  In this paper, a novel approach based on robust stability concepts and tools
is introduced to evaluate the unconditional stability of microwave active
$\textit{n}$-port devices. An efficient calculation of the Structured Singular
Value of the $\textit{n}$x$\textit{n}$ scattering matrix is proposed to obtain
the stability characteristics of the device. The presented method is validated
in two ways. First, it is applied to a referential 4x4 scattering parameter set
for independent verification. Second, the method is applied to a 4-port GaAs
FET amplifier fabricated in hybrid technology. The results confirm the validity
and computational efficiency of the proposed approach.
