Time domain envelope characterisation of power amplifiers for linear and high efficiency design solutions

Abstract : This paper focuses on the time domain envelope measurements based analysis of power amplifiers in order to improve both linearity and efficiency of microwave transmitters. First of all, a versatile time domain envelope test bench is presented. Then, two applications related to those RF PA measurements are reported. The first application concerns linearity characterization that can be specified in term of Noise Power Ratio (NPR) and Error Vector Magnitude (EVM). Those figures of merit are essential to compute the final bit error rate in a system level simulation tool. A relationship between NPR and EVM is presented and validated by measurements. As a result it can be advantageously used to simplify simulation and measurement procedures for the linearity characterisation of devices and subsystems. The second application is regarding enhancement of the trade-off between linearity and energy efficiency. In order to have a good efficiency, a 10 W class-B GaN Power amplifier is considered. The work presented here, proposes a solution for linearity specifications at large output power back-off thanks to a dynamic gate bias control related to low instantaneous envelope power level. Meanwhile, a dynamic drain bias control handles good efficiency at a constant gain for high instantaneous envelope power level. The measurement-based gate and drain bias trajectory extraction will be fully detailed and take into account GaN dispersive effects such as thermal and trapping effects.