Power amplifiers (PAs) have always been a pivotal front-end building block in wireless communication transmitters. As signal modulation schemes become more spectrally efficient and cellular traffic increasingly intense, the radio frequency (RF) performance of PAs should be constantly improved. This can be achieved by improving the analog circuit design of the PA and by including additional software and system features to balance energy consumption and performance.
This thesis focuses on several aspects related to power efficiency enhancement and bandwidth extension of wireless Doherty PAs using baseband digital signal processing (DSP) techniques. A variety of digital signal conditioning algorithms are proposed to enable optimal Doherty PA operation at and beyond its nominal frequency bandwidth. An innovative dual-input Doherty PA architecture is also presented in this work to enable the implementation of these advanced DSP techniques.
Analog circuit design and optimization approaches are investigated and proposed for the enhancement of Doherty PA performance. Experimental implementation is carried out to validate the proposed techniques. Critical issues related to complex gain imbalance as well as energy waste within the RF building blocks of the Doherty PA are studied and mitigated. Two advanced methodologies, namely digital adaptive phase alignment and digital adaptive input power distribution, are developed to effectively address the above-mentioned problems.
The frequency response of Doherty PAs is analyzed; and, a novel digital domain based precompensation mechanism is derived to mitigate the bandwidth limitations of Doherty PAs, resulting in substantial bandwidth extension. An original architecture of a digitally equalized Doherty PA based RF front-end is proposed to allow the use of Doherty PAs in the context of wide bandwidth and multistandard radios.