Necessary and sufficient conditions for the stability of microwave amplifiers with variable termination impedances

New criteria to check conditional stability of microwave amplifiers with input and output terminations varying in pre-specified circular regions surrounding complex nominal values are proposed. Necessary and sufficient conditions are provided both in terms of the immittance parameters Lambdaij (i.e., zij , yij , gij or hij) and in terms of the scattering parameters sij. Proposed conditions can be easily implemented in any commercial computer-aided design tool and are suitable to be used as design goals within an optimization routine: this allows to optimize the tradeoff between stability constraints under termination variations and performance in a yield-oriented design flow. A case study, in which a distributed amplifier has been designed, shows the advantages of the proposed approach.