Linearity performance of an RF power amplifier under different bias-and load conditions with and without DPD

Adjusting the biasing- and loading conditions in a power amplifier affects the linearity and efficiency of the PA. If an accurate nonlinear model for the transistor does not exist, load-pull measurements are necessary for a high-performance PA design. In this work load-pull measurements are performed under different biasing conditions in order to utilize the fully potential of the transistor in the PA. Two different linearity criteria, one strict at -40 dB ACPR and one more relaxed at -33 dB for a 16 QAM signal, should be met by the PA, and the best bias point and load are found for these criteria from load pull measurements with single tone and digitally modulated signal. The impact on the results when linearizing the PA with DPD are also investigated. The measurements on a 1 W pHEMT transistor show that by utilizing sweet spots when biasing in deep class AB the most output power and PAE could be achieved with an ACPR level of -33 dB. With an ACPR limit of -40 dB biasing toward class A maximizes the output power. At this criterion there is much to gain by performing load pull measurements with a digitally modulated signal. When applying DPD the output power is almost independent of the biasing conditions for both linearity criteria and the load should be chosen to maximize output power at 1 dB compression.