4.2 A Broadband Switched-Transformer Digital Power Amplifier for Deep Back-Off Efficiency Enhancement

Sophisticated OFDM modulation schemes with high spectrum efficiency and data throughput in modern wireless communication systems often result in a large peak-to-average power ratio (PAPR). Besides, wireless standards like LTE, WLAN, NB-IoT, etc., require wide transmission power range to accommodate various communication environments, and devices often function at low average output power. For better battery lifetime, it is critical to improve the power amplifier (PA) efficiency at deep power back-off (PBO) levels (e.g., 12/18dB or higher). Recently, several digital-style techniques have been employed to enhance PA PBO efficiency, such as dynamic power control [1], Class-G, and Doherty [2-4], as well as multilevel outphasing [5]. Class-G or Doherty techniques usually provide an efficiency peaking at 6dB PBO, and when combined together [2,3] or cascaded [6] they can further enhance the efficiency beyond 6dB PBO by introducing two efficiency peaks at 6/12dB PBOs. However, most of the Class-G Doherty PAs suffer from large area overhead with two power supply paths and glitches due to mode transitions. The dynamic power control or multi-level outphasing PA requires multiple phase modulators and amplitude-level transitions, which cause inherent discontinuities and degrade the linearity. In this work, a switched-transformer digital-PA technique is proposed for wide-range PBO efficiency enhancement. This topology does not require multiple power supplies and does not introduce AM/PM discontinuities. The PA achieves multiple efficiency peaks at 0/6/12/18dB PBOs and wide frequency coverage with a single-transformer footprint and only one supply voltage.