2.8 A Class-G voltage-mode Doherty power amplifier

Abstract

In modern communication, wideband and high-spectral-efficiency modulation results in high peak-to-average power ratio (PAPR), up to 8 to 10dB. Well-known PA-efficiency-enhancement techniques, such as Doherty and outphasing, offer reduced efficiency improvement beyond 6dB back-off, limiting the efficiency enhancement obtainable with high PAPR modulation. Recent works have shown that a combination of different techniques can result in improved efficiency well beyond 6dB back-off. However, these combined techniques have come at a cost of glitches due to mode-transitions, when power supply voltage or load impedance undergo large variations at critical power levels. Switching between power supply voltages causes significant glitches, which degrade the EVM and ACPR of the transmitted signal. Reasonable EVM is achieved, by reducing the average output power so that power supply switching is less frequent. A “skipping window” technique is proposed in the work of Tai, et al. (2012) to skip high-frequency mode-transitions reducing overall glitching. While this improves the ACPR, the efficiency is degraded since there is no enhancement during a skipped transition. In this work, the authors demonstrate the combination of the voltage-mode Doherty (VMD) and Class-G switched-capacitor power amplifier (SCPA) techniques. Together, these techniques provide efficiency peaking at both 6 and 12dB backoff over a wide bandwidth without introducing the glitches present in previous works. The Class-G VMD with integrated matching network achieves a PAE of 24% and better than -32dB EVM for 256-QAM OFDM signals.