Nested feedback is a way to apply more NFB around the output stage without increasing the global feedback factor. The output has an extra voltage gain stage bolted on, and a local feedback loop is closed around these two stages. This NFB around the composite output bloc reduces output stage distortion and increase frequency response, to make it safe to include in the global NFB loop.

Suppose that bloc A1 (Figure 7.2a) is a Distortionless small-signal amplifier providing all the open-loop gain and so including the dominant pole. A3 is a unity-gain output stage with its own main pole at 1 MHz and distortion of 1% under given conditions; this 1 MHz pole puts a firm limit on the amount of global NFB that can be safely applied. Figure 7.2b shows a nested-feedback version; an extra gain-bloc A2 has been added, with local feedback around the output stage. A2 has the modest gain of 20 dB so there is a good chance of stability when this loop is closed to bring the gain of A3 + A2 back to unity. A2 now experiences 20 dB of NFB, bringing the distortion down to 0.1%, and raising the main pole to 10 MHz, which should allow the application of 20 dB more global NFB around the overall loop that includes Al. We have thus decreased the distortion that exists before global NFB is applied, and simultaneously increased the amount of NFB that can be safely used, promising that the...