Voltage-Combined CMOS Doherty Power Amplifier Based on Transformer

Abstract

This paper presents a voltage-combined CMOS Doherty power amplifier (PA) based on a transformer. The dynamic load modulation of the two-way transformer is analyzed in detail. A comparison between the current- and voltage-combined Doherty PAs shows that the Doherty PA based on a voltage-combining method delivers a broad bandwidth (BW) with a larger load tolerance. The proposed Doherty PA achieves close to ideal load modulation without additional offset line by manipulation of the output capacitances. For demonstration purposes, the PA and output combining transformer are implemented using a 0.18- $\mu \text{m}$ CMOS process and an FR4 printed circuit board, respectively. The PA is tested at 880 MHz using a fully loaded long-term evolution signal with 16-quadrature amplitude modulation, 7.5-dB peak-to-average power ratio, and 10-MHz BW. The PA delivers a gain of 13.3 dB, a power-added efficiency of 47.4%, an error vector magnitude of 3.85%, and $\mathrm {E-UTRA_{ACLR}}$ of −33 dBc at an average output power of 27 dBm. This simple voltage-combined Doherty PA without any additional circuitry achieves good performance. These results show that the proposed CMOS Doherty PA is suitable for handheld PA applications.