Loop gain in feedback circuits: A unified theory using driving point impedance

The analysis of analog circuits containing feedback has long been confusing due to incomplete definitions of fundamental terms such as loop gain, open loop gain, and port impedance with feedback zeroed, Z₀´. In this paper, following a driving point impedance perspective, a unified model is developed that can be translated into the Black´s block diagram to properly identify the feedback analysis parameters and definitions. With this approach there is no ambiguity of how to terminate the sub-circuits used and can be done anywhere in the outer loop. This approach fully accounts for bi-directional properties in the amplifier and in the feedback net as well as loading effects. The approximation inherent in the properly loaded open-loop method is shown.