Feedback in Amplifiers

With negative feedback factor $\beta$ around an amplifier of open-loop gain $A$:

$A_f = \frac{A}{1 + A\beta} \approx \frac{1}{\beta} \quad \text{when } A\beta \gg 1$

The closed-loop gain then depends only on the (precise, passive) feedback network. Negative feedback reduces distortion and sensitivity to component variation and extends bandwidth by the same factor $1 + A\beta$ that reduces gain.

Positive feedback creates oscillators and Schmitt triggers. Too much phase shift can turn negative feedback positive — the subject of stability analysis.