Linearity improvement of HBT based Doherty power amplifiers based on a simple analytical model

A simple analytical model is proposed and shown to be effective in predicting the nonlinear behavior of single-ended amplifiers as well as Doherty amplifiers implemented with GaAs HBTs for handset applications. The analytical model is based on linear and nonlinear components extracted from a VBIC model for Skyworks InGaP/GaAs HBT devices. Equations derived from the model provide insights into effects of individual components on the gain and phase of the amplifier. It is found that tuning the phase delay inserted in front of the auxiliary PA within the Doherty can improve linearity at high input power. The efficacy of the model is demonstrated by experimental results, in which for a Doherty PA with a tuned phase delay at the auxiliary PA side, the measured gain and phase agree with the simulation results, and the IMD3 performance is improved as much as 8dB when compared with a Doherty PA without phase delay tuning.