Implementation of dual gate and drain dynamic voltage biasing to mitigate load modulation effects of supply modulators in envelope tracking power amplifiers

This paper presents a combination of dynamic gate and drain biasing techniques applied to a S-Band - 10 W GaN power amplifier. A GaN-based drain supply modulator (DSM) and an integrated gate biasing circuit have been built and connected to a GaN RF power amplifier (RFPA). The complete circuit architecture is then implemented in a test bench for the study of the envelope tracking power amplifier (ETPA). The work reported here focuses on the nonlinear coupling between the drain supply modulator and the RF power amplifier and proposes a solution to mitigate the load modulation effect of the supply modulator that is prejudicial for the overall efficiency and linearity performances. It consists in implementing an appropriate dynamic gate bias control of the RF power amplifier. The experimental validation of the study is demonstrated here for a QAM-16 (2 MSymb/s) modulated carrier at 2.5 GHz. At 38 dBm output power, dynamic load variations of the drain supply modulator (in the 30-500Ω range) versus instantaneous input power level variations have been drastically reduced and maintained to a 40Ω average value. The measurement of the signal constellation diagram at the ETPA output was found to be a well suited way to optimize the tuning of bias trajectories. A measured minimal error vector magnitude (EVM) of 2% and a power added efficiency (PAE) of 40% are obtained when optimal tuning is reached.