A Doherty Power Amplifier with Extended Bandwidth and Reconfigurable Back-off Level

Emerging wireless standards are designed to be spectrally efficient to address the high cost of licensing wireless spectra. Unfortunately, the resulting signals have a high peak-to-average ratio that reduces the base station power amplifier efficiency at the back-off power level. The wasted energy is converted to heat that degrades the device reliability and increases the base-station’s carbon footprint and cooling requirements. In addition, these new standards place stringent requirements on the amplifier output power, linearity, efficiency, and bandwidth. To improve the back-off efficiency, a Doherty amplifier, which uses two device in parallel for back-off efficiency enhancement, is deployed in a typical base station. Unfortunately, the conventional Doherty amplifier is narrowband and thus cannot satisfy the bandwidth requirement of the modern base station that needs to support multiple standards and backward compatibility. In this thesis, we begin by studying the class F/F−1 high efficiency mode of operation. To this end, we designed a narrowband, harmonically-tuned 3.3 GHz, 10 W GaN high efficiency amplifier. Next, we investigate how to simultaneously achieve high efficiency and broad bandwidth by harnessing the simplified real frequency technique for the broadband matching network design. A 2 to 3 GHz, 45 W GaN amplifier and a 650 to 1050 MHz, 45 W LDMOS amplifier were designed. Finally, we analyze the conventional Doherty amplifier to determine the cause of its narrow bandwidth. We find that the narrow bandwidth can be attributed to the band-limited quarter-wave transformer as well as the widely adopted traditional design technique. As an original contribution to knowledge, we propose a novel Doherty amplifier configuration with intrinsically broadband characteristics by analyzing the load modulation concept and the conventional Doherty amplifier. The proposed amplifier uses asymmetrical drain voltage biases and symmetrical devices and it does not require a complex mixed-signal setup. To demonstrate the proposed concept in practice, we designed a 700 to 1000 MHz, 90 W GaN broadband Doherty amplifier. Moreover, to show that the proposed concept is applicable to high power designs, we designed a 200 W GaN broadband Doherty amplifier in the same band. In addition, to show that the technique is independent of the device technology, we designed a 700 to 900 MHz, 60 W LDMOS broadband Doherty amplifier. Using digital pre-distortion, the three prototypes were shown to be highly linearizable when driven with wideband 20 MHz LTE and WCDMA modulated signals and achieved excellent back-off efficiency. Lastly, using the insights from the previous analyses, we propose a novel mixed-technology Doherty amplifier with an extended and reconfigurable back-off level as well as an improved power utilization factor. The reconfigurability of the proposed amplifier makes it possible to customize the back-off level to achieve the highest average efficiency for a given modulated signal without redesigning the matching networks. A 790 to 960 MHz, 180 W LDMOS/GaN Doherty amplifier demonstrated the extended bandwidth and reconfigurability of the back-off level. The proposed amplifier addresses the shortcomings of the conventional Doherty amplifier and satisfies the many requirements of a modern base station power amplifier. iii Acknowledgements First and foremost, I want to thank my parents Ruby Pao-Chen and Teng-Tsan for encouraging me to follow my interests and to always be driven by curiosity. I also want to thank my sisters Maxine and Julie for their support and encouraging words. Also, this thesis would not have been possible without the support of my girlfriend Xenia Kant who filled my life with laughters. I’ve been blessed to be part of an amazing research group. I would like to thank my supervisor Professor Slim Boumaiza for identifying my thesis topic as an important area of research and for providing feedback throughout the years. I’m especially grateful to have access to many stateof-the-art equipments without which many ideas in this thesis would’ve never been realized in practice. I also want to thank everyone in the EmRG research group. A special thanks to Hassan Sarbishaei for all the engaging technical discussions throughout the years. I will no doubt miss the invigorating discussions that greatly enhanced my PhD experience. I’d also like to thank Farouk Mkadem, Houssem Medini, and Bilel Fehri who took the time to help me with the DPD linearization of the amplifiers. I also want to thank Traian Antonescu who fabricated the printed circuit boards with care and great attention to detail. I’d like to thank Rogers Corp. for providing the PCB substrate samples, ATC Ceramics Corp. for providing the capacitor samples, Cree Inc. and Freescale Semiconductor Inc. for providing the GaN and LDMOS devices and their large signal models, and Ericsson Inc. for providing feedback on this research. Lastly, I’d like to thank Professor Raafat Mansour, Professor David Nairn, Professor Patricia Nieva, and Professor Carlos Saavedro from Queen’s University for reviewing this thesis. I must also thank Canadian taxpayers who sponsored my research through NSERC. This research would not have been possible without your support.