Analog Linearization of Highly Efficient Supply-Modulated Power Amplifiers

Licentiatavhandling, 2025

The increasing demand in the global wireless data traffic imposes a set of new requirements and challenges for the development of the wireless communication infrastructure. At microwave frequencies the Power Amplifier (PA) tends to suffer from increased Radio Frequency (RF) losses and a reduction in output power levels, effectively yielding reduced energy efficiency. Thus, it is crucial to investigate efficiency enhancement techniques to improve the back-off efficiency. 

In the first part of this thesis the concept of supply modulation is introduced. An analysis of the benefits and drawbacks of various supply modulator topologies is discussed. Based on this analysis two reconfigurable laboratory-grade supply modulator implementations are presented. The first design employs a push-pull output stage, while the second design exhibits a parallel load-sharing output stage topology. The manufactured Printed Circuit Boards (PCBs) achieve a high half-power (3-dB) bandwidth of 130 MHz with a maximum linear output current exceeding 2.5 A. 

The second part of this thesis discusses a novel analog phase correction method for a K-band (18 – 27 GHz) 4 W PA in a 150 nm GaN-on-SiC technology, implemented with a custom tuneable phase modulator Monolithic Microwave Integrated Circuit (MMIC) directly in the PA RF input path. The phase modulator exhibits an inverse response compared to the PA for linearization. Modulated measurements are performed with a 33.75 MHz 16-QAM input signal with 6 dB Peak-to-Average Power Ratio (PAPR) at a frequency of 20.5 GHz. With supply modulation the average Power-Added Efficiency (PAE) is improved by 6.5 – 9.5 percentage points (pp) for a variety of tracking functions. The PA output phase variation is reduced from 30° to 14° with phase correction. Simultaneously, the Adjacent Channel Power Ratio (ACPR) improves by 2 – 5 dB, resulting in similar or even better linearity performance compared to a static 20 V bias, while a significantly higher average PAE is achieved. 

Overall, this thesis provides a comprehensive overview of the benefits and drawbacks of supply modulation for energy efficiency enhancement. The main findings contribute to the development of highly efficient and linear RF transmitters for future wireless communication systems.