Switching amplifier

A class-D amplifier or switching amplifier is an electronic amplifier in which the amplifying devices (transistors, usually MOSFETs) operate as electronic switches, and not as linear gain devices as in other amplifiers. The signal to be amplified is a train of constant amplitude pulses, so the active devices switch rapidly back and forth between a fully conductive and nonconductive state. The analog signal to be amplified is converted to a series of pulses by pulse width modulation, pulse density modulation or other methods before being applied to the amplifier. After amplification, the output pulse train can be converted back to an analog signal by passing through a passive low pass filter consisting of inductors and capacitors.

Class-D amplifiers work by generating a train of square pulses of fixed amplitude but varying width and separation, the low-frequency portion of whose frequency spectrum is essentially the signal to be amplified. The high-frequency portion serves no purpose other than to create a two level waveform. Because it has only two levels, it can be amplified by simple switching. The output of such a switch is an identical train of square pulses, except with greater amplitude. Such amplification results in a wave-form with the same frequency spectrum, but with every frequency uniformly magnified in amplitude.

A passive low-pass filter removes the unwanted high-frequency components, i.e., smooths the pulses out and recovers the desired low-frequency signal. To maintain high efficiency, the filter is made with purely reactive components (inductors and capacitors), which store the excess energy until it is needed instead of converting some of it into heat. The switching frequency is typically chosen to be ten or more times the highest frequency of interest in the input signal. This eases the requirements placed on the output filter. In cost sensitive applications the output filter is sometimes omitted. The circuit then relies on the inductance of the loudspeaker to keep the HF component from heating up the voice coil. It will also need to implement a form of three-level (class-BD) modulation which reduces HF output, particularly when no signal is present.

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