A Study on Efficiency and Linearity Enhancement of Differential Power Amplifier for Mobile Applications

Abstract

A Study on Efficiency and Linearity Enhancement of Differential Power Amplifier for Mobile Applications Sungwoon Hwang Department of Electrical and Electronic Engineering The Graduate School Hanyang University This dissertation presents a comprehensive study on enhancing efficiency and linearity in RF power amplifiers (PA) based on a differential structure. With the advancement of mobile communication generations, the allocation of high- frequency bands poses network coverage challenges, while the increasing complexity of signals aimed at achieving higher data rates leads to signal distortion. To address these issues, providing high output power and linearity in the RF transmitter is crucial in handset applications. The PA plays a vital role in the RF transmitter, consuming the most power from the battery and significantly impacting the overall output power and signal linearity. As signal complexity and bandwidth increase, the peak-to-average power ratio (PAPR) of the signal also increases. Therefore, PAs face the challenge of achieving high efficiency to maximize battery life in mobile handset devices while meeting the demands for high output power and linearity. Furthermore, there is a growing need for broadband PAs to reduce the usage of multiple PAs due to the size challenge of front-end modules. As a result, PAs should meet high-performance requirements with enhanced linearity and efficiency in backed-off power using various modulation schemes such as the fifth- generation new radio (5G NR) and sixth-generation Wi-Fi (802.11ax) standard signals. This dissertation delves into three main studies that aim to improve linearity and efficiency and the studies are verified the PAs in the 5 GHz and 6 GHz bands, which are the edge frequencies of the InGaP/GaAs heterojunction bipolar transistor (HBT) process. Firstly, to improve efficiency at back-off output power, a switchless dual-power- mode (DPM) PA based on a fully differential structure is presented. The switchless DPM PA operates independently in high-power mode (HPM) and low-power mode (LPM) and is constructed using a proposed bidirectional output matching structure. This structure utilizes a transformer (TF) to optimize the impedance for each operating mode and provides high resistance in the non-operating mode, eliminating the need for switches. The mathematical analysis and implementation methods of the bidirectional TF are described, including the use of a parallel capacitance and variation of transistor size. This ensures high passive efficiency in the operating mode and low losses in the non-operating mode. As a result, the proposed DPM PA requires only additional small area for the LPM bidirectional TF compared to a single-mode PA. The proposed PA is tested for error vector magnitude (EVM) using a 160 MHz 64-QAM signal. Secondly, a TF-based second harmonic trap network is presented to improve the linearity and efficiency of differential PAs. The proposed TF-based second harmonic trap selectively terminates the second harmonic without affecting the fundamental frequency. Unlike general second harmonic traps that use series inductors and capacitors in parallel with transistors, which increase the transistor output capacitance (𝐶 ), the proposed trap is implemented using a multi-coupled TF on a multi-layer PCB. This trap only represents a short circuit for the second harmonic. The proposed second harmonic trap is verified with 5-GHz band PA and linearity is tested using a new radio (NR) 100 MHz QPSK CP 273 RB signal with a PAPR of 10.7 dB. Finally, a novel dynamic cross-coupled capacitor (𝐶 ) is proposed to improve linearity and efficiency at the average output power region. The dynamic 𝐶 compensates for the nonlinear intrinsic parameters of transistors according to the output power of the PA, thereby enhancing linearity in the large signal region. The dynamic 𝐶 technique is verified in two PAs operating in the 5.15-5.925 GHz and 5.15-7.125 GHz bands. Additionally, a multi-coupled transformer (MCT) is proposed as a broadband 4-way matching method in the 5.15-7.125 GHz band, suitable for implementation on a multi-layer PCB with low insertion loss. The MCT structure allows for the implementation of a broadband TF with a low turn ratio while maintaining high symmetry in the PA. The methods for implementing 4-way PAs in the 5.15-5.925 GHz and 5.15-7.125 GHz bands are presented, and PAs capable of achieving high linear output and efficiency are derived through compensated linearity using dynamic 𝐶 . The 5.15-5.925 GHz band PA is verified using an NR 20 MHz CP signal with a PAPR of 10.7 dB. The 5.15-7.125 GHz band PA is tested with various signals, including NR 100 MHz CP and DFT signals with PAPRs of 11.4 dB and 8 dB, as well as an 802.11ax MCS11 80 MHz signal with a PAPR of 10.7 dB. Keywords: Cross-coupled capacitor, differential, efficient, heterojunction bipolar transistor (HBT), linearity, multi-mode, power amplifier, transformer.