The Design of a Gallium Nitride Doherty Amplifier Using Waveform Measurements and Active Harmonic Load-Pull

The Doherty power amplifier (PA) architecture is rapidly gaining popularity in the mobile communications system application domain. Its principle of operation relies upon the intentional interaction between two transistors, and in order to obtain optimum performance the large signal characteristics of both devices need to be considered. This poses a significant challenge as firstly, the current nonlinear models available within CAD-based design environments generally lack the required accuracy, and secondly the established trial-and-error based design approaches are less effective and tend to be impractical for complex, multiple device PA structures. This paper describes how active harmonic load-pull systems and the direct measurement of voltage and current time-domain waveforms can be directly utilised in the design and optimisation of a GaN Doherty PA. As part of this process a large signal measurement system is employed to synthesise the precise dynamic environment that exists around a single, 'main' device within a GaN Doherty structure