Theory and Design of Efficient Active Load Modulation Power Amplifiers

Licentiate thesis, 2021

The increasing demand for mobile data traffic has put new challenges and requirements for the development of the wireless communication infrastructure. The performance of the RF power amplifier (PA) is, in particular, of great importance, since it is the key building block for microwave transmitters in base stations and radio link equipment. The energy and bandwidth efficiency of the PA is vital for maximized channel capacity, reduced operational cost, and further integration. Among the efficiency enhancement techniques, active load modulation is one of the most widely used techniques. The overall objective of this thesis is to improve the average efficiency and bandwidth performance in active load modulation PAs for future wireless systems.

In the first part of the thesis, an analytically based combiner synthesis approach for the three-stage Doherty PA (DPA) is proposed and presented. A compact output combiner network, together with the input phase delays, is derived directly from transistor load-pull data and the PA design requirements. The technique opens up new design space for three-stage DPAs with reconfigurable high-efficiency power back-off levels. The utility of the proposed technique is demonstrated by the implementation of a 30-W gallium nitride (GaN) three-stage DPA prototype at a center frequency of 2.14 GHz. Measurement results show that the prototype circuit can linearly reproduce 20-MHz long-term evolution signals with 8.5- and 11.5-dB peak-to-average power-ratio (PAPR), providing average efficiencies of 56.6% and 46.8% at an average output power level of 36.8 and 33.8 dBm, respectively.

In the second part of the thesis, a novel PA architecture, the circulator load modulated amplifier (CLMA) is proposed and demonstrated. The CLMA is able to maintain high efficiency over large output power dynamic ranges. Moreover, the availability of wideband and low-loss circulators makes this architecture promising for wideband applications. Consequently, it has the potential to overcome many of the drawbacks of other architectures. The fundamental operational principle and theoretical performance of the CLMA are studied and presented. As a proof of concept, a demonstrator circuit based on GaN transistors is designed and characterized at 2.09 GHz. Measurement results show that the peak output power is 43.1 dBm and the drain efficiency is 73\% at 6-dB output power back-off level.

In summary, the thesis presents two promising PA architectures for efficiency enhancement. The results of this thesis will therefore contribute to the development of energy efficient PAs for future mobile communication systems.