Q.Why is positive feedback seldom used in amplifier ?

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:

$V_i = V_s - V_f.$ …