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 converter-based supply modulators now come in a variety of topologies. One approach is the discrete-level supply modulation (class-G) of RF power amplifiers (PAs), at which the supply voltage is switched at certain discrete envelope levels of the modulated signal. Class-G modulated PAs 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 supply voltage-levels and cover the full bandwidth at all supply voltages. Compared to continuously supply-modulated systems, a significantly lower switching frequency is required for the supply modulator. It is in the same range as the IBW. 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 is one charge trapping effect that has a large impact on the performance in the RF PA of 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. Supply-modulated GaN-MMIC-designs with RF PA and class-G supply-modulator on the same chip show promising performance. Nevertheless, 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.