Once you know how amplifiers work, it’s easy to understand oscillators. All oscillators are amplifiers with positive feedback. In radio communications, oscillators generate the waves, or signals, that are ultimately sent over the air. Audio-frequency oscillators find applications in such devices as music synthesizers, modems, doorbells, sirens, alarms, and electronic toys.
Feedback can be in phase or out of phase. For a circuit to oscillate, the feedback must be in phase (positive). Out-of-phase (negative) feedback reduces the gain of an amplifier. In fact, negative feedback is used in some amplifiers to prevent oscillation.
The output of a common emitter or common source amplifier is out of phase from the input. If you couple the collector to the base through a capacitor, you won’t get oscillation. It is necessary to reverse the phase in the feedback process in order for oscillation to occur. In addition, the amplifier gain must be high, and the coupling from the output to the input must be good. The positive feedback path must be easy for a signal to follow. Most oscillators are common emitter or common source amplifier circuits with positive feedback.
The output of a common base or common gate amplifier is in phase with the input. But these circuits have limited gain, and it’s hard to make them oscillate. Common collector and common drain circuits don’t have enough gain to make oscillators.
Feedback at a Single 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 in radio transmitters and receivers; the RC method is more often used in audio work. The tuned circuit makes the feedback path easy for a signal to follow at one frequency, but hard to follow at all other frequencies. As a result, oscillation takes place at a predictable and stable frequency, determined by the inductance and capacitance, or by the resistance and capacitance.