A 24–28-GHz GaN MMIC Synchronous Doherty Power Amplifier With Enhanced Load Modulation for 5G mm-Wave Applications

In this article, a load-modulation enhanced wideband compact high-efficiency millimeter-wave (mm-wave) gallium nitride (GaN) monolithic microwave integrated circuit (MMIC) synchronous Doherty power amplifier (DPA) is presented. A synchronous DPA architecture is used to ensure the optimal saturation performance at the mm-wave band. Based on the analysis of load-modulation behavior, the bandwidth of the DPA can be extended by decreasing the phase dispersion factor $\alpha $ . A simple optimal tuning-based method is proposed to design an equivalent $\lambda $ /4 transmission line (EQWTL) with the minimal $\alpha $ for a given topology. Furthermore, the influence of the power division ratio $\sigma $ on the proposed DPA's load-modulation behavior has also been analyzed and a proper load modulation can be realized by splitting more power to the peaking branch properly. For verification, a 24–28-GHz GaN MMIC synchronous DPA has been designed using a 0.15- $\mu \text{m}$ GaN on silicon carbide high-electron-mobility transistor process. Both the load modulation and the matching of the fundamental and the second harmonic load impedances can be realized by a modified bandpass-type EQWTL with a minimal $\alpha $ easily. Experimental results show that the fabricated DPA can achieve the output power of 35.4–36 dBm and the power-added efficiency (PAE) of 27.8%–36.8% at saturation. The PAE at 6-dB power back-off (PBO) is 18.3%–30.1% and the 9-dB PBO PAE is higher than 14.3% in the whole frequency band. An average PAE of 27.4% with good linearity is obtained when excited by a 400-MHz modulated signal after linearization.