I. Introduction
RF power amplifiers (PAs) play an important role in modern wireless communication systems, and the key part of RF PAs is the RF power transistor. Recently, transistors have been pushed to work under strong nonlinear conditions in newly developed PA architectures and operating modes, i.e., the Class PA, the Doherty PA [1] etc. The main motivation for these modern designs is to improve power added efficiency (PAE) when the PAs are excited with high peak-to-average power ratio (PAPR) digitally modulated signals.