ier INTRODUCTION TO THE FERRISTOR Reliability is an ever-growing problem for the tiny iron core, all encased in epoxy resin, the electronic design engineer today. In evidence FERRISTOR offers the design engineer reliability are hundreds of published articles specifying that is --all but absolute. FERRISTORS do notde- long-life requirements for military and com- teriorate with use or age and cannot be damaged merical utility and communication s y s t e m s . by shock, vibration or moisture. Virtually the Berkeley's contribution toward meeting such re- only thing that candamage a FERRISTOR-- aside quirements-- the culmination of several ye a rs from a sharp blow with a hammer-- is excessive research and development-- is the FERRISTOR, current in the windings, and this solitary pos- a tough new component designed to replace the sibility is quite remote because FERRISTORS fragile, short-lived vacuum tube in m a n y ap- are operated at only a small fraction of rated plicat ions. "FERRISTOR"is the Berklely trade current. Lastly, as a bonus in reliability, the name for a miniature saturable reactor which life span of associated components is also pro- will operate at high carrier frequencies. Con- longed because FERRISTORS do not generate sisting simply of two windings of fine wire on a abundant heat as do vacuum tubes. CIRCUITS YOU CAN BUILD WITH FERRISTORS FERRISTORS may be used in two distinct ways. pactive reactance of the c i r c u it in which it is Used in the first way, current in one of the wind- placed, a resonant circuit is formed which pas- ings controls the reactance of the other winding. ses so much carrier current that the FERRISTOR If a carrier current is flowingin the other wind- saturates and "latches" in this condition. This ing, it will be modulated linearly in response to phenomenon is c a 11 e d "ferro-resonance". A the small controlling current. When the mod- ferro-r e s o na n t circuit has two stable states ulated carrier is demodulated, the result is an ("latched" and "unlatched") each of which will amplified replica of the controlling c u r r e n t . persist indefinitely in the absence of an input sig- This elementary amplifier may be modified by nal. From 2 to 20 of these circuits can be can- simple f e e db a c k arrangements to produce an nected in a ring in which the particular circuit impressive array of familiar circuits such as "latched" will revolve around the ring in response oscillators, free-running multivibrators, one- to successive input pulses, thus forming a ring shot multiuibrators, and current discriminators counter. (the magnetic counterpart of the familiar Schmitt trigger circuit). Two amplifiers may be com- Most of the circuits described in the section en- bined to produce a balanced amplifier or a dif - titled "Detailed Examples of Magnetic Circuits" ferential amplifier. And, by adding s eve r a 1 are similar-- if not identical-- to those used input currents to obtain the controlling current, successfully in Berkeley equipment. This in- a coincidence gate can be built. formationis offered with the thought that an en- gineer contemplating magnetic design may wish The other way of using FERRISTORS takes ad- to construct experimental circuits in order to vantage of the fact that when the inductive re- acquaint himself with their advantages and pos- actance of the controlled winding equals the ca- sibilities . THE BASIC AMPLIFIER Figure 4 is a schematic diagram of a magnetic ing in it. Since the carrier current variations circuit which is comparable to a vacuum tube are much larger t ha n the control current var- amplifier. The Ferristor itself consists of two iations, the Ferrietor amplifies. in Figure 4 windings of fine wire on a tiny iron core, all en- the a -c c a r r i e r current is drawn through C1. cased in resin, The esiential characteristics of The voltage developed across C1 is the carrier the Ferristor arise from the fact that the small frequency modulated in accordance with control core can be a a t w r a t e d by minute currents in current variatione. This signal is rectified and either windiilg. For this reason it is sometimes filtered to produce a demodulated output. called a "saturable reactor" --abbreviatedr'SR". With no current the carrier winding has the rel- atively high inductance of a iPon-core inductor. Notice that this circuit is roughly analog- to But, when the sum of the currents in both wind- a vacuum tube amplifier. Control current cor- ing increases beyond a certain point, the core responds to control grid voltage; carrier supply saturates and the inductance of the carrier wind- voltage, to B+ supply; carrier current, to plate ing falls greatly. Operated as an amplifier cur- current; and load impedance, to plate load. The rent in the control winding is used to vary the main differences a re that the controlling quantity inductance of the carrier winding. If the car- is a current r a t he r than a potential and that a rier wlnding is energized by a carrier. supply carrier frequency must be intrduced, modulated (an a-c voltage source), its changing inductance and finally removed. w will cause changes in the carrier currbnt flow- FIGURE 4. AW AMPLIFIER I I q CAMIER WINDING 7 OUTPUT rI T * I I L----.---J F l LTER 2.2K CAPAC l TOR - ( 500pp f 1 - --. -- I-- i-- The transfer characteristics of this amplifier A CURRENT DISCRIMINATOR are diagramed in Figure 5. This figure shows Figure 6 shows an amplifier circuit which has how carrier winding inductance and output volt- b e e n converted to a current discriminator by age vary with input current. The inductance is adding positive resistive feedback. This circuit nearly constant from zero current until saturat- is the m a g n e t i c equivalent of the well-known ion begins. This range will be called the "non- Schmitt trigger vacuum tube circuit. The out- I saturated:lf region. From the beginning of sat- put voltage Eo draws current through the control uration until the Ferristor is fully saturated the winding and feedback resistor in a direction that inductance falls rapidly. This will be called the re-enforces the input current. As the input cur- region of "partial saturation. " Beyond full sat- rent rises from zeroto the point where saturat- uration the inductance stays nearly constant again ion begins, the output voltage remains almost constant. When the Ferristor begins to saturate, BEGIN SATURATION the gain from input to output rises sharply. At a certain point the feedback loop gain becomes greater than one and the control current and out- 4 OUTPUT VOLTAGE / put voltage increase rapidly in a regenerative circuit untilthe Ferristor is fully saturated. At this point the gain from input to output decreases again tonearly zero andfurther increases in in- put current have no effect. When the input cur- SATURATION COMPLETE rent decreases to a value slightly lower than that at which sudden saturation occurred, the total current in the control winding becomes too low CARRIER WINDING to maintain saturation. At this point the output INWCTANCE voltage begins to drop, and a reverse regener- rI b CONTROL CURRENT ative action returns the circuit to the original unsaturated state. Due to this action the circuit FIGURE 5. AMPL IF IER CHARACTERI'STIC'S switches rapidly back and forth from one to an- other of two comparatively constant sta t e s as the input current rises and falls. Because the at a lower value. This will be called simply the state it assumes depends only upon whether the I1 saturated" region. Notice t ha t the transfer- input current is' above or below a certain value, characterisitc curve (output voltage VS control it is called a "current discriminator". The dif- current) is analogous to that of a vacuum tube. ference between the input current value at which The point at which saturation begins corresponds the Ferristor switches from non-satura tion to to cut-off and the f u 11 saturation point corres- saturation and the lower value at which it switches ponds to plate saturation. The gain is apprec - back from saturation to non-saturation is called iable only when the control current varies within the hysteresis of the circuit. In elec tronic count- the region of partial saturation. The amplifier ers this circuit is usedmainly to generate sharp will have nearly the same characteristics if C1 pulses from slowly varying waveforms. is removed. In this case the carrier current is 'I .-. - predominately pblsating d-c. GND 1 * ( INPUT CURRENT) t 0 FIGURE 6. A CURRENT D I'SCR IMlNA TOR A CIRCUIT BI-STABLE AT ZERO INPUT If the value of the feedback resistor in the cur- the circuit to saturation, where it will remain rent discriminator described above is lowered, when the input returns to zero. With the circuit the hysteresis of the circuit will increase until in the saturated state a pulse of negative input the transfer characteristic curve assumes the current will switch it back to the non-saturated shape shownin Figure 7. This happens because state, where it will stabilize when the input again the feedback current in the saturated state be- returns to zero. comes so gre a t that it will hold the Ferristor saturated without the aid of any input current, and even when some input current is "bucking" the feedback current in the control winding. In this discussion "positive" input current means current in a direction which a ids feedback and "negative" input current means current which "buc ksl'feedback.