A 28-GHz Beamforming Doherty Power Amplifier With Enhanced AM-PM Characteristic

This article presents a beamforming Doherty power amplifier (B-DPA) for the 28-GHz fifth-generation (5G) new-radio band. The proposed B-DPA is based on a new combiner topology that allows the current profile in the auxiliary branch of the DPA to be relaxed while simultaneously canceling the amplitude-to-phase (AM-PM) distortion exhibited in the active devices. This approach enables efficiency and linearity enhancements. In addition, the DPA is augmented with high-accuracy digitally assisted vector multipliers at its input stage to provide the required relative and absolute phase adjustments for proper Doherty operation and the overall phase shift required for beamforming. A proof-of-concept prototype was implemented in the 45-nm silicon on insulator (SOI)-CMOS technology. Measurement results reveal that the proposed B-DPA demonstrated good Doherty operation with drain efficiencies of 18%-20% and 33%-35% at 6-dB back-off and saturation power (Psat, 18 dBm) levels, respectively, between 28 and 30 GHz. In particular, excellent AM-PM characteristics were noted with a peak value of only 1.5 degrees up to Psat, which remained below 2.5 degrees as the B-DPA was swept with a 0 degrees to 360 degrees phase shift. Finally, modulated-signal tests revealed error vector magnitudes of 3.1% and 0.9% for 4-MHz x 100-MHz orthogonal frequency division multiplexing (OFDM) signals before and after applying digital predistortion, respectively.