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This paper presents a rigorous way to quantify the role played by higher baseband impedances in determining baseband electrical memory effects observed in power transistors under two-carrier excitation. These effects typically appear not only as asymmetrical distortion terms in the frequency domain, but also more reliably as a recognizeable hysteresis or looping in the dynamic transfer characteristics extracted from measured input voltage and output current envelopes of a power device. Investigations have been carried out using a commercially available 10W GaN HEMT device characterised at 2GHz within a high-power modulated wave for measurement system. Active IF loadpull has been employed to present specific baseband impedance environments, allowing the sensitivity of IMD symmetry to baseband impedance variations to be investigated.