A Highly Linear GaN MMIC Doherty Power Amplifier Based on Phase Mismatch Induced AM-PM Compensation

This article presents a highly linear Doherty power amplifier (DPA) based on phase mismatch. When output phase mismatch (OPM) is introduced, i.e., the phase shift of the output impedance transformer deviates from 90 degrees, the power-combining network (PCN) will exhibit a certain amplitude-to-phase (AM-PM) characteristic. When input phase mismatch (IPM) is introduced, i.e., the main and auxiliary branches are not phase-aligned, the AM-PM of the PCN can be further finely tuned. By choosing proper OPM and IPM, the AM-PM of the overall DPA can be compensated by that of the PCN while maintaining reasonable back-off and saturated performances. Moreover, the PCN with phase mismatch shows gain expansion, and thus the amplitude-to-amplitude (AM-AM) distortion of the DPA can also be improved to some extent. A fully integrated DPA is implemented in a 0.25 mu m gallium nitride (GaN)-HEMT process to validate the proposed method. The fabricated DPA realizes an AM-PM of 2 degrees and an AM-AM of 0.3 dB at 6.3 GHz, with a saturated power of 41.1 dBm and a 6 dB back-off drain efficiency (DE) of 45%. Applying a 200 MHz signal with a 7.8 dB peak-to-average power ratio (PAPR), a raw adjacent channel power ratio (ACPR) of -42 dBc and an average DE of 37.4% are measured at the output power of 33.1 dBm. When the carrier frequency is swept from 6.1 to 6.5 GHz, a raw ACPR below -39 dBc and an average DE better than 37% are maintained.