Multi-Frequency Measurements for Supply Modulated Transmitters

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 500 MHz. The low-frequency (LF) dynamic impedance of the supply terminal of a power amplifier (PA) is often unknown and available nonlinear transistor models are unable to predict dynamic LF effects required for design of wideband efficient supply modulators (SMs). This paper describes a technique to calibrate and measure multi-port multi-frequency parameters of a transistor and PA under supply modulation conditions. The measurement setup is used to characterize the complex drain impedance of GaN transistors and PAs in large-signal operation at X-band with 1-500-MHz LF excitation on the drain terminal, over a range of input power levels. In addition, the LF drain impedance of a 10-GHz monolithic microwave integrated circuit PA with 4-W output power and 60% peak power-added efficiency is measured when the PA is connected to a simple switched resonant SM. The main motivation for this work is to obtain knowledge of the dynamic supply-port impedance that can enable improved PA and SM co-design.