Harmonically tuned GaN-HEMT Doherty power amplifier for 6 GHz applications

In this paper a GaN-HEMT Doherty amplifier targeted for vehicular applications at 6 GHz is presented. The harmonic tuning of the selected bare-die transistors was investigated and implemented in order to improve the overall large-signal performance. The Doherty amplifier was studied and optimized on device reference plane level using two equally sized 8-Watt GaN devices for the main class-AB and the peaking class-C amplifiers. The measured maximum output power was 41.5 dBm with 63 % peak drain efficiency (50 % power added efficiency). At 6 dB output power back-off a drain efficiency of 49 % (42 % power added efficiency) was obtained. The linearity of the amplifier was tested by applying modulated signals according to the IEEE 802.11p standard. At 34 dBm average output power a drain efficiency of 43 % was achieved for this high dynamic range OFDM signal. After digital pre-distortion, the related adjacent channel power ratio could be reduced from -31 dBc to values below -43 dBc without significant impact on the efficiency performance.