A comparison of polar transmitter architectures using a GaN HEMT power amplifier

In this paper three transmitter architectures are compared, that each use the low-frequency envelope and high-frequency phase component of an RF signal. A power amplifier (PA) with Pulse Width Modulation by Variable Gate Bias (PWMVGB) is compared to an Envelope Elimination and Restoration (EER) and Envelope Tracking (ET) configuration. The test circuit is implemented using a discrete GaN HEMT power amplifier and discrete surface-mount passive components assembled on a PCB. Measurements show that the EER architecture maintains a drain efficiency of 56 to 69% for a wide output power range, while the PA with variable gate bias shows a significant drop in efficiency for lower output powers (from 59 to 6%). Other comparison issues are modulation of the supply voltage and transmitter complexity.