Processes of AM-PM Distortion in Large-Signal Single-FET Amplifiers

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.