A Linear Millimeter-Wave GaN MMIC Doherty Power Amplifier With Improved AM-AM and AM-PM Characteristics

This article proposes a new method to improve the linearity of millimeter-wave (mm-wave) gallium nitride (GaN) Doherty power amplifier (DPA) while maintaining high efficiency. By analyzing the nonlinear characteristics of the Doherty output combining network (OCN) under various complex-valued load modulation and network parameter combinations, analytical design equations are derived to enable an independent and flexible manipulation of the OCN amplitude and phase distortions to effectively counteract the main transistor ones, thus resulting in high-linearity and high-efficiency mm-wave DPAs. For validation, the proposed method is applied to design an mm-wave GaN DPA in a 0.15 $\mu $ m GaN-on-SiC process. Measurements under continuous-wave (CW) excitation reveal that, over the frequency band of 25-26.5 GHz, the proposed DPA is capable of delivering a saturation output power ( $=P-out ) of 32.8-33.2 dBm with 8 and 6 dB back-off and saturation power-added efficiency (PAE) of 23.4%-27.2%, 29.1%-32.2%, and 33.7%-37.4%, respectively, while maintaining excellent linearity with small amplitude and phase distortion of < 1 dB and < 3 degrees , respectively. Due to this excellent linearity, when fed with a 64-quadrature amplitude modulation (QAM) signal with a modulation bandwidth of 400 MHz and a peak-to-average power ratio (PAPR) of 7.22 dB over 25-26.5 GHz, the DPA is able to deliver well-balanced efficiency-linearity performances with excellent average PAE of > 28% and root-mean-squared error vector magnitude (EVMRMS) of < 4.3% at an average P-out of 25.3-26.3 dBm without using any digital predistortion (DPD). In addition, when excited with a single-carrier 400-MHz 5G NR FR2 64-QAM signal with a large PAPR of 9.65 dB, DPA can provide good average PAE of > 20% and EVMRMS of < 4% without DPD.