A PRACTICAL APPROACH TO TRANSIST VACUUM TUBE AMPLIFIERS BY F. J. BECKETT TEKTRONIX, INC. ELECTRONIC INSTRUMENTATION GROUP DISPLAY DEVICES DEVELOPMENT Two articles published in past issues of Service Scope contained information that, in our experience, is of particular benefit in analyzing circuits. The first article was "Sim- plifying Transistor Linear-Amplifier Analysis" (issue #29, December, 1964). It describes a method for doing an adequate circuit analysis for trouble-shooting or evaluation pur- poses on transistor circuits. It employs "Transresistance" concept rather than the compli- cated characteristic-family parameters. The second article was "Understanding and Using Th6venin's Theorem" (issue #40, October, 1966). It offers a step-by-step explanation on how to apply the principles of ThBvenin's Theorem to analyze and understand how a circuit operates. Now we present three articles that will offer a practical approach to transistor and vacuum-tube amplifiers based on a simple DC analysis. These articles will, by virtue of additional information and tying together of some loose ends, combine and bring into better focus the concepts of "Transresistance." Part I THE TRANSISTOR AMPLIFIER INTRODUCTION Tubes and transistors are often used to- there is an analogy between the two. It 50, it seems pointless not to pursue this ap- gether to achieve a particular result. Vacu- is true of course, that the two are entire- proach to its logical conclusion. um tubes still serve an important role in ly different in concept; but, so often we In this first article we will look at a electronics and will do so for many years come across a situation where one can be transistor amplifier as a simple DC model ; to come despite a determined move towards explained in terms of the other that it is and then, in the second article, look at a solid state circuits. very desirable to recognize this fact vacuum-tube amplifier in a similar light. We will assume that both devices are op- Whether a circuit is designed around Transistor and vacuum tube data give us erated as linear amplifiers and then use vacuum tubes or transistors or both, it is very little help in the practical sense. Pa- the results in a practical way. important to recognize the fact that the rameter Curves and electrical data show One must bear in mind that this ap- two are in many ways complementary. It the behavior of these devices under very proach cannot be assumed in all cases. is wrong to divorce vacuum tubes and defined conditions. In short, they are more It is, as it is meant to be, a simple analy- transistors as separate identities each pe- useful to the designer than the technician. sis but the results will prove to be a valu- culiar to their own mode of operation. \Ye are often reduced to explaining most able tool in trouble-shooting and under- Indeed, as this series of articles will sho~ circuits in terms of an ohms law approach; standing circuits. Let us consider the general equation fo~ One is quite justified in looking at a current through a P.N. diode junction. ORJECTIVE transistor in terms of the two-diode con- cept, refer to Figure 3. Therefore, assum- v The objective of this paper is to present (1) ing diode A to be forward biased and di- a practical approach to Transistor and ode B to be reverse biased, as would be the Vacuum-tube amplifiers based on a simple where V = applied volts case if we were to operate the transistor DC analysis. I, = reverse bias current as a linear amplifier, the current through = constant between 1 & 2 The articles will be published in the fol- diode A will conform to equation (1). kT lowing sequence. Let us take a closer look at Figure 2. and V, = - where k = Boltzmans cl 1. The Transistor Amplifier. We define conductance in the gener*, Const., 1.38 s lO~'~oule/"Kelvin 2. The Vacuum-tube Amplifier. case as I T = absolute temperature in degree Kel- 3. An analysis of a typical Tektronix hy- g=- vin at room temperature, i.e., T = 300°K brid circuit (Type 545B vertical) based v on conclusions reached in (1) and (2). q = electronic charge 1.602 x 10-'You- and therefore at our operating point "A" the dynamic conductance lo111b. As a corollary they will bring forward 300 some important relationships between vacu- V, r -= 0.026 volts um tubes and transistors. 11600 hence v I, esp --- pV, g' = pVc Figure 1 Figure 1 shows a typical forward volt/ amp characteristic for germanium and sili- con diodes. Figure 2 is a plot of the col- lector current or the base cyrrent versus Figure 2. Line (1 ) is a plot of the base cur- the base-to-emitter voltage of a transistor; rent versus the base-to-emitter voltage (VBE). Line (2) is o plot of the collector current point A on this curve is a typical operating versus the base-to-emitter voltage (VBE). Point Figure 3. Illustration of the two-diode con- point. "A" is a typical operating point. cept of o transistor. @ 1967, Tektronix, Inc. All Rights Reserved. I tlie common emitter antl the common I~aw sisted of resistances alone, it would be pas- but I 10 tlien g' = - >> ,,,V,> coniigurations. sive; i.e., it could supply no energy of its own. But a transistor can amplify energy Firstly, let us define tlie tern] p (tlie sm:tll-signal current gain) as to the signal. To represent tliis we liave sl~o\vn a current generator shunting R,. AI<. The term takes into account the re- p = -- The value of R, will depend 011 tlie circuit coml)ination of carriers in the junction re- AT,, configuration; i.e., tens of kiloh~iis for a gion It is approximately unity for ger- :tnd since In = I, -t 11, comnion emitter configuration, to many tnaniuni antl ;~pprosiniately 2 for silicon megolinis for a common base configuration. At a typical operating point tliis term can 111 our approach it is not necessary to pur- mually be neglected. Therefore, we may sue this matter any further since we will say that usually p >> 1 tlien 11: =: 1, not he considering a transistor in any ex- treme condition. Now resistance is tlie reciprocal of con- Kow in a more pr:tctical sense, let us tlie enlitter current flows into tlie hase. ductance antl therefore the value of con- look at Figure 5, a typical common-emitter Hence, it is reasonable to suppose that any (luctance at point "A" can be given In term\ con£iguration. impedance in tlie emitter, wlien viewed from of resistance tlie lnse, will be p times as great; and, :my impetlance in tlie base, wlien viewed iron1 tlie emitter, will be p times as small. This resistance (r,) is commonly laio\vn Tliat is to say, the dynamic resistance mul- as the dynamic emitter resistance. tiplied 1)y ,Cj niust equal R, in our equiva- lent "Tee" circuit. At this point we will tlepart from our simple model and look at the transistor in Hence R,. = pre another form; but, Ixar in mind our first Our equivalclit circuit slio\vs :t resistance thoughts. Transistor parameters are derived It,,. This resistance is known as the base- from various equivalent circuits depending sprcatling resistance. It is :L physical qun~i- upon tlie configuration i.e., common emitter, tity and can Ix expressed in terms of resis- comnion base, or common collector. We will tivity associated with tlie Ixw-emitter junc- not consider any detailed analysis in this lion. It can vary hetween :t few olinis to approach ; hut, to untlerstantl tlie approach Iiuntlretls of ohms, depending upon the type :f is necessary to know how tliese param- of transistor ; :tnd therefore, nus st be taken 2rs are derived. It will 11e simple enough Figure 5. A typical common-emitter circuit. into consit1er:ttion. T.ooking into tlie emitter to derive another set of parameters once we see it ns an impeclance wliose value is we liave our basic model constructed. tlivitled Ily P :tnd appears in series wit11 The simplest and easiest equivalent cir- the t1yn:lmic emitter resistance (r,.) . Hence cuit of a transistor is the "Tee" equivalent. the emitter current encounters an impetlancc Tt is a very good approximation about the in tlie I):lse/emittcr junction wliicli is equal l~eliavior of a transistor, especially at DC lo tlie sum of tlie dynaniic resistance plus and low frequencies. \\re can also represent RI, either tlie comnion emitter or the common - , the latter term we will designate R, lnse simply hy interchanging RI, and Re. P ,Ihe. input impetlance we see looking into Figure 1 is a "Tee" equivalent circuit of and the sum of tliese two resistances we the base of a transistor in tlie conimon emit- will designate Rt. ter co~ifiguration is The value of R, can vary anywhere be- EMITTER Re Rc COLLECTOR tween 2 R to 24 R depending on tlie value AV,,,, also 411, = --- of RI,. RI, is difficult to measure and rare- RI,, ly given in electrical data 0x1 transistors. A figure of 250 R's is a typical value at low BASE frequencies. Therefore, if P were 50 tlien R, \vould he 5 R's. COMMON BASE we also recall that Now if we look into tlie base in tlie com- mon emitter or the comnion collector con- p = aIc i~guration it is reasonable to suppose we art, will see tlie resistance (Rt)-plus any other impedance which may be wired to the emit- ter terminal-multiplied hy P, then Tlierefore substituting equation (13) in RI,, = P(Rt -1 RI:) (10) equation (14) EMITTER where R,: = the external emitter resistance.