Analytical Approaches to Load Modulation Power Amplifier Design
Sammanfattning: In future mobile communication networks, there will be a shift toward higher carrier frequencies and highly integrated multiple antenna systems. The system performance will largely depend on the available radio frequency (RF) hardware. As such, RF power amplifiers (PAs) with improved performance, e.g. energy efficiency, are needed. Active load modulation (ALM) is one of the most common PA efficiency enhancement techniques. Unfortunately, different ALM techniques come at the cost of degrading other PA attributes. Through investigation of new ALM design techniques, the overall objective of this thesis is to improve upon different attributes and performance trade-offs in ALM PAs for future wireless systems. The working principle of ALM PAs is determined by both how the individual transistors are operated and how their outputs are combined. In the first part of the thesis, an analytical approach, where the output combiner is assumed to be an arbitrary black-box, is applied to the Doherty PA. The fundamental interaction between the main and auxiliary transistors is analyzed and generalized. New solutions with improved performance are identified, such as higher gain and an improved efficiency-linearity trade-off. This approach also introduces improved integration possibilities, which are demonstrated by a transmitter where the antenna acts as both the radiator and the Doherty combiner. Additionally, the analytical approach is applied to an isolated two-way power divider. This unlocks many new possibilities, such as improved integration and layout flexibility. In the second part, one embodiment of the emerging ALM architecture, the load modulated balanced amplifier (LMBA), is proposed: the RF-input Doherty-like LMBA. Design equations are derived and the fundamental operation is studied. This variant presents several advantages over other known architectures, such as higher gain and device periphery scaling of the different transistors. The third part proposes a new measurement-based ALM PA design procedure, which emulates the full behavior of the transistors in any ALM architecture using active load-pull measurements. This method can predict the intricate behavior in ALM PAs and it gives measurement-based insights into the internal operation of the circuit already at the design stage. This facilitates the design for optimal ALM PA performance. The thesis contributes with several promising techniques for reducing performance trade-offs and improving the overall performance of ALM PAs. Therefore, the results will contribute to the development of more energy efficient and high capacity wireless services in the future.
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