A 0.85 – 2.7 GHz two-cell distributed GaN power amplifier designed for high efficiency at 1-dB compression

High performance broadband distributed power amplifier (PA) design relies on the co-design of input signal distribution and non-isolated output combiner to take maximum advantage of the resulting load-pulling effects at both fundamental and higher harmonics. Due to the load-pulling and the need to consider higher harmonics, PA operation is not readily expressed analytically. In this work this is addressed by using a numerical linear multi-harmonic calculation method for rapid circuit evaluation and optimization. This linear approach targets good broadband performance without relying on nonlinear saturation effects. An initial investigation is performed to find suitable input network and output combiner topologies for a two-cell PA configuration. A demonstrator is then designed based on the findings, where a 3-dB hybrid coupler with an offset transmission line in one branch is used to produce a frequency dependent phase difference between transistor cells. Measuring the PA at 40-V supply and 1-dB compression the output power is in the 43-46 dBm range with corresponding efficiencies of 46-58% across 0.85-2.7 GHz, whereas saturated efficiencies are typically in the 60%-range. The simultaneous combination of high output power and high efficiency reported across 100% fractional bandwidth is state-of-the-art and clearly shows the merit of the proposed PA design approach.