Design and performance of sequential power amplifiers

This paper presents an analysis of a sequential power amplifier (SPA) in a transmitter with an FPGA-circuit at the input and a directional coupler at the output. The nonlinear combining characteristics of the output coupler are investigated. The coupling strength and the ratio of maximum power of peaking and main amplifier are discussed as design parameters. A switched coupler is examined as the combiner. Due to the sequential transmitter concept the back-off efficiency can be significantly increased. E.g. using conventional class-B amplifiers a 10 dB back-off efficiency of 38% can be achieved. The average power added efficiency of the whole transmitter may be improved by 30% compared to conventional amplifiers. Design tradeoffs, theoretical performance and measurement results of the proposed transmitters are presented.