A quasi-invariant single-parameter criterion for linear two-port unconditional stability

This letter presents a novel unconditional stability criterion for linear two-port networks. The criterion involves a new symmetrical single-parameter defined as. It is shown that K_t>1 if and only if a two-port network is unconditionally stable. Moreover, K_t is invariant under arbitrary lossless termination and reactive matching at either port when unconditional stability exists. Such invariant property is retained so long as |Δ|≤1, which occurs more frequently than |Δ|>1 for most devices. The parameter can also be aptly related to the maximum available gain of an unconditionally stable device.