 2N7746 10011 2 0. 2 /JF scribed in section 6.2. The system is simi- 20 W. 5 W. 50 W.V.

111F 1.1.1F lar to the transistor power converter. How- DELCO 2N773 200.0. SD. ever, no rectifier -filter system is employed 20 W. 5 W. 50 W.V. and, of course, the frequency is reduced to 60 cycles.In military aircraft installations Figure 6.3-B the operation frequency is 400 cycles. In either case the conversion device is usually referred to as an inverter. A buffer capacitor is connected across The circuit for a 120 -watt inverter is a portion of the secondary to absorb tran- shown in Fig. 6.3-A.It operates from a sients generated during the switching cycle. source of 12 volts d.c. and delivers 115 This capacitor is mandatory when using volts and 60 cycles at 1 ampere maximum. the Triad TY-75-A transformer. The sec- Note that the common -collector configu- ondary also includes two additional output ration is used to eliminate the need for in- connections to compensate for low battery sulating the collectors from the chassis. voltage, etc. This is compatible with modern automo- The transistors should be mounted on biles which use negative ground, 12 -volt heat sinks with a minimum surface area of ignition systems. at least 50 sq. in.Caution must be exercised to prevent exceeding the 1 ampere Resistor R1 applies starting bias to the maximum secondary current rating. Fur- switching transistors, while R2 completes ther, the transistor heat sinks must be vent- the divider network. Capacitor C1, across ilated during prolonged operation. R2, minimizes attenuation of the base drive A chart of the recommended transistor signal.A 14 ampere fuse protects the pri- types and the associated component values, mary circuit. is shown in the chart, Fig. 6.3-B.

Ra ERN 6.4 A Low Cost Power 87.K-GRNM BROWN Converter

BLA1 This popular power converter was orig- RI' inally developed by John Specialny of Phil - co Corporation(ApplicationReport 11511. - 60'b 1 A, Y EL .C) #317).It uses no special switching or Ry step-up transformers. Rather, two 6.3 volt center -tappedfilamenttransformers are Figure 6.3-A used, one for voltage step-up and the other A power inverter which develops 117 for impedance matching of the base drive volts a.c., 60 cycles from a 12 volt d.c. signal.This technique is particularly use- supply. Caution must be exercised ful in power converters delivering 10 to not to exceed a one -ampere load cur- 40 watts output. rent.For additional protection, con- Transformer T1 ,in conjunction with nect a 14 -ampere fuse in the supply R1 and R2 , provides the feedback current line. necessary to sustain oscillation. The feed- 172 Power Supplies

2N386's A The transistors should be mountedon Ti heat sinks with a minimum surfacearea of r 5 sq. in.Since the transistors are connected in the common -emitter configuration, mica oranodized aluminum washers .02 600 V. should be used to insulate the transistors R2,1.2 I< from the heat exchanges.

RECTIFIERS: +12V.- + 250-300- SARNES-rAPZIAN M-500 V. D.C. If desired, the power convertercan be SILICON DIODES used at lower input voltages. The output voltage and available power will be almost proportional to the supply voltage. Figure 6.4-A A low power converter for receiver or The circuit is not at all critical withre- QRP transmitter applications. Theout- gardtothe transistor type used.Any put ratings are nearly ideal forpower- power transistor with two or more watts ing the Command Set receiver series. dissipation could switch the 4amperes when the supplyis operated at full load current. back polarity, from points A and B, must be correct or the supply will neither start Thereareafew precautions which nor operate properly.As shown, the should be observed when working with power supply oscillates at approximately power converters of this type.The leak- 200c.p.s.Resistors R3 and R4 provide age reactance of the transformers intro- starting bias for the transistors.Trans- duces severe spikes on the leading edge of former T2 steps up the primary voltage, the switching waveform. These transients which isessentially a square wave, by an have been measured with a variety oftrans- amount determined by the turns ratio. A formers and found to be almost exactly simple bridgerectifier system provides four times the supply potential, peak -to- nearly pure d.c. peak.Unless transistors with a peak col- The required load current will deter- lector rating of 50 volts or more are used, the transient will punch through the thin mine how much feedback voltage isre- quired.The feedback can be increased by junction and destroy the transistors.In reducing the size of R1 and R2 while keep- such cases the builder has no alternativebut ing both values equal. to use high voltage transistors or to install some form of transient protection. Itis As shown, the supply has the following usually easiest to place a 0.02 /dd. buffer characteristics.At 60 ma. the output volt- capacitor across the primary ( 117 volt side) age will be 310 volts (18.6 watts), while of the feedback transformer.As pointed the primary current will be 2.5 amperes out earlier, this does slow the switching for an efficiency figure of 60%. At 110 ma., time and increase transistor heating. How- the output voltage will be 265 volts (29.2 ever, at the 30 to 40 watt level more than watts), and the supply will consume 4 am- adequate dissipation is available with the peres for an overall efficiency of 67%. transistors listed. As a further precaution, the value of R1 and R2 should not bere- The output transformer will run warm duced without checking the base current. due toIRlosses. The transistors run rela- Reducing the resistor value increases the tively cool indicating an ability to switch base drive and excessive currentcan dam- greater current levels. age the transistors.

INDEX

A B A.C. to D.C. Conversion. 156 Barrier 16 A.g.c. 62 Barrier potential. 16, 17 All -band converter 106 Base current curves 35, 125 Construction of 107 Base injection 61 Adjustment 109 Beta 21, 34 Transistors 110 Bias 29 Output coil 110 Emitter 31 Alpha 21, 34 Starting 165 Alpha cut-off 34, 55 Voltage dividers 31 Amplification. 21 Bifilar wound balanced modulator coil.. 132 Current 21 Bond. 13 Voltage 22 Broadcast receiver . 86 Amplifier 28 Audio 28 C Class A, 5 -watt 52 Capacitive coupling 32

Class B. 37, 68 Cathode follower 29 Class C 68 Circuits 55 Common base 28 R.F. 55 Common collector. 29 Sliding bias 54 Complementary 39 VHF 150 Considerations 36 Circuit, vibrator alternator 162 D.C. linear 39 Clapp Oscillator 64 Linear 125 Class A amplifier, 5 -watt 52 Modulator, Mini 46 Class B amplifier 37, 68 Modulator, 10 -watt 48 Class C amplifier 68 Neutralized, r f 58 Coil data for converters 115 R.F. Power 120 Colpitts oscillator . 64 Semi -conductor speech 40, 42 Common -collector amplifier 29 Sliding bias 53 Common -emitter amplifiers 29 S -meter 48 Communications receiver. 94, 103 Stability, r.f. and if 58 A.g.c. amplifier 98 Transistor 34 Alignment 102 Vacuum tube 29 Audio section 99 Atom 9, 11, 12, 15, 16 Battery 101 Audio amplifiers 28 B.F.O. and amplifier 99 Audio amplifier stage 36 Coils 101 Audio circuits 39 Construction hints 100 Audio compressor . 43, 44, 45 Crystal filters 103, 105 Autodyne converters 111 Front end protection 101 F.M.. 112 Heath GC -1A . 67 Third overtone 113 IF amplifiers 98 V.H.F. 112 IF filter 103 Automatic gain control 62 Mechanical filters 105 Avalanche diode 19, 157 Mixer and oscillator. 97

174 Nuetralization 102 Variable capacitance 19 Product and A.M. detector 98 Zener 18,19, 21 Technicalities 100 Diode balanced modulator 69 Tuning capacitor 101 Diode detector 60 Use of receiver 102 Dissipation 18 Complementary amplifiers 39 Dividers, bias voltage 31 Compounds 9 Doping 13 Compressor, deluxe audio 43 Conversion E A.C. to D.0 156 Efficiency 37 D.C. to A.0 171 Elements 9 D.C. to D.0 161 Emitter bias 31 Converters. 60 Emitter -follower 29, 32 Autodyne 111 Emitter -follower amplifier 49 Coil data 115 Emitter injection 61 Low power 172 220 Mc 117, 119 F Power. 171 Feedback, D.0 30 Six -meter 113 Feedback, negative 32 Stage. 61 Filter systems 166, 167 Transistor all -bond. 106 Filter exciter, Transistor 136, 137 Two -meter. 115 Alignment 138 Coupling, capacitive 32 Construction hints 138 Coupling, transformer 56 Operation of 136 Crystal controlled oscillator 66 Flip-flop 66 Crystal set, simplex 73 F.M. autodyne converter 112 Crystal filters 103 Forming 23 Crystal lattice 12 Forward bias. 16, 20 Current amplification 21 Forward bias resistor 30 Current bias 30 Full -wave rectifier system 166 Current carriers 28 Current gain 34 G Cut-off, alpha 34, 55 Gain control, Auotmatic 62 Gain, current 34 D Germanium diode . 13, 14 D.C. amplifier 39 D.C. to A.C. conversion 171 H D.C. to D.C. conversion 161 Half -wave rectifier system 166 D.C. feedback 30 Hartley oscillator 64 Decoupling capacitance 59 Heath Kit Amplifier Model AA -80 ..50,51, 52 Detection 59 Heath GC -1A 67 Detector High level modulator. 67 Diode. 60 Infinite impedance 60 i Product 60, 192 I.F. Stage 62 Superregenerative 71, 72 I.F. Transformers, Construction of 85 Diode Impedance matching 32, 56 Avalanche 19, 157 Impedance matching transformer ..... 85 Germanium 13, 14 Impedance, input..37 Sil icon 13 Impedance, output . 37 Tunnel 60 Impedance transformation 57

175 Impedances, Transistor 32 Impurities . 13, 14, 23, 24 O Infinite impedance detector . 60, 62 One -transistor receiver 76 Injection 61 Operating point 35 Base. 61 Oscillator Emitter. 61 Clapp 64 Input impedance 37 Colpitts 64, 67 Inverse feedback 38 Crystal -controlled 66 Hartley 64 J Overtone 68 Junction 15, 17, 18, 20 R.C. Phaseshift 65 Junction capacitance 19 R.F. 123 Self-excited 63 I Tickler coil 63, 67 Leakage current 17, 26 Ultra audion 64 Linear amplifiers 125 Output impedance 37 Emitter follower 126 Output, Power 37 Bias considerations 126 Overtone oscillator 68 Link coupling. 56 Load lines for transistors 34 P Low level modulator 69 Phasing Exciter, Transistor 128 Adjustments ...... 134 M Amplifier. 135 Matching, impedance 32, 56 Audio amplifiers 133 Matter 9 Balanced modulators 134 Mechanical filter. 105 Batteries 135 Mobileer 89 Coils 134 Converter 90, 91 Construction 133 Construction of 91 Gain control 135 Microphone preamplifier 41 Operation of 129 Mini -amplifier -modulator 46 P -N -P type transistor 29 Mini -amplifier #2 47 Potpourri of Circuits 149 Mixer 60 Power amplifier 151 Modulation 127 Power converter efficiency 163 Low level system 127 Power converter, low cost 171 High level system 127 Power output 37 The modulator 128 Power pack, Simplex Audio 74, 75 Modulating R.F. Circuits 69 Power Supplies. 156 Modulator 47 Power supply, Heavy duty 159 Diode balanced 69 Preamplifier, microphone 41 High-level 69 Product detector 60, 92 Low-level 69 Professional communications Transistor balanced 70 receiver 94, 103 10 -watt kit 50 Molecule 9 R Mu ltiv ibrator 65 Receivers 71 Receiver, regenerative ...... 79, 80, 81 N Recombination 16 Negative feedback 32 Rectifier 17 Neutralization 57 Selenium 18 N -P -N type transistor 29 Rectifiers for power converters 169

176 Rectifier systems . 166 Third overtone autodyne converter. 113 Rectifier system, half wave 166 Operation 113 Rectifier system, full wave . 166 Performance. 114 Rectifiers, vacuum tube 163 Third overtone oscillator. 151 R.C. Phaseshift oscillator 65 Three transistor superheterodyne 81 Reflex circuits 40 Tickler coil oscillator. 63 Regenerative receiver 79, 80, 81 Tone control 38 Regulated power supply 159, 160 Transceiver, 40 -meter s.s.b 138, 140 R.F. Circuits 55 Alignment 143 R.F. circuits, Modulating 69 The Audio 139 R.F. oscillators 123 TRB-1. 141 Circuit configuration 124 TRG1 . 141 Crystal controlled. 123 TRO-1H 141

R.F. Power Amplifiers 120 TRT-2 Linear amplifier board ... 142 Bias considerations 121 VFO 141 Input circuit 122 Transformer coupling 56 Neutralization and Unilateralization .122 Transformers, I.F., construction of . 85 Output circuit 122 Transformer output impedance...37 Resistor, Forward bias 30 Transient protection 170 Reverse bias 17, 20 Transmitters 120 Transmitter, C.B. or 10 -meter 146 S The circuit 146 Saturation 31, 38 The output network 146 Selenium rectifier 18 Transmitter, Two -meter 147 Self-excited oscillator 63 Construction hints 148 speech amplifier. 40, 42 Testing 149 Silicon diode. 13, 19 Transmitter, Tunnel diode . 153 Simplex Audio Power Pack 74, 75 Transmitter, 10 -meter phone 154 Simplex Crystal Set 73 Transmitting R.F. amplifiers . 68 Sliding bias amplifiers 53 Transistor Sliding bias circuit 54 Action 19 S -meter amplifier 40 Alloy 24 Solar cell 76 Balanced modulator 70 Square -wave generator 65 Base 19, 22 Squegg 68 Collector 19, 22 Stability, R.F. and I.F. amplifiers 58 Drift . 25 Stabilization.. 30 Emitter 19, 22 Temperature 79 Epitaxial . 25 Thermistor 32 Gain 20 Stage, reflex 40 Grown.. 24 Standardized symbols, PNP and NPN High frequency 26 transistors 29 Impedances . 32 Starting bias, Setting 165 Life 27 Superheterodyne, 4 -transistor 86, 88 Load lines for 34 Superregenerative detector 71,72 MADT 25 Super Three. 81,83 Mesa 25 Noise 27 NPN 20, 24 T Point contact 23 Temperature stabilization 79 Power 26 Thermistor stabilization 32 Power converter 161, 162

177 PNP 19, 24 Surface barrier 25 Transistor data, Interpretation of..

Transistors for power supply ...... 168 Transistor vs. Vacuum tube 28 Transit time 55 TR One 76 1 TR Two 77 Tunnel diode 66, 152 Tunnel diode transmitter 153 Two -meter converter 115 Two -transistor receiver 77

U Ultra audion oscillator ...... 64 Unilateralization 58 V Vacuum tube amplifier 29 Vacuum tube rectifiers 163 VFO for A.M., C.W., or SSB 143 Alignment 145 Construction hints 144 Diagram of VFO 143 Operation of 144 The capacitors 144 Voltage stabilization 145 V.H.F. autodyne converter 112 Vibrator alternator circuit 162 Voltage amplification 22 Volume control. 38 Voltage doubler 167 Voltage regulation 157

W Waveforms 164

Z Zener diode 18, 19, 21, 157

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