Design of High-Efficiency Outphasing Power Amplifiers for Wireless Communications

Abstract

5G and future 6G wireless communications have extensively implemented advanced modulation schemes that result in baseband signals with high peak-to-average power ratios (PAPRs). The reason is to increase the throughput and improve spectrum efficiency. These modulated baseband signals have further exacerbated the challenges in the design of radio-frequency (RF) power amplifiers (PAs), which need to operate at back-off with high efficiency. Outphasing power amplifiers (OPAs) based on the load modulation technique are promising techniques to overcome these challenges because of their high efficiencies at back-off. However, conventional OPAs suffer from narrow operating bandwidth, limited output back-off (OBO) range, complex design procedures and poor linearity. To overcome these limitations, four novel OPAs covering the popular wireless communication n41, n78 and n79 bands are proposed in this thesis.

Chapter 1 covers background, motivation, contribution and organization of the thesis. Operating bands of conventional OPAs are limited by the quarter-wavelength transmission lines in the Chireix combiners. Therefore, Chapter 2 proposes a new design methodology using a generalized combining network (GCN) to realize a dual-band OPA. The proposed GCN consists of a post-matching network and two output matching networks, rather than the Chireix combiner. Two commercial 10-W gallium nitride (GaN) high electron mobility transistor (HEMT) CG2H40010F devices are used in this OPA resulting in output measured peak powers of 44.3-44.9 and 43.5-43.8 dBm. The implemented OPA achieves drain efficiencies (DEs) of 64.1%-70.5% and 51.6%-56.9% at saturation and 40.3%-53% and 40%-44.5% at 6-dB back-off over the 3.45-3.55 and 4.75-4.85 GHz, respectively.

Although an efficient dual-band OPA is realized in Chapter 2, it has a limited OBO range of 6 dB. Chapter 3 proposes a new asymmetric OPA architecture to extend the OBO range by introducing an asymmetric output power for outphasing power combining. To validate the proposal, an OPA is designed using a modified Chireix combiner to combine two asymmetric sub-PAs with synthesized harmonic terminated networks. Two commercial 10-W GaN HEMTs (CG2H40010F) are adopted to generate a peak output power of 44.1 dBm at 2.58 GHz. The implemented OPA achieves measured DEs of 60.6% and 60% at saturation and at the extended 8-dB back-off under continuous wave excitation, respectively.

As the design of the modified Chireix combiner is found to be highly complex in Chapter 3, two new outphasing-like topologies using coupler-based power combining techniques are proposed in Chapter 4. Theoretical analysis provides a straightforward realization of OPAs without additional reactive compensation elements based on the output coupler, thereby increasing the bandwidth and significantly simplifying the OPA design procedures. For validation, outphasing-like PAs (OLPAs) I and II are designed using output branch-line and rat-race couplers with synthesized direct-matching networks for high efficiency, respectively. By adopting two 6-W GaN HEMT (CGH40006P) devices, the implemented OLPA-I with peak output powers of 39.7-39.8 dBm achieves measured DEs of 61.3%-64.1% at saturation and 42%-48% at 6-dB back-off over 3.4-3.5 GHz. The implemented OLPA-II with two 10-W HEMTs (CG2H40010F) delivers peak output powers of 43.5-44.3 dBm over 3.2-3.6 GHz, achieving DEs of 61.8%-70% at saturation and 41.5%-51.1% at 8-dB back-off.

In addition to the OLPAs presented in Chapter 4, an outphasing energy recovery amplifier (OPERA) is proposed and is another attractive topology with improved linearity that also uses a coupler for power combining. The rectifier used in the OPERA is used to replace the isolation resistor, thus recovering otherwise wasted power to boost system efficiency. However, rectifier performances are generally limited at high input power in the RF band. A load-modulated OPERA using a complex-impedance rectifier is proposed with modulated load impedances introduced to further enhance efficiency. For validation, an OPERA system using two 6-W GaN HEMTs (CGH40006P) and Schottky diodes (HSMS-2822) is implemented to deliver a peak output power of 39.4-39.8 dBm over 3.35-3.5 GHz, with measured efficiencies of 65.7%-70.4% at saturation and 40%-42.2% at 6-dB back-off. The implemented OPERA also achieves a low adjacent channel power ratio (ACPR) of below -37.2 dBc at 3.5 GHz without digital pre-distortion, demonstrating high linearity suitable for wireless communications.

Date of Award	24 Jun 2026
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Wing Shing CHAN (Supervisor)