Stability Criterion for Two-Port Network With Input and Output Terminations Varying in Elliptic Regions

New criteria to check the conditional stability of microwave amplifiers with input and output terminations varying in prespecified elliptic regions surrounding complex nominal values are proposed. Necessary and sufficient conditions are provided in terms of the immittance parameters lambda
_ij
(i.e., z
_ij
, y
_ij
, g
_ij
, or h
_ij
): these conditions can be implemented in commercial computer-aided design tools and are suitable to be used as design goals within optimization routines. Since elliptic shapes accurately fit the real variability region of input and output terminations, the proposed criterion allows to impose stability in the actual tolerance range of terminations and therefore to improve the tradeoff between performance and stability under termination variations