![]() A switching amplifier must not be confused with linear amplifiers that use an SMPS as their source of DC power. Whereas buck converters usually function as voltage regulators, delivering a constant DC voltage into a variable load, and can only source current (one-quadrant operation), a class-D amplifier delivers a constantly changing voltage into a fixed load, where current and voltage can independently change sign (four-quadrant operation). The structure of a class-D power stage is somewhat comparable to that of a synchronously rectified buck converter (a type of non-isolated switched-mode power supply (SMPS)), but works backward. The circuit then relies on the inductance of the loudspeaker to keep the HF component from heating up the voice coil. In cost-sensitive applications the output filter is sometimes omitted. A simple low-pass filter consisting of an inductor and a capacitor provides a path for the low frequencies of the audio signal, leaving the high-frequency pulses behind. This is the main reason for their high efficiency. Since the transistors are either fully "on" or fully "off", they spend very little time in the linear region and dissipate very little power. Great care is taken to ensure that the pair of transistors are never allowed to conduct together, as this would cause a short circuit between the supply rails through the transistors. The output of the modulator is then used to gate the output transistors on and off alternately. The modulator clock can synchronize with an incoming digital audio signal, thus removing the necessity to convert the signal to analog. Basic operation Ĭlass-D amplifiers work by generating a train of rectangular 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. The first class-D amplifier based integrated circuit was released by Tripath in 1996, and it saw widespread use. The availability of low-cost, fast-switching MOSFETs led to Class-D amplifiers becoming successful in the mid-1980s. There were subsequently rapid developments in VDMOS ( vertical DMOS) technology between 19. In 1978, Sony introduced the TA-N88, the first class-D unit to employ power MOSFETs and a switched-mode power supply. Practical class-D amplifiers were later enabled by the development of silicon-based MOSFET (metal-oxide-semiconductor field-effect transistor) technology. As a result, these early class-D amplifiers were impractical and unsuccessful. The Sinclair X-20 in 1966 produced 20 watts, but suffered from the inconsistencies and limitations of the germanium-based BJT (bipolar junction transistor) transistors available at the time. However, it had an output power of only 2.5 watts. The first commercial product was a kit module called the X-10 released by Sinclair Radionics in 1964. The first Class-D amplifier was invented by British scientist Alec Reeves in the 1950s and was first called by that name in 1955.
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