Feedback is a network which takes sample of output signal which may be voltage or current and fed back to input if feedback signal (Vf) is in phase with the input signal (Vs) then such feedback is called positive feedback.

Positive feedback is used in oscillator to satisfy Barkhausen’s Criteria in order to produce sustain oscillations.

Oscillator produces sinusoidal waveform without any input signal hence starting voltage is noise signal which may be due to resistor’s, power supply. The noise signal gets amplified and fed back to the input. Hence amplifier introduce phase shift of 180 º & feedback circuit also introduce phase shift of 180 º, hence total phase shift become 360º or 0º which satisfy barkhausen’s criteria.

Barkhausen’s Criteria:

In order to produce sustain oscillation following criteria should be satisfied:

The total phase shift around close loop should be 360º or 0º i.e. the total phase shift afford by the circuit as signal proceeds from input terminal to output terminal and to the feedback network and back to input terminal is 360º or 0º.

Oscillations will be developed only if the product of magnitude of gain of amplifier i.e. A and feedback factor of the feedback network i.e. $\beta$ is unity. Hence for sustained oscillation $|A \beta|=1.$