Contains research objectives and reports on one research project

Adler, Richard B.

Hurtig, C. R.

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XIII. TRANSISTOR CIRCUITSProf. H. J. Zimmermann J. Blair A. LipskyProf. R. B. Adler J. B. Cruz R. B. MartindaleProf. S. J. Mason J. Gross R. F. MeyerC. R. HurtigRESEARCH OBJECTIVESThis research is aimed at problems of interest in communication systems. Prin-ciples of transistor circuit design and practical limitations imposed by available devicesare being investigated for circuits useful in a wide variety of applications. At thepresent time problems pertaining to the tuning of RC networks are being investigated,with semiconductor diodes used as the control elements.The study of noise in semiconductors is a closely related program.R. B. AdlerA. TRANSISTOR CIRCUITRYi. Dual Terminations as a Guide to Practical RC Active Filter DesignThe use of RC active filters to achieve a desired pole configuration in the complex-frequency plane is well known in conventional vacuum tube circuitry. Two of the commonRC networks used in feedback amplifiers are the Twin Tee and the Wien bridge. Net-works of this class are designed for voltage-source drive and open-circuit load. Quiteoften this combination of drive and load impedance is inconvenient for practical designof transistor amplifiers. A complete dual to the desired network may be employed toyield a current-source drive and short-circuit load, a combination which yields a morepractical amplifier design from the point of view of transistor work. Unfortunately, acomplete dual to an RC network requires inductors with their consequent basic limita-tions in respect to Q and self-resonant frequency. A dual circuit, especially at lowfrequencies, is, consequently, not an acceptable solution. One promising solution fortwo terminal-pair networks is the use of dual terminations with a reversed network, inthe particular manner outlined below.It can be shown quite readily by the use of the Reciprocity theorem and Thevenin'stheorem that the open-circuit voltage-transfer ratio of a two terminal-pair network andthe short-circuit current-transfer ratio of the same network, with its input and outputterminals interchanged, are identical. Two commonly used RC networks and the inter-changed forms suggested above are shown in Fig. XIII-1.An interesting, and sometimes useful, corollary of the foregoing result is, that ifthe original network were driven from a voltage source es in series with a resistor Rs'and the output voltage e appeared across a load resistor R L , the reversed networkdriven by e' in series with RL will yield an output voltage e' across a load R so thate'/e' = (Rs/RL)(e /e).0os s L 0os116REVERSED NETWORKR, R,TWIN TEE e C eoC R(WIEN-BRIDGE ei C2  R2  e 2DERIVED)eFig. XIII-1.INPUTesSF(jw)isR, C,2 HFig. XIII-2. Twin Tee amplifier with voltage-tuned center frequency.117Two basic networks and their corresponding reversedforms with dual terminations.NETWORK1004fI I 1 I I0 2 4 6 8 10 15 20 25CONTROL VOLTAGE (VOLTS)Fig. XIII-3. Center frequency versus control-voltage character-istic of the Twin Tee amplifier.0520CONTROL VOLTAGE (VOLTS)Fig. XIII-4. Q versus control voltage for Twin Tee amplifier.1181500r.w 1000W0a:LL50030 35k2 I I I I I^^^^UUU200Co-i I- 1- -(XIII. TRANSISTOR CIRCUITS)2. Voltage-Tuned RC Active FiltersIn an RC active filter the pole locations are moved from the negative real axis tosome other location in the complex plane by the use of feedback. In ordinary circuitdesign, the magnitude of the vector from the origin to the pole is inversely proportionalto an RC product, but the angle of the vector with respect to the coordinate system isindependent of the RC product. By the use of forward-biased junction diodes as theresistive components of the RC filter, the magnitude of the vector from the origin tothe pole may be made a linear function of a control signal over a large range (ten to one)of control signals. This circumstance follows because the dynamic resistance of thediodes is inversely proportional to the direct current flowing through them.The design of voltage-controlled variable-frequency RC active filters is predicatedon the use of the incremental resistance of junction diodes. The range over which thisincremental resistance is inversely proportional to the direct-current bias extends froma few hundred microamperes to fractions of a microampere. The nominal range ofcontrol signals used in circuits thus far is from 20 famp to 200 4amp. There are twobasic limitations to extending the lower value of the range of control current. Bothlimitations arise because the ac signal must be small compared to the direct-controlcurrent. Transistor noise and the required dynamic range place one limitation on theac signal current. Allowable distortion, which is also a function of the configurationof the RC filter, may place a conflicting requirement on the value of the ac signalcurrent. In the circuits designed up to the present time a dynamic range of 40 db withdistortion less than 5 per cent seems consistent with the range of control signals quotedabove.The circuit diagram of a voltage-controlled Twin Tee amplifier that makes use ofthe concept of dual terminations (see Section XIII-1), is shown in Fig. XIII-2. Thecontrol current for the junction diodes is obtained from a variable voltage source inseries with fixed resistors of large value. The first transistor is employed to furnisha current-gain amplifier, the second to provide an approximation to a current-sourcedrive for the Twin Tee network, and the third to approximate a short-circuit loadingfor the Twin Tee. A curve relating the center frequency to the control voltage is shownin Fig. XIII-3. The linearity of this characteristic was within the measurement accuracyof the voltmeter (±2 per cent). The Q of the Twin Tee amplifier was nominally 7. 8,and its variation with control signal is shown in Fig. XIII-4.A frequency-modulated, low-frequency oscillator employing an RC network derivedfrom the Wien bridge has been built and is now being tested. The frequency range isfrom 160 cps to 1600 cps, with a linearity, as a function of control voltage, of the sameorder of magnitude as that shown in Fig. XIII-3.C. R. Hurtig119

Transistor Circuits

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Transistor Circuits

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