The Doherty power amplifier (DPA) has been extensively explored in the past and has become one of the most widely used power amplifier (PA) architectures in cellular base stations. The classical DPA suffers intrinsic bandwidth constrains which limit its application in future 5G wireless transmitters. In this paper, we present a comprehensive review of the DPA bandwidth enhancement techniques proposed in literature in order to provide a thorough understanding of the DPAs broadband design for high-efficiency 5G wireless transmitters. We elaborate on the main bandwidth limitation sources and provide circuit design insights. We then follow with an overview of bandwidth enhancement techniques developed for the DPA, including modified load-modulation networks, frequency response optimization, parasitic compensation, post-matching, as well as distributed DPA, dual-input digital DPA, transformer-based power-combining PA, and transformer-less load modulated PA architectures. Furthermore, challenges and design techniques for integrated circuit (IC) implementation of broadband DPAs are discussed, including a review of circuits developed in CMOS, SiGe, and GaN processes, and operating in RF and mm-Wave frequencies.
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