Checking Rollett's Proviso for Degenerated Devices through S-Parameter Analysis

An analysis is presented of the small-signal stability of the class of three-port networks derived by making the reference node of a two-port accessible. That class of networks is of particular interest since it includes, under rather mild conditions, any high-frequency active device when viewed as a three-port, i.e., when accessed also from the source (or emitter) terminal. The discussion here presented is completely based on a black-box (S-parameter) representation of the original two-port network, rather than on its small-signal equivalent circuit (SSEC). In a sense, this study complements a previous paper addressing small-signal active device stability from a SSEC standpoint. On the other hand, it is more general (not being based on a specific circuit topology) and more easy-to-use (not requiring the knowledge of the SSEC).