Generalized Theory and Design Methodology of Wideband Doherty Amplifiers Applied to the Realization of an Octave-Bandwidth Prototype

This paper derives a novel continuum of Doherty power amplifier (PA) solutions with enhanced load modulation over an extended RF bandwidth. It is analytically shown that Doherty PAs with efficiency peaks at maximum power and at an arbitrary back-off level can be achieved with numerous configurations of output power combiner. Generalized governing equations for carrier and peaking transistor current ratios, voltage profiles, and biasing conditions are derived in order to assess the performance of any Doherty amplifier configuration. The generalization demonstrates the impact of the selection of the output combiner on PA performance and highlights the tradeoffs between the different solutions. A wideband Doherty PA was designed based on the proposed methodology and implemented using packaged gallium nitride transistors. Over the frequency band of 0.55-1.10 GHz, the Doherty PA exhibited drain efficiency in excess of 40% and 56% at 6-dB output power back-off (OPBO) and peak power, respectively. The Doherty PA was successfully linearized when driven with modulated stimulus and achieved average drain efficiency above 40% at 7.1-dB OPBO.