Asymmetrical Doherty amplifier using GaN HEMTs for high-power applications

In this paper, we present an asymmetrical Doherty amplifier (DA) that can enhance the efficiency at a 9.5 dB backoff from its saturated output power (P_SAT). The asymmetrical DA consists of a peak amplifier and a carrier amplifier with two and one 210 W gallium nitride (GaN) high-electron-mobility transistors (HEMTs), respectively. A Wilkinson combiner at the output of the peak amplifier enables the stable operation of the GaN HEMTs for high-power applications. The asymmetrical DA is designed and fabricated for the ultrahigh-frequency (UHF) band. Measurement results using a continuous wave (CW) signal indicate a drain efficiency (η_d) of 60% at a P_SAT of 57.5 dBm and a η_d of 56% at a 9.5 dB backoff from P_SAT. For a 6 MHz orthogonal frequency-division multiplexing (OFDM) signal with a high peak-to-average power ratio (PAPR), the measurement results indicate a η_d of 52% and an adjacent-channel leakage power ratio (ACLR) of -41 dBc at an average output power (P_AVE) of 48 dBm using a digital predistorter. To the best of our knowledge, this is the highest η_d and output power for an OFDM signal.