A MM-Wave Current-Mode Inverse Outphasing Transmitter Front-End: A Circuit Duality of Conventional Voltage-Mode Outphasing

This article presents a millimeter-wave (mm-wave) transmitter (TX) front-end employing a new inverse outphasing architecture with current-mode power amplifiers (PAs). This study on the conventional outphasing operation reveals its strong dependence on its voltage-mode driving source assumption and thus exhibits major limitations at mm-wave, since efficient and linear mm-wave PAs often behave as current sources. To address this challenge, we propose a current-mode inverse outphasing architecture that employs current-mode driving sources and is inherently compatible with mm-wave linear PAs. Moreover, the conventional series outphasing combiner is replaced by a much simpler and low-loss parallel power combining scheme. Closed-form mathematical expressions are derived for both the outphasing and inverse outphasing operations, including the active load modulation, Chireix compensations, and outphasing efficiency, which fundamentally explains the limitations of the conventional approach and the advantages of the inverse outphasing architecture. Further theoretical analysis discovers a circuit duality relationship between outphasing and inverse outphasing architectures, which offers unique circuit intuitions. As a proof-of-concept, we implement a 28-GHz TX front-end using cascode PAs, based on the inverse outphasing architecture. In the measurement, the TX achieves 22.7-dBm saturated output power with 42.6% PA drain efficiency. The TX exhibits effective PA efficiency enhancement at 6-dB power back-off (PBO), validating the high-efficiency inverse outphasing active load modulation. Modulation tests with high-order quadrature amplitude modulation (QAM) signals demonstrate that the TX supports >10-Gb/s high-fidelity complex signals and achieves the state-of-the-art modulation performance.