Processes of AM-PM Distortion in Large-Signal Single-FET Amplifiers
S. Golara, S. Moloudi and A. A. Abidi, "
in IEEE Transactions on Circuits and Systems I: Regular Papers,
vol. 64, no. 2, pp. 245-260, Feb. 2017; doi: 10.1109/TCSI.2016.2604000
Abstract: Using an appropriate formulation of field-effect transistor (FET) current as a nonlinear function of terminal voltages, and a simplified model of gain compression in common source amplifiers, we are able to identify four principal sources of amplitude-to-phase (AM-PM) distortion. A new analysis shows the varactor effect of gate-source capacitance on AM-PM distortion, and the changing Miller-multiplied gate-drain capacitance as the field-effect transistor (FET) is driven into compression. The phase shift in the load impedance at the frequency of operation and incomplete suppression of 2nd harmonic by a resonant load of limited Q are explained and analyzed. We are able to identify the dominant mechanism of AM-PM distortion in various practical circuits, which then suggests methods of remediation. The analysis was put to test by predicting the measured AM-PM distortion of power amplifiers reported in the literature. Good agreement is found in all cases, with insights gained into the dominant cause of distortion in each case. In this paper, AM-PM distortion is first defined in Section II. In Section III, the EKV model of the MOSFET is briefly presented and dominant mechanisms are explained. In Section IV the analysis is compared against measured data to validate the theory and Section V summarizes the conclusions [read more...]