Quasi-invariant single-parameter criterion for unconditional stability: Review and application

This paper presents a review of the recent quasi-invariant single-parameter criterion for linear two-port unconditional stability. The criterion involves a symmetrical parameter K_t, which is invariant under arbitrary lossless termination and reactive matching at either port when unconditional stability exists or |Delta| les 1. This parameter is hereby derived from the classical matrix invariants and the familiar stability criteria, leading to the straightforward proof of its unconditional stability condition. Application of the quasi-invariant single-parameter criterion is extended for simplified graphical analysis of linear three-port stability.