Efficient X-Band Transmitter With Integrated GaN Power Amplifier and Supply Modulator

In this paper, we present a high-efficiency transmitter based on an integrated circuit (IC) supply modulator implemented in the same 0.15-μm gallium nitride (GaN)-on-SiC RF process as the power amplifier (PA) monolithic microwave IC. The X-band 10-W two-stage PA is designed for stable operation with minimal drain capacitance, which enables fast supply modulation. The multilevel supply modulator provides eight voltage levels with 3-bit digital control [(power digital-to-analog converter (pDAC)], achieving a state-of-the-art slew rate of 5 kV/μs. Characterization of the dynamic RON of the GaN switches allows the development of an efficiency model for the pDAC and an investigation of the effects of the pDAC internal resistance on the PA performance, resulting in a comprehensive efficiency model for the supply-modulated PA. The flexible compact transmitter consisting of the PA and pDAC ICs shows highh efficiency in backoff for a variety of signals, both for radar and communications. Measured results for amplitude- and frequency-modulated radar pulses show a composite power-added efficiency (CPAE) of 44% with a peak power of 10 W at 9.57 GHz, with simultaneous spectral confinement and 52-dB improvement of the first time sidelobe. For a 20-MHz high peak-to-average ratio LTE signal, the CPAE increases from 11% to 32% compared to a fixed supply voltage transmitter, while linearity under dynamic supply operation is maintained through digital predistortion.