Broadband Doherty Power Amplifiers with Enhanced Linearity for Emerging Radio Transmitters

Abstract

The ever-increasing demand for utilizing wireless spectra has led to development of spectrally efficient radio systems. While these systems offer much higher data throughput, they employ more sophisticated modulation schemes, which result in wideband signals with high peak-to-average power ratios. These signal characteristics significantly complicate the design of RF transmitters, particularly power amplifiers, in terms of power efficiency and linearity requirements. Furthermore, upcoming wireless standards, such as long term evolution advanced (LTE-A) require adoption of carrier aggregation which incorporates multiple component carriers to yield aggregated channels of larger bandwidth (up to 100 MHz). On the other hand, the emerging systems are expected to support legacy standards with minimum area, cost, and power overhead, and thus call for highly-efficient linear broadband power amplifiers capable of efficiently amplifying concurrent modulated signals located over a broad carrier frequency range. This thesis focuses on Doherty power amplifiers (DPAs) with extended high-efficiency range, enhanced bandwidth and improved linearity as a solution for high-efficiency multi-band multi-standard transmitters. It addresses three major concerns associated with DPAs, namely, back-off efficiency, bandwidth, and linearity. The Thesis begins with a detailed theoretical analysis of two-way and three-way Doherty configurations from which the governing equations are derived. This is followed by a comprehensive study of bandwidth limitation in DPA variants. As the first contribution, it is shown that the two existing three-way Doherty structures, i.e., conventional and modified DPAs have inherently broadband characteristics and thus are promising solutions for multi-standard base station transmitters. As a proof of concept, a 30-W three-way modified Doherty amplifier was designed and implemented using packaged GaN transistors over 0.73-0.98 GHz. The prototype was successfully linearized under modulated signals with up to 20 MHz modulation bandwidth. To further improve the linearizability of the DPAs under wideband and multi-band modulated signals, this thesis investigates major sources of static and dynamic nonlinearity in two-way DPAs both at device and circuit levels and explores circuit techniques to mitigate them. Furthermore, the challenges of applying the Doherty technique for concurrent transmission of multiple modulated signals are tackled. The most significant contribution of this thesis is to develop a novel waveform engineering approach to designing ultrawideband DPAs. This approach completely reformulates the DPA's output combiner conditions in order to accommodate complex-valued load modulation. Moreover, it relaxes the harmonic termination requirements of the DPAs to further enlarge the Doherty design space, thereby enhancing the bandwidth. A 50-W waveform-engineered two-way DPA prototype was designed for 1.5-2.5 GHz range and was successfully linearized under intra- and inter-band carrier-aggregated signals with up to 600 MHz carrier spacing. Lastly, an input matching network design methodology is proposed for broadband DPAs. This methodology uses the novel concept of ``current contours'' to minimize the bandwidth, efficiency and linearity degradation of DPAs caused by device input non-idealities.