Frequency Reconfigurable and Linear Power Amplifiers Based on Doherty and Varactor Load Modulation Techniques

Licentiate thesis, 2016

In future mobile communication networks, there will be a shift towards higher carrier frequencies and highly integrated multiple antenna systems. The system performance will largely depend on the available RF hardware. As such, RF power amplifiers (PAs) with improved efficiency, linearity, and bandwidth are needed. Dynamic load modulation (DLM) is one of the most common PA efficiency enhancement techniques. By investigation of new DLM design techniques, the overall objective of this thesis is to improve the efficiency-linearity and efficiency-bandwidth trade-offs in PAs for future wireless systems. In the first part of the thesis, a method for improving the frequency agility of varactor-based DLM PAs is proposed. It is demonstrated that class-J operation provides the possibility of enhancing the efficiency for a large dynamic range of powers by means of purely reactive load modulation across a large bandwidth. This allows for very simple realization of a varactor-based tunable output matching network. This ideal analysis of the class-J operation establishes a profound theory behind wideband capabilities of varactor-based DLM PAs. The theory is experimentally verified with a prototype PA using a GaN HEMT and SiC varactors. In the second part, a method for improving the efficiency-linearity trade-off in Doherty PAs is proposed. The fundamental way the main and auxiliary transistors in the Doherty PA interact with each other is analyzed and generalized. The output combiner is treated as a black-box and its parameters are solved for arbitrary current profiles for the main and auxiliary branches. Solving for maximum efficiency and scaling the conventional current ratios results in new solutions with significantly higher gain. Solving for linear gain and high efficiency, and combining current scaling with reactive mismatch results in the possibility of controlling the phase response in the high power region. This control can be used to compensate the severe inherent phase distortion in Doherty PAs. The theory is experimentally verified with a highly efficient and highly linear prototype PA using GaN HEMTs. The thesis has presented two promising techniques for improving the efficiency-bandwidth and efficiency-linearity trade-offs in PAs. The results will therefore contribute to the development of more energy efficient and high capacity wireless services in the future.