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°, 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 \text{m} gallium nitride (GaN)-HEMT process to validate the proposed method. The fabricated DPA realizes an AM-PM of 2° 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.