Broadband Sequential Power Amplifier With Doherty-Type Active Load Modulation

This paper presents the concept and the detailed analysis of a sequential power amplifier with Doherty-type active load modulation (SPA-D). By introducing a Doherty-type output combiner into the conventional sequential power amplifier (PA), highly efficient amplification over broadband and wide dynamic range for the reconfigurable output power back-off can be achieved. The proposed amplifier mimics both the sequential and the Doherty PAs by exploiting the active load modulation region that has not been considered in the design of Doherty amplifiers. A broadband SPA-D prototype having 8-dB output back-off power (OBO) is designed and implemented. The experimental results exhibit drain efficiencies of 50%-75% at 7-10-dB OBO and 57%-70% at peak power with Doherty-like behavior over 700-MHz bandwidth centered at 2.35 GHz. The ability to easily reconfigure the breakpoint is experimentally verified, providing a convenient means for optimizing the amplifier efficiency according to various wireless standards. In addition, by using a memoryless digital predistortion, the spectrum emission mask required for long-term evolution down link can be fulfilled, making it well suited for applications in modern wireless transmitters.