The primary concern in the design of a high-frequency amplifier is to ensure that the active device does NOT oscillate at any frequency, especially at the frequency of operation.
A two-port active device is unconditionally stable if both its input and output impedances
have positive real part for ANY passive source and load terminations. In S-parameter
terminology, it is to say
It can be proved, will be shown in below, that the necessary and sufficient conditions for unconditionally stable operation are
Note that a unilateral device, , is always unconditionally stable.