Q.Define positive feedback.

Feedback is classified by whether the returned signal increases or decreases the net input, and this single phase difference sets all their contrasting properties.

In positive (regenerative or direct) feedback the feedback voltage $V_f$ is in phase with the source $V_s$, so the net input to the amplifier increases:

This raises the gain but worsens the amplifier in every other respect — more distortion, poor gain stability, poor frequency response and more drift — and can drive the circuit into oscillation. Its in-phase, self-reinforcing action is exactly what an oscillator needs to sustain its own output, so positive feedback is used there.

In negative (degenerative or inverse) feedback the feedback voltage is $180^\circ$ out of phase with the source, so the net input decreases:

The gain falls, but in return the amplifier gains highly stabilised gain, reduced distortion, reduced noise, increased bandwidth and adjustable input and output impedances. Because a good amplifier must reproduce its input faithfully with a constant, predictable gain, negative feedback is the type used in amplifiers, and positive feedback is avoided there. This trade-off — a little gain given up for large gains in quality — is why negative feedback is the focus of the rest of the chapter.