A Coupler-Based Differential mm-Wave Doherty Power Amplifier With Impedance Inverting and Scaling Baluns

Abstract

Designing Doherty power amplifiers (PAs) at high millimeter-wave (mm-Wave) range remains a major challenge because most Doherty combiners at this frequency band are inefficient. To resolve this bottleneck, we analyze balun response under load variation and subsequently propose the Doherty PA architecture with impedance inverting and scaling baluns that perform active load modulation on differential PAs and delivers combined power to a single-ended load. At the PA output/input stage, instead of utilizing the transformer-based balun commonly used at RF and low mm-Wave frequencies, we explore the coupler-based balun to resonate out the device parasitic capacitance and achieve well-balanced differential to single-ended conversion at high mm-Wave frequencies. Based on the coupled-line theory and our analysis, we derive the [Y] matrix of the coupler-based balun and originate various solutions for baluns that can invert or scale impedance. From this derivation, we conceive a canonical form of impedance inverting baluns when the electrical length of coupled lines equals 45°. We also propose a close-formed solution for the coupler-based impedance scaling balun that is electrically equivalent to an idealistic transformer in series with a small inductance. We then combine the proposed impedance inverting and scaling balun structures to construct a Doherty network that exhibits desirable Doherty active load modulation and occupies a single-transformer footprint. On the active circuit design, we employ the adaptive biasing circuit for both driver and PA stages of the auxiliary PA to enhance the main and auxiliary cooperation. Using a 45-nm GlobalFoundries CMOS SOI process, we fabricate a proof-of-concept Doherty PA that demonstrates 20.1-dBm $P_{\mathrm {sat}}$ , 19.3dBm OP1,dB, 26% peak power-added efficiency (PAE), and 16.6% PAE at 7-dB power back-off (PBO) from OP1 dB at 60 GHz. Our results indicate a substantial PAE enhancement at PBO and a state-of-the-art modulation efficiency achieved when transmitting digital pre-distortion (DPD)-free 64-QAM-modulated signals.