Efficient and Wideband Power Amplifiers for Wireless Communications

Doctoral thesis, 2012

The rapid evolution of wireless communication systems and the development of new standards require that wireless transmitters process several types of standards across multiple bands. Power amplifiers (PAs) are key components in wireless transmitters because they have a big impact on the overall system performance in terms of their bandwidth, efficiency, and linearity. This thesis presents various design techniques that improve bandwidth and efficiency characteristics of the PA. For narrowband transmitters, a circuit design methodology that enables first-pass design of high efficiency single-ended PAs is presented. The method, based on employing bare-die transistors, specialized modeling technique, and optimization of harmonic impedances, is validated with excellent experimental results. A class-F−1 PA at 3.5GHz and a harmonically tuned PA at 5.5GHz are designed and implemented demonstrating 78% and 70% PAE respectively. For broadband transmitters, a design methodology for single-ended PAs with octave bandwidth is presented and verified. The method is based on a harmonic tuning approach combined with a systematic design of broadband matching networks. The demonstrator PA achieves 50-63% PAE across 1.9- 4.3GHz. Then, extending the bandwidth beyond one octave while maintaining high efficiency is investigated by adopting a push-pull configuration. For this reason, a novel push-pull harmonic load-pull measurement setup is proposed and a push-pull PA operating between 1-3GHz is implemented. The investigation demonstrates the proposed setup as an important tool for understanding and optimizing PAs and baluns for wideband push-pull microwave PAs. For multi-band transmitters, using signals with large peak-to-average power ratio, the design of dual-band Doherty PAs (DPAs) is considered. A detailed analysis of each passive structure constituting the DPA is given, leading to different configurations to implement dual-band DPAs. One of the configurations is implemented, leading to state-of-the-art results for dual-band DPAs. Finally, the multi-band branch-line coupler (BLC) is a key component for also extending the design of DPAs to multi-band in the future. A closed form design approach for multi-band BLCs operating at arbitrary frequencies is presented and validated by the successful design of dual-band, triple-band, and quad-band BLCs. The excellent results obtained demonstrate the success of the developed design methodologies for high efficiency and multi-band/wideband PAs. These methods will contribute to the design of future wireless systems with improved performance in terms of efficiency, bandwidth and hence cost.