A Super-Resolution Mixed-Signal Doherty Power Amplifier for Simultaneous Linearity and Efficiency Enhancement

Abstract

This article presents a mixed-signal Doherty power amplifier (MSDPA) architecture for simultaneous linearity and efficiency enhancement. The MSDPA comprises one analog power amplifier (PA) as the main PA and one binary-weighted digital PA as the auxiliary PA. The MSDPA input is a generic envelope-varying complex-modulated signal. Based on the realtime amplitude-modulated (AM) envelope, auxiliary digital PA weightings are dynamically turned-on to perform optimum Doherty load modulation for superior linearity and back-off efficiency. Moreover, quantization noise is largely suppressed by the mixed-signal Doherty operation and nonuniform quantization (NUQ), while spectral images are substantially reduced by the quasi-first-order hold (quasi-FOH) operation, which together achieves super-resolution over conventional digital PAs; this makes the MSDPA conducive to millimeter-wave (mm-Wave) or compound PA designs by obviating the need for a large effective number of bits (ENOB) on AM digital controls. As a proof of concept, a 3-bit MSDPA is implemented at 27 GHz in a 45-nm silicon-on-insulator (SOI) CMOS process. The prototype PA achieves 40.1% peak power-added efficiency (PAE), 23.3-dBm saturated output power (Psat), and 39.4% PAE for 22.4-dBm P1dB at 27 GHz in continuous-wave (CW) measurements. The PAE at 6-dB power back-off (PBO) is 33.1%, which corresponds to a 1.68× improvement over a normalized Class-B PA. With only three control bits, the MSDPA PA supports a 12-Gb/s 64-QAM signal at -24.5-dB rms error vector magnitude (EVM) and average Pout/PAE of +15.6 dBm/27.8% without digital predistortion (DPD).