Development of a high-efficiency power amplifier for envelope tracking applications

Complex and spectrally efficient modulation schemes present a power-efficiency challenge to base station power amplifiers due to the time-varying envelope and high peak-to-average power ratios involved. The envelope tracking architecture is one way to address this issue, where an envelope amplifier provides a dynamic, modulated supply to the power amplifier to reduce power consumption. While most research into envelope tracking focuses on the envelope amplifier, this work focuses on optimising the power amplifier design for envelope tracking using the waveform engineering approach. It studies the behaviour of a highly-efficient power amplifier mode of operation (class-F) using a relatively low-cost high voltage laterally diffused metal oxide semiconductor (HVLDMOS) technology in an envelope tracking environment. A systematic design process is formulated based on identifying the optimum amplifier load and the envelope shaping function, and then applied in the development of an actual class-F power amplifier. The fabricated power amplifier is integrated into an envelope tracking system and is able to produce one of the highest recorded efficiencies compared to current state-of-the-art envelope tracking amplifiers, which are mostly based on Gallium Nitride technology. The limitation of this design is its linearity performance, and the efficiency-linearity trade-off is analysed in detail in this work. The use of continuous mode power amplifiers in envelope tracking is also explored for high-bandwidth operation. The limitation of such a technique is posed by the device nonlinear output capacitance, and this is analysed through the use of a novel characterisation approach called voltage-pull, which is derived from an active load-pull system but uses voltage waveforms as the target instead of loads. This method is also used to investigate the possibility of exploiting the device nonlinear output capacitance as a 2nd harmonic injection source to improve power amplifier efficiency, as predicted in a novel mathematical analysis presented in this work