Base- and Collector-Controlled Oscillators

Author: Leonard Krugman

Fig. 6-18. (A) Base-controlled relaxation oscillator (B) Collector-controlled relaxation oscillator.

Base-controlled and collector-controlled relaxation oscillators are illustrated in Figs. 6-18 (A) and 6-18 (B). Both operate very much like the emitter-controlled type, and are analyzed on the basis of their respective operating characteristics, illustrated in Fig. 6-9 (A) and (C). The main difference is that the base-controlled type uses an inductance for the storage and release of circuit energy.

The fundamental difference between the sine wave oscillator and the relaxation oscillator is determined by which of the circuit parameters control the repetition rate. This, in turn, is determined by which has the lowest period of oscillation. For example, if in Fig. 6-12 (A) the time constant of the emitter network C₁R_E or the collector network C₂R_c is greater than that of the base L-C tank, the circuit becomes a relaxation oscillator. If a properly designed base-controlled high frequency sinusoidal oscillator suddenly switches to a different frequency and produces a distorted waveform, the trouble is most likely in the base resonant circuit.

While the R-C time constant of the collector- and emitter-controlled relaxation oscillator is fixed by the required operating frequency, the C to R ratio should be as high as possible. This causes minimum degeneration in the circuit, and, at the same time, increases the surge current handling capacity of the condenser. As before, the value of the base resistor R_B is determined by the amount of positive feedback required for sustained operation.