A scattering parameter theory of the performance characterisation for a bilateral transistor is developed, where mismatching at the input port V_i is considered as a degree of freedom, and its combination with noise and gain is mapped as circles in the Γ_in-plane. Stability analysis is based on the unconditionally stable working area (USWA) concept, and all possible USWA configurations are determined by the necessary and sufficient conditions.' For each USWA configuration, the constrained maximum stable gain G_Tmax and its termination couple (Γ_s, Γ_L) are expressed as functions of the input VSWR V_i noise figure F and the scattering S and noise N parameter vectors. Furthermore, the possible incompatible cases for the (F V_i G_Tmax ) triplets are determined by their necessary and sufficient conditions. A computer program based on this formulation is developed, and cross- relations among the (F V_i, G_Tmax) triplets have been utilised in obtaining the performance contours at an operating frequency and bias condition.
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