GaN MMICs for microwave backhaul: Doherty vs. combined class-AB power amplifier

The Doherty power amplifier is competing with class-AB amplifiers in mobile base-stations, thanks to its superior performances in terms of back-off efficiency. However, in case of microwave backhaul applications, where higher center frequencies and wider bandwidths are needed, it is still not clear if the linearity-efficiency trade-off is in favour of the DPA or of the classical class-AB. In this work, this point is treated reporting the design and experimental characterization of two GaN MMIC power amplifiers for microwave radio links: a combined class-AB and a Doherty amplifier fabricated with same technology and total active device periphery. The modules have been characterized from 6.6 GHz to 7.3 GHz under large signal condition. The obtained results show that the Doherty amplifier can have a beneficial effect on the energy optimization of the system at the expense of larger chip area with respect to the class-AB. Both circuits achieve 40 dBm saturated output power, but at 6 dB back-off the Doherty shows an efficiency improvement of about 50 %. The amplitude and phase distortions of the two PAs have also been measured, and a system level evaluation is provided highlighting pros and cons of each architecture in terms of linearity.