A 42–62 GHz Transformer-Based Broadband mm-Wave InP PA With Second-Harmonic Waveform Engineering and Enhanced Linearity

Indium phosphide (InP) heterojunction bipolar transistors (HBTs) with $f_{t}/f_{\max}$ of 350/675 GHz are studied and explored for a linear, high efficiency and broadband power amplifiers (PAs) at mm-wave frequencies. Unlike the conventional transmission-like-based design, this article presents a compact, broadband transformer-based power combining and impedance matching using the waveform engineering approach. We present, for the first time, mm-wave (40–60 GHz) InP topologies incorporating the following: 1) on-chip transformer for broadband, efficient and compact impedance matching and power combining; 2) synthesis of optimal second-harmonic impedance through transformer center tap to achieve high-efficiency differential PA operation; and 3) biasing techniques to reduce AM–PM distortion for linearity enhancement. This work reports a transformer-based push–pull InP PA in $0.25~\mu \text{m}$ technology across 42–62 GHz demonstrating a peak power added efficiency (PAE) of 39.5% and peak $P_{\text{sat}}$ of 20.6 dBm. The PA supports 4 GHz bandwidth at 52 GHz with an EVM of −22.9 dB and an adjacent channel leakage ratio (ACLR) of −32 dBc for an 8 Gb/s QPSK signal at 13.3 dBm average output power. This work presents one of the highest efficiency with wide bandwidth and highest linearity mm-wave PAs in integrated technology.

I. Introduction

The 5G new radio (NR) band allocation is primarily divided into two frequency ranges: FR1, which operates below 6 GHz (sub 6 GHz), and FR2, which includes bands above 24 GHz extending into the frequency range above 50 GHz. There is an increasing interest in making wide use of the mm-wave frequency spectrum, specifically the U-band (40–60 GHz) and the V-band (50–75 GHz) for high-data-rate wireless communication, including point-to-point and wireless back-haul radio link applications [1]–[3]. Existing silicon device technologies face inherent limitations in terms of low breakdown voltages, power density, and relatively lower [4]–[7]. On the contrary, within III–V technologies, gallium nitride (GaN) offers promising power density but is often limited in terms of gain above 40 GHz [8]. In contrast to CMOS, SiGe, and GaN, the indium phosphide (InP) heterojunction bipolar transistor (HBT) technology offers promising performance advantage in terms of output power, efficiencies, and gain above 40 GHz due to better Johnson figure of merit (FOM), high RF output power density, large intrinsic transconductance, excellent threshold control and high levels of integration [9].