A Digital Power Amplifier With Built-In AM–PM Compensation and a Single-Transformer Output Network

Abstract

This paper presents a digital power amplifier (DPA) with a built-in AM-PM compensation technique and a compact single-transformer footprint. The AM-PM distortion behavior of the current mode/voltage mode PAs are detailed and an AM-PM compensation technique for both modes is introduced. The proposed design utilizes one current-mode digital power amplifier as the main path PA and a class-G PA voltage mode digital PA as the auxiliary path PA, combined through a single transformer footprint. It provides enhanced linearity through built-in adaptive biasing and hybrid current/voltage mode Doherty-based power combing. As a proof-of-concept, a 1.2-2.4 GHz wideband DPA is implemented in the Globalfoundries 45nm CMOS SOI process. The measurements show a 37.6% peak drain efficiency (DE) at 1.4GHz, and 21.8dBm saturated output power (Psat) and 1.2×/1.4× PBO efficiency enhancement, compared to the ideal class-B at 3dB/6dB PBO at 1.2GHz. This proposed digital PA supports 20MSym/s 64-QAM modulation at 14.8dBm average output power and 22.8% average PA DE while maintaining error vector magnitude (EVM) lower than -23dB without any phase predistortion. To the best of our knowledge, this is the first demonstration of hybrid current-voltage mode Doherty power combining on a single footprint transformer over a broad bandwidth (BW).