RF Class-D amplification with bandpass sigma-delta modulator drive signals

The power efficiency of a RF Class-D amplifier with a bandpass sigma-delta (Sigma Delta M) modulator is analyzed for a complementary voltage-switched configuration. The modulator broadens the application of the amplifier to include signals with time varying envelopes such as W-CDMA. The addition of a modulator introduces new design variables which affect amplifier power efficiency including coding efficiency and the average transition frequency of the pulse train. Design equations are derived for the optimum load impedance, output power, conduction losses, capacitive switching losses, and drain efficiency. The general design equations are consistent with both periodic and aperiodic drive signals. Analytic and simulated results are compared for an example design with pseudomorphic high-electron mobility transistor and metal-semiconductor field-effect transistor switches with a fourth-order bandpass Sigma Delta M. The results show a drain efficiency of 52% with a 10-dB peak-to-average power ratio W-CDMA source signal at a frequency of 500 MHz.