A dynamic bias control technique of PHEMT SSPA for optimised PAE and EVM applied to MQAM satellite communication systems

This paper presents a method for the optimisation of both PAE and EVM of a millimeter wave power amplifier applied here to MQAM modulations. A first key point lies in that both input and output biasing voltages of the SSPA are dynamically controlled versus the RF power level associated with the symbol to be transmitted. The leading idea here is that the dynamic biasing control is designed and implemented to keep fixed AM/AM and AM/PM conversion values while RF input power level changes. By doing so, the power gain of the PAs can be dynamically tuned to a fixed power gain corresponding to the compression gain behaviour for which the PAE is optimum either at low, medium and high input RF power level. As a main consequence, PAE performances can be drastically improved (from 1% to 20% at Ka band) as compared to classical back-off solutions and optimised while keeping a very good EVM. A Ka band hybrid amplifier has been developed using an 8*75 /spl mu/m Power PHEMT. The proposed linearisation technique is validated by comparisons between measured PAE and EVM on the SSPA when a fixed bias and a controlled bias are used.