Transistor Basics is a free introductory textbook on transistors and their basic applications. See the editorial for more information....

Self-Quenching Oscillator

Author: Leonard Krugman


Fig. 6-19. Basic self-quenching oscillator.

The relaxation oscillator in combination with the regular base-controlled type can be used to form the self-quenching oscillator. Figure 6-12 (A) illustrates a self-quenching type if the value of either C1 or C2 is increased sufficiently to make the emitter or collector time constant appreciably greater than that of the L-C tank circuit.

Figure 6-19 represents the basic self-quenching oscillator. Due to its time constant, the R-C emitter network has primary control of the circuit and produces the sawtooth voltage and pulsed current waveforms illustrated in Fig. 6-17 (C). The operation of the relaxation section of the circuit is independent of the base tank. The base network, however, depends entirely on the relaxation operation. Assume the cycle is moving in the charging direction (B of Fig. 6-17), operation from point C to point A. When the operation reaches the negative-resistance region where sufficient regenerative energy is supplied, the base tank oscillates at its resonant frequency. The amplitude of the resulting wave is small initially, but rises to a peak at the point when CE starts its discharge cycle (B of Fig. 6-17), operation from point A to point D. The duration of the oscillation in the base tank is a function of the Q of the network, the amount of stored energy and the loading effect on the tank by the rest of the circuit. The relaxation or quench frequency in this case is transistor_basics_06-55.gif, while the resonant frequency of the tank is transistor_basics_06-56.gif. Notice that fQ must be less than fT for proper operation. The basic circuit becomes collector controlled if capacitor CE is moved into the collector circuit. The circuit operation is exactly the same.

Last Update: 2010-11-17