Pade-Approximation Based Behavioral Modeling for RF Power Amplifier Design

Radio frequency (RF) power amplifier (PA) design using Gallium Nitride (GaN) transistor technology requires accurate device models in order to maximise performance and reduce development time. The current state-of-the-art frequency-domain behavioural models focus on linear and quadratic approximations to the polyharmonic distortion (PHD) formalism. However, the linear approximation suffers from poor accuracy under load mismatch conditions, while the quadratic approximation suffers from poor extrapolation beyond the measured range, leading to erroneous predictions of the optimum load impedances for maximum output power and maximum drain efficiency. In this work, a rational Pade-based approximation is proposed as the model core, and it is shown, through experimental validation, that the Pade approximation-based model can provide superior results in a more scalable format. It can mitigate problems found in the existing PHD models when applied to the matching problem. Specifically, the proposed model produces fewer erroneous solutions for the optimum load points, due to the well-behaved nature of Pade approximants. In addition, for the first time, results are reported on using the behavioural model to determine the optimum impedance for maximum transducer gain in a two-port device model. All results show the Pade model has high potential when compared to the established PHD-derived models in RF PA design.