Gain-controlled resistors in analysis of amplifiers featuring accurate input and output impedance

An amplifier featuring accurate input and output impedance can be considered as a device that includes two models of gain controlled resistors. The models are introduced, and it is shown how they can be used in the analysis and design of such an amplifier. Then the analysis is applied to two-transistor amplifiers used in practice, and a new configuration is added. The relationship that should exist between passive elements of these amplifiers to obtain input and output matching is formulated for all three circuits.