Analysis and Optimization of Transformer-Based Series Power Combining for Reconfigurable Power Amplifiers
Transformer-based series power combiners are analyzed within the context of reconfigurable, watt-level, wideband digital envelope tracking transmitters. A model is developed for designing and analyzing transformer-based switchable series combiners. This model takes into account losses in the windings and non-ideal mutual coupling, as well as the effect of switching and scaling the input amplifiers on the combiner efficiency, input impedance, and power combining ratio. Optimum combining efficiency is derived for a combiner with scaled sources. Based on the developed model, a four-element 29.5-dBm power amplifier with 31% peak efficiency at 1.9 GHz, using a 1.5-V supply is designed and simulated in IBM 130-nm CMOS technology.