Outphasing power amplifiers

Abstract

This chapter presents a comprehensive introduction to the outphasing architecture, with particular focus on nonisolating outphasing techniques. When efficiency at output power back-off is required, the outphasing technique presents a significant advantage over conventional single-ended PAs. This performance benefit comes, as is typical in efficiency enhancement architectures, at the expense of complexity, linearity, and bandwidth. Nonetheless, when digital predistortion and the associated complexity can be tolerated, outphasing is clearly beneficial for systems using high peak-to-average power ratio signals. A direct comparison to other efficiency enhancement techniques is made complicated by the large trade-off space in efficiency, linearity, complexity, and so on. It is of interest, however, to consider a comparison to the popular Doherty PA (DPA). The main difference between these two techniques lies in how the branch PAs are operated. In outphasing systems, the branch PAs are driven identically, with both branch PAs operating in saturation in the outphasing regime and in back off at low output power levels. In Doherty PAs, on the other hand, in the higherpower regime only the main PA is operated at full voltage swing, while in back-off the auxiliary PA is completely off. As a result, at high output powers the outphasing PA has theoretically higher achievable efficiency, and better utilization of the device periphery at low power levels.