Class D amplifiers work in an entirely different way. The output transistors don’t partially open and close in a continuously variable manner; they switch fully on or fully off. This is why Class D amplifiers are also called “switching” amplifiers. Although it may seem intuitive that devices operating in an on/off mode are digital, Class D amplifiers are actually purely analog in nature. Many people mistakenly refer to Class D amplifiers as “digital,” or even believe that the “D” in “Class D” signifies digital operation. They are in fact analog, even though some use digital circuitry in the process of computing the switch-control signal.
This method of converting a low-level audio signal into a high-level signal with power-switching transistors that can drive loudspeakers is extremely efficient. If a Class AB amplifier is generally about 25 percent efficient (25 percent of the power it consumes is delivered to the loudspeaker), Class D amplifiers are generally 90 percent(or more) efficient. This dramatically reduces the need for huge power transformers, banks of output transistors, and huge extruded aluminum heatsinks. These are the largest, heaviest, and most expensive elements of a Class AB power amplifier—and at least 75 percent of their function is to dissipate power from your wall outlet in the form of heat.
The Class D amplifier’s tremendous efficiency is why it can be small, light, and relatively inexpensive for its output power. The photo below shows a Class D output-stage module that can produce four channels of 250W into 8 ohms. It is 3- by 4-inches and just 1.5 inches in height. The output stage (and power supply) from a conventional linear amplifier of this power would be 50 times this module’s size and weight.
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