Input impedance of class E switching amplifiers

This paper studies the effect of feedback capacitance on the input matching of a class E switching power amplifier. The on/off switching causes the input to see the feedback capacitance either amplified by the Miller effect or not and therefore, this feature is dependent on the gain of the amplifier. The time-varying feedback appears at the gate as negative input resistance, as a cause of severe second harmonic injection, and - if the supply voltage is varied to modulate amplifier gain as in envelope elimination and restoration (EER) - also as a cause for supply dependent input impedance. The paper discusses ways to minimize these effects.