On the performance of class-D power amplifiers with RF Pulse-Width Modulation

The application of intrinsically nonlinear class-D power amplifiers can be significantly broadened to amplify a wider class of time-varying envelope signals provided a suitable pulse train is synthesized for amplification. Radio-Frequency Pulse-Width Modulation (RFPWM) is one method of encoding the source signal into a binary level pulse train. The performance of class-D power amplifiers with RFPWM are evaluated in terms of important pulse modulation parameters: coding efficiency, average transition frequency, and minimum pulse width. Compared with other pulse modulation methods such as Neoteric, RFPWM has the advantage of higher coding efficiency and lower average transition frequency which benefits the amplifier´s output power and efficiency. On the other hand, it demands higher switching speed on the transistors to produce very narrow pulses generated for a low envelope level. Thus, for the same class-D amplifier the operation frequency with RFPWM is much lower than with Neoteric modulation. Tested with a W-CDMA signal on a class-D amplifier constructed using GaN FET devices, it is found that for below -40dBc output spectral growth the amplifier can operate up to 200 MHz with RFPWM, while it can operate up to 800 MHz with Neoteric modulation. At 200 MHz it achieves 2W output power with a power efficiency of 40.7% for RFPWM, compared to 35.6% for Neoteric modulation.