Wideband GaN-Based Supply Modulated Power Amplifier Systems

In recent years supply modulation has emerged to a very promising efficiency enhancement alternative for systems operating with large peak-to-average power signals. Fast DC/DC converters now come in a variety of topologies. Discrete level supply modulation (class-G) of RF power amplifiers (PA) i.e. switching of the supply voltage with the envelope of the modulated signal reach state-of-the-art performance in terms of efficiency and instantaneous modulation bandwidth (IBW) for 4G wideband signals. The PA must be optimized for multiple voltage level supply and cover the full bandwidth at all supply voltages. Compared to continuously supply modulated systems, a significantly lower switching frequency, in the same range as the IBW, is required. The key enabler for this development has been the introduction of GaN technology. It has improved in recent years, but still suffers from charge trapping effects. Drain-lag has a large impact on the performance in supply modulated systems particularly at lower supply voltage levels. Work is now pursued to bring the class-G efficiency enhancement topology into the 20 to 30GHz range targeting space and 5G MIMO-applications. Integrated supply modulated GaN-MMIC designs with RFPA and class-G supply modulator on the same chip show promising performance but the very large IBW is challenging. Increased trapping effects and the reduced maximum supply voltage for the short gate-length technology add further challenges to the work.