10.2.1.1 Amplifier Output Filtering

Output FETs drive the amplifier outputs in an H-bridge configuration. These transistors are either fully off or on. The result is a square-wave output signal with a duty cycle that is proportional to the amplitude of the audio signal. The amplifier outputs require a low-pass filter to filter out the PWM modulation carrier frequency. People frequently call this filter the L-C filter, due to the presence of an inductive element L and a capacitive element C to make up the 2-pole low-pass filter. The L-C filter attenuates the carrier frequency, reducing electromagnetic emissions and smoothing the current waveform which the load draws from the power supply. See the Class-D LC Filter Design application report, SLOA119, for a detailed description on proper component selection and design of an L-C filter based on the desired load and response.