I. Introduction
Nowadays, any nonlinear microwave circuit design process requires harmonic balance (HB) computer-aided design (CAD) techniques to be effective. Among those circuits, power amplifiers (PAs) used for wireless base stations are particularly reliant on these techniques to reduce the time and cost of their design. However, HB CAD is constrained by the accuracy of the nonlinear device models employed in the simulation. In some instances, these models may not even be available, which is particularly common for the latest transistor technologies or very high-power devices [1]. In the latter case, it may not even be possible to extract equivalent-circuit models due to the measurement difficulties imposed by involving the high currents and low impedances, as well as complex electromagnetic (EM) extrinsic structures used in their packages.