Simultaneous linearity and efficiency enhancement of a digitally-assisted GaN power amplifier for 64-QAM

The first dynamic 4-bit, digitally-assisted GaN high power amplifier (DAPA) system transmitting 7.68Msymbol/s with 64-QAM modulation is presented. An FPGA is programmed to generate the pulse-shaped 64-QAM signal, perform envelope estimation, and time-align the RF and digital control signals arriving at the DAPA. A high-speed, level-shifting circuit converts the FPGA's low-voltage differential signaling (LVDS) DAPA control signals into single-ended logic levels required for the depletion-mode GaN HEMT DAPA auxiliary cells. Measured results show 9.6% DC power savings, 23% improved PAE, and 23% higher output power at 4% EVMRMS compared to the static PA configuration.