Transformer-Based Predistortion Linearizer for High Linearity and High Modulation Efficiency in mm-Wave 5G CMOS Power Amplifiers

Abstract

This paper presents a new predistortion linearization technique for high linearity and high modulation efficiency in millimeter-wave (mm-wave) CMOS power amplifiers (PA) for fifth-generation (5G) mobile communications. Our proposed linearizer adopts a transformer-based (i.e., inductive) self-compensated predistortion network at the input of the PA whose amplitude-modulation to phase-modulation (AM–PM) response is opposite compared with the AM–PM response of a CMOS PA, resulting in an AM–PM cancellation effect. This proposed inductive linearization method mitigates the large gain reduction problem in traditional capacitor-based linearization approaches while consuming no extra dc power or without introducing additional control circuitry. As a result, a significant improvement in power-efficiency and linearity is achieved with high-order complex modulation signals. To validate the proposed linearization method, a PA prototype in 65-nm CMOS technology was fabricated and tested, and it exhibited $\vert $ AM–PM $\vert $ distortion at $P_{\mathrm {o,1\, dB}}$ over 4 GHz of bandwidth (27–31 GHz). At 28 GHz, the measured saturated $P_{\mathrm {o}}$ and peak power-added-efficiency (PAE) was 15.6 dBm and 41%, respectively, while achieving a 6-dB $P_{\mathrm {o}}$ back-off PAE of 25%. To assess PA’s large-signal performance for 5G communications, the prototype was measured with the 64-quadratic-amplitude modulation (QAM) signal at 2-Gb/s data rates at 28 and 30 GHz, and the PA achieves modulated-PAE of 18.2%/17.6% and average- $P_{\mathrm {o}}$ of 9.8 dBm/10 dBm, respectively, while maintaining $2\times $ improvement in comparison with the recently reported 28-GHz linear CMOS PAs. Also, the PA occupies a compact active-area of 0.24 mm2.