<P> 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 . They are rapidly switching back and forth between the supply rails, being fed by a modulator using pulse width, pulse density, or related techniques to encode the audio input into a pulse train . The audio escapes through a simple low - pass filter into the loudspeaker . The high - frequency pulses, which can be as high as 6 MHz, are blocked . Since the pairs of output transistors are never conducting at the same time, there is no other path for current flow apart from the low - pass filter / loudspeaker . For this reason, efficiency can exceed 90% . </P> <P> Class - D amplifiers work by generating a train of square pulses of fixed amplitude but varying width and separation, or varying number per unit time, representing the amplitude variations of the analog audio input signal . It is also possible to synchronize the modulator clock with an incoming digital audio signal, thus removing the necessity to convert it to analog, The output of the modulator is then used to gate the output transistors on and off alternately . Great care is taken to ensure that the pair of transistors are never allowed to conduct together . This would cause a short circuit between the supply rails through the transistors . Since the transistors are either fully "on" or fully "off", they spend very little time in the linear region, and dissipate very little power . This is the main reason for their high efficiency . A simple low - pass filter consisting of an inductor and a capacitor are used to provide a path for the low - frequencies of the audio signal, leaving the high - frequency pulses behind . 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 . </P>

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