Class-D Amplifier

Encode a signal as pulse width, then recover its amplified shape with an LC filter.

V1 is a 0.5 V peak, 100 Hz signal. VT supplies a ±1 V, 20 kHz triangle. A1 compares them and produces an approximately ±12 V pulse-width-modulated output. L1 = 10 mH, C1 = 1 µF and RL = 100 Ω form a reconstruction filter.

$$D\approx\frac12\left(1+\frac{V_{\mathrm{signal}}}{1\,\mathrm{V}}\right)$$ $$\begin{aligned}\overline{V_{\mathrm{PWM}}}&\approx12V_{\mathrm{signal}}\\f_{LC}&=\frac{1}{2\pi\sqrt{LC}}\approx1.59\,\mathrm{kHz}\end{aligned}$$

Signal is green, recovered Audio red, PWM teal and load current blue. The recovered signal is about 6 V peak with residual switching ripple. This is a functional modulation example: the limited-output amplifier represents an idealized switching stage. It does not model a transistor half-bridge, dead time, switching loss or efficiency.

Read the results. The saved transient chart uses separate voltage and current panels; the table provides numerical samples. The first voltage probe is green and the second red. The first current probe is blue; a second current probe is purple.

Model limits. The three-terminal op-amp model has implicit ±12 V output limits, 200,000 low-frequency gain and 10 MHz gain-bandwidth product. Supply pins are intentionally absent in this functional model. In Schmitt-trigger and relaxation/integrator oscillator examples, the switching amplifier uses a 100 kHz gain-bandwidth setting to resolve transitions cleanly at the saved time step. It omits detailed slew-rate, input common-mode, output-current and thermal limits.

Try it. Set V1 peak voltage to 0.25 V; the recovered output amplitude approximately halves. Restore the original values after comparing the results.