A High Efficiency and Low Distortion 6 W GaN MMIC Doherty Amplifier for 7 GHz Radio Links

This contribution presents a novel design methodology for Gallium Nitride (GaN) Monolithic Microwave Integrated Circuit (MMIC) Doherty Power Amplifiers (DPAs). The proposed solution allows to mitigate the relevant phase distortion (AM/PM) and gain penalty, typically registered in GaN DPAs, without worsening other key features such as back-off efficiency and amplitude distortion (AM/AM). To this purpose, a non-linear driver stage is introduced in both Carrier and Peaking branches, with the aim to provide a chip-level phase distortion compensation mechanism. The idea is to almost zeroing the overall phase distortion by achieving, in the driver stage, an opposite AM/PM behaviour with respect to the one of the final stage. The theoretical formulation is verified presenting the design and experimental characterization of a GaN MMIC DPA for 7 GHz radio links. The chip has been developed in a commercial 0.25 mu m GaN power process, resulting in a 3 x 3mm(2) chip area. The realized DPA shows 38 dBm of saturated output power, 16 dB of gain, and less than 3 degrees of phase distortion, with a power added efficiency higher than 41% in 6 dB of output power back off.