Sequential power amplifiers with adaptable combiners

This paper discusses design aspects of sequential power amplifiers (SPAs). After derivation of theoretical performance limits, an optimum practical realisation is suggested, using an FPGA-circuit at the input and an adaptable directional coupler as the signal combiner at the output. Design trade-offs between the optimum coupling and the output power capability of the involved amplifiers are outlined. Using SPAs efficiencies of more than 50% can be achieved under up 10 dB back-off from maximum power. The combining characteristics of the adaptable combiner are investigated. Off-the-shelf power amplifiers may be employed, which can be linearized in a conventional manner. The paper closes with measurement results.