Subharmonic Oscillations in High-frequency Switched-Mode Power Amplifiers

Switched Mode Power Amplifiers (SMPA) are preferable in high power applications as its advantage of high-efficiency. While the inevitable effects of parasitic elements hinder its applications in higher frequencies. In this paper, the specific manifestation and mechanism of subharmonic oscillations, which will cause high peak voltage pressure and significant subharmonic distortions, were disclosed and analyzed at the first time in SMPA. Different with linear PA, where the subharmonic oscillation was caused by the input capacitance of transistor, the origination of the phenomenon in SMPA was found to be the non-linear output capacitance of switches in this paper. The time-domain analytical method based on the solutions of Mathieu Equations was carried out to estimate the system stability in Class-E circuit. And it concluded that in Class-E circuit, the parameters of input tank and the bias voltage dominates the phenomenon, which was verified by the experimental results.