Loop Gain, Input Impedance and Output Impedance of Feedback Amplifiers

Abstract

Introductory treatments of feedback amplifiers commonly contain inconsistencies, or present results as universal when in fact they need qualification. Loop gain, overall gain, input impedance and output impedance are instances. A case can therefore be made for rigorous development, even in a first course, provided mathematical rigor can be combined with physical insight. A feedback factor H is first defined for ideal circumstances. The corresponding forward-path gain G has an obvious physical interpretation: the amplifier without feedback, the external source and load impedances, and the feedback network are all involved, and the form of G is such as to suggest best practice for the various feedback configurations. Stability considerations, and precise values for overall gain, sensitivity, input impedance and output impedance follow directly from GH.