On the Optimal Input Voltage of a Class-E Power Amplifier with GaN HEMTs at MHz Frequency Operation

Class-E power amplifiers have regained academic interest over the past decades thanks to the introduction of new high-performance wide-bandgap semiconductor devices. While these power devices, notably GaN HEMTs, have exceptional performance at megahertz operation, they also display an additional loss component due to the C _oss at this frequency. Unfortunately, the dependency of this loss term on the peak voltage is the opposite of that of the conduction loss. In this paper, we mathematically analyze the operation of a Class-E amplifier when facing with this C _oss loss term. We have found that the conventional wisdom of using the highest input voltage to get the best efficiency no longer holds above a certain frequency. Furthermore, to allow this new loss term to be easily simulated, we propose a distributed loss model for the C _oss loss based on a generalized Steinmetz equation (GSE). To demonstrate the applications of the proposed GSE-based C _oss loss model, a design example is provided for a variable-resistance Class-E amplifier at 40.68 MHz. The experimental result on this design example shows good agreements with our analysis.