Decreasing Distortion with Feedback

The use of negative feedback can discriminate against sources of noise or distortion within an amplifier. Considering a two-stage amplifer (after Simpson) with sources of distortion v_d1 and v_d2 inside the feedback loop.

Distortion reduction takes the form below for this amplifier, showing that distortion within the feedback loop is discriminated against, with more reduction of distortion which arises near the output.

Increasing the Bandwidth

Amplifier gain will generally decrease at higher frequencies, but the contribution of negative feedback in stabilizing voltage gain and making it nearly independent of the open-loop gain is a major contribution to extending the useful frequency range of amplifiers.

The input impedance is

which without negative feedback is Zin0 .

But with feedback, the current is reduced to