A Millimeter-Wave Two-Way Series-Doherty Power Amplifier With Transformer-Based Second-Order Output Network

This article presents a two-way series-Doherty power amplifier (PA) for fifth-generation (5G) millimeter-wave (mm-wave) communications. The design integrates a transformer-based second-order output network, which replaces the quarter-wave transmission line (QTL) with a second-order$\pi $-network to achieve wideband operation and a compact footprint. To realize wideband quadrature signal generation, the proposed phase-insensitive quadrature hybrid coupler explicitly incorporates the effect of the inductor quality factor ($Q$factor) based on the ideal coupler model, achieving$I$/$Q$phase mismatch$P_{\text {sat}}$) of$20.2~\pm ~1$dBm, output 1 dB compression point (OP${}_{1{\,}\text {dB}}$) of$19.8~\pm ~1$dBm, peak PAE (PAE${}_{\max }$) of 24.8%$\pm ~2.4$%, and PAE at 6 dB power back-off (PBO) (PAE${}_{6{\,}\text {dB}}$) of 16.7%$\pm ~2.5$% from 25 to 30 GHz. Modulation measurements demonstrate average output power ($P_{\text {avg}}$) of 10.1/9.9 dBm and average efficiency (PAEavg) of 13.3%/13.1% for a 100/400 MHz single-carrier 64-quadrature-amplitude-modulation (QAM) signal with an error vector magnitude (EVM) below −24 dB at 28 GHz.