Linearization of a 500-W L-Band GaN Doherty Power Amplifier by Dual-Pulse Trap Characterization

This paper describes the linearization of a base-station L-band 500-W GaN Doherty high power amplifier (HPA) driven by OFDM signals. Pre-pulsing characterization is used to extract the gain dispersion of the carrier and peaking PAs due to trap-induced degradation of GaN-on-SiC transistors. Peak drain voltages reached by PA load-lines mainly set the trap states of the carrier and peaking PAs, while the recovery is longer with a dominant time constant of 100 µs for this specific GaN technology. When the peak occurrences are below this dominant time constant of 100 µs, such as for symbol periods of 16.7 to 66.7 µs (i.e., LTE/5G OFDM), the HPA trap-state remains approximately constant in the time interval between voltage peaks, allowing low-complexity linearization of the HPA. With a 10-MHz OFDM signal with peak-to-peak intervals shorter than 100 µs, a memory-less digital pre-distortion (DPD) is shown to improve ACLR by 4 dB and NRMSE by 1.6 percentage points, as compared to peak-to-peak intervals longer than 100 µs when significant trap recovery takes place.