Hurtig, C. R.

Lipsky, A. H.

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XI. TRANSISTOR CIRCUITSProf. H. J. Zimmermann Prof. S. J. Mason C. R. HurtigProf. R. B. Adler J. B. Cruz A. H. LipskyA. TRANSISTOR CIRCUITRY1. Voltage-Tuned RC Active FiltersIn the Quarterly Progress Report of January 15, 1956, a voltage-tuned amplifier thatemployed twin-T networks with a Q of 8 was described. During the past quarter, tunedamplifiers employing twin-T networks and Wien-bridge networks with a range of Q's ashigh as 30 were considered. The current gain of a tuned amplifier employing a Wien-bridge network is related to Q in the following manner:IAil (1)Q 3where A. is the current gain of the amplifier. To obtain a Q of 25, a current gain of1approximately 75 is required. Thus, two stages of current amplification, together withother stages, are necessary to provide the proper drive and load impedance to the RCnetwork. Except at the center frequency, the amplifier has highly degenerate feedback.This feedback, coupled with the high gain and multistage form of the amplifier, causesstability problems. To provide stable operation, single, dominant time-constants wereincluded externally at both low and high frequencies. A study of the effects of the loca-tion of these dominant time-constants showed that a ratio of dominant cut-off frequencyto maximum or minimum center frequency of 5 or 1/5, respectively, produced negligibleeffects on the magnitude or phase characteristics of the amplifier.A voltage-tuned amplifier with a Q of 30 was constructed and tested. Two inter-esting observations are worth mentioning. The first is the observation of phase jitterat the output of the amplifier that was caused by resistance modulation of the diodes.An approximate relation between the phase jitter, Q, and the resistance fluctuation perdiode in a tuned amplifier that employs a Wien-bridge network is0 z6Q AR (z)ON8 60 Q (2)N Rwhere 0N is the rms phase shift in degrees, and AR/R is the rms deviation in the dioderesistance caused by resistance modulation. The second observation is one of condi-tional stability. Manual tuning of the oscillator, monotonically in one direction, occa-sionally induced transient oscillation. This oscillation can be stopped by interruptingthe loop in order to establish equilibrium conditions in the RC network. Tests of sinu-soidal voltage tuning have not yet been made.(XI. TRANSISTOR CIRCUITS)2. Voltage-Tuned OscillatorThe major factors that limit the frequency stability of a voltage-tuned Wien-bridgeoscillator were studied. The long-time frequency stability is influenced primarily bythe temperature dependence of the incremental resistance of the junction diodes thatare employed, and also by the temperature dependence of the capacitors. The tem-perature dependence of the diodes is approximately 0. 3 per cent per degree centigrade.Since the capacitors that are required can be in the range of 0. 1 to 1.0 4f, a tempera-ture dependence of +500 parts per million per degree centigrade can be expected. Themajor cause of short-time instability of the frequency of oscillation is, apparently, aresistance modulation of the junction diodes that are employed. Several tests withdifferent diodes under different bias conditions indicate an rms fluctuation in dioderesistance from 1 part in 1000 to 1 part in 10, 000. The rms fluctuation increases withdirect bias current. The measurements that have been made are not of sufficient accu-racy for us to state the dependence specifically; it is not, however, a linear relationship.C. R. Hurtig, A. H. Lipsky

Transistor Circuits

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Transistor Circuits

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