Positive feedbackFeedback can be in phase or out of phase. For a circuit to oscillate, the feedback mustbe in phase, or positive. Negative feedback (out of phase) simply reduces the gain.The output of a common-emitter or common-source amplifier is out of phase fromthe input. If you couple the collector to the base through a capacitor, you won’t get os-cillation.The output of a common-base or common-gate amplifier is in phase with the input.But these circuits have limited gain. It’s hard to make them oscillate, even with positivefeedback.Common-collector and common-drain circuits don’t have enough gain to make os-cillators.But take heart: There are lots of ways to make circuits oscillate. Obtaining oscillationhas never been a problem in electronics. Public-address systems do it willingly enough!Concept of the oscillatorFor a circuit to oscillate, the gain must be high, the feedback must be positive, and thecoupling from output to input must be good. The feedback path must be easy for a sig-nal to follow. The phase of a fed-back signal can be reversed without any trouble, so thatcommon-emitter or common-source amplifiers can be made to oscillate.Feedback at a single frequencyRecalling the public-address fiasco, some variation of which you’ve doubtless heardmany times, could you know in advance whether the feedback would have a low pitch,a midrange pitch, or a high pitch? No. The oscillation was not intended, and might havestarted at any audio frequency.The frequency of an oscillator is controlled by means of tuned, or resonant, circuits.These are usually inductance-capacitance (LC) or resistance-capacitance (RC)combinations. The LC scheme is common at RF; the RC method is more often used foraudio oscillators.The tuned circuit makes the feedback path easy for a signal to follow at one fre-quency, but hard to follow at all other frequencies (Fig. 25-1). The result is that the os-cillation takes place at a predictable and stable frequency, determined by theinductance and capacitance or by the resistance and capacitance.458 Oscillators25-1Basic concept of the oscillator.