A Millimeter-Wave Four-Way Doherty Power Amplifier With Over-GHz Modulation Bandwidth

This article presents the design and analysis of a millimeter-wave (mmWave) four-way Doherty power amplifier (PA), aiming to enhance the PA energy efficiency when amplifying orthogonal frequency-division multiplexing (OFDM)-based 5G new radio (NR) signals with a 10-12-dB peak-to-average power ratio (PAPR). We first introduce a systematic approach to extending a conventional two-way Doherty PA to N ways, followed by a new transformer-based N-way Doherty network synthesis flow. The proposed network synthesis achieves N-way Doherty load modulation using ( N - 1) transformers, one fewer transformer and thus lower loss than conventional designs. In addition, it enables the desired impedance transformation from RANT to ROPT and effectively absorbs the parasitic capacitance of the power cells. Along with the Doherty network, we also introduce a high-speed adaptive biasing circuit, addressing the modulation bandwidth bottleneck in prior Doherty PA demonstrations. As proof of concept, a four-way Doherty PA prototype is implemented in the 47-GHz 5G band (band n262) using the GlobalFoundries 45-nm CMOS silicon-on-insulator (SOI) process. It achieves 24.0-dBm saturated power ( PSAT), 23.7-dBm output 1-dB compression point ( P1 dB), 26.8% peak power-added efficiency (PAEPEAK), 26.3% PAE at P1 dB (PAE1 dB), 21.7% PAE at 6-dB back-off (PAE6 dB), and 13.1% PAE at 12-dB back-off (PAE12 dB), demonstrating state-of-the-art performance. In the modulation tests, the PA achieves 14.1-dBm average output power ( Pavg) and 13.7% average efficiency (PAEavg) when amplifying a 2000-MHz 5G NR 64-QAM OFDM signal. To the best of our knowledge, this is the first silicon PA demonstration of 2000-MHz channel modulation bandwidth for 5G NR OFDM along with back-off efficiency enhancement up to 12-dB back-off.