A robust modeling and design approach for dynamically loaded and digitally linearized Doherty amplifiers

In this paper, an active load-pull-based large-signal modeling approach suitable for designing and optimizing Doherty amplifiers is proposed. The optimization of the drive dependant dynamic loads seen by both carrier and peaking amplifier´s transistors was carried out using a large-signal load-pull-based behavior model built into CAD software. The latter was also used to optimize the biasing conditions of the peaking and carrier amplifier transistors. The simulation versus measurement results demonstrated the strong ability of the constructed model to predict the nonlinear behavior of 16 watts peak-envelope power designed Doherty amplifier in terms of power added efficiency, AM/AM, and AM/PM distortions. In addition, a complex digital predistortion technique was used to significantly enhance the linearity of the Doherty amplifier and to preserve an ACPR level better than 50 dBc under a WCDMA signal while attaining a power added efficiency of about 44% at the same time.