The active feedback amplifier. A versatile analog building block

Abstract

The active-feedback amplifier (AFA) topology approaches the ideal voltage-controlled voltage source (VCVS) with distinct input and output common-mode levels. This topology is very useful whenever it is required to sense a differential voltage at the input whose common-mode level is different to that at the output. With the AFA no ambiguity about the input and output grounds arises; compare this to op-amp based applications where one always has to decide where the 'ground' should be referenced to - input or output? Furthermore, when the op-amp is viewed as an integrator and not as an infinite-gain, infinite bandwidth block, then at frequencies not too remote from the unity-gain frequency the op-amp will be nothing like the infinite-gain block it is supposed to be. The AFA in contrast provides accurate performance right up to its unity-gain frequency. This paper explains these benefits.