feedback    In  an input output device feedback is transfer of a portion  of  the output back to its input. Feedback can be either positive-in  phase  with applied signal or negative- out of phase with applied signal. In an amplifier negative feedback reduces distortion, and makes its operation more stable in respect to variation in  gain or aging.

Fig.  f1  shows  schematically an amplifier with feedback. Total input signal is

e_s + b e₀ ,

where  b  is the fraction of output fed into the input, which can be positive or negative. If A is the gain of the amplifier,

(e_s + b e₀)A = e₀

or e₀ /e_s = a / [ 1 - bA]

For an amplifier with very large negative feedback.

e₀ / e_s = 1/b

The  gain  of  an amplifier with a large negative feedback is small but extremely stable, and is independent of fluctuations in the supply voltage. On the other hand positive feedback increases the gain with the result that an amplifier becomes an oscillator.