Carrier and Envelope Frequency Measurements for Supply-Modulated Microwave Power Amplifiers

Scott R. Schafer, University of Colorado Boulder


Transmitters for high peak-to-average power ratio communication are increasingly using supply modulation to improve efficiency. In addition to a dc component, the dynamic supply may contain ac components up to 500MHz. The signal envelope dynamic impedance of the supply terminal of a power amplifier (PA) is often unknown and available nonlinear transistor models are unable to predict dynamic low frequency effects required for design of wideband efficient supply modulators. This thesis investigates envelope frequency effects on nonlinear behavior of microwave transistors and PAs under supply-modulated conditions. A measurement setup is created to characterize multi-frequency large-signal excitation of GaN transistors and PAs at carrier frequencies in the 10GHz range with 1-500MHz low frequency excitation on the drain terminal. A novel method for multi-frequency analysis of nonlinear circuit components based on describing functions is developed. It is shown that the describing functions agree with simulation and measurements. In addition, the measurement setup is used to characterize the low frequency drain impedance of a MMIC PA when connected to a simple resonant supply modulator. The main motivation for this work is to obtain knowledge of the dynamic supply terminal in the low frequency regime (1-500MHz) that can enable power amplifier and supply modulator co-design for very broadband signals.