Frequently referred to as a digital amplifier, the class-D amplifier is an electronic device that increases the size of electrical signals. What distinguishes the class-D amplifier from other types of amplifiers is its design, which trades off the risk of increased distortion for a very high level of efficiency, allowing it to be smaller and lighter than other amplifiers. Class-D amplifiers are most commonly used as speaker amplifiers in audio applications and are especially popular in situations where either cost or size is a major factor.
Typical speaker amplifiers use at least one transistor, a small electronic part with three wires, which is typically made of silicon, or integrated circuit (IC) chips which contain transistors. A constant power feed goes into its "collector" while a small, line-level, signal is fed into its "base." The base signal controls how much of the collector's power passes through to the "emitter," the lead from which the amplified signal comes. Class-A, class-B, and hybrid class-AB designs constantly draw power for the collector, whether it needs it or not. This tends to make them very inefficient, and the high quantities of unused power are converted into heat, which the amplifier dissipates through large and bulky heat sinks. The design of a class-D amplifier eliminates much of this inefficiency.
Class-D amplifiers still use transistors or IC chips, but instead of simply making the signal from the base bigger with power from the collector, they use the base's signal to switch the collector on and off. For example, to create a low level of light in a room without a dimmer switch, theoretically, the light switch could be flicked on and off multiple times per second, causing the eye to register light that is dimmer than a fully illuminated bulb. Class-D amplifiers work on the same principle, and when they are "off," they essentially draw no power.
The misnomer "digital amplifier" is applied to many class-D amplifier components because the short pulses that they use could come from a digital source. On the other hand, they can also amplify analog signals. In fact, the class-D amplifier design predates digital audio technology.
When they were first introduced, class-D amplifiers made a number of compromises in sound quality to achieve high power output levels from a small, inexpensive component. Due to this limitation, they first gained popularity in applications like subwoofer amplifiers where the type of distortion that the design created was harder to hear. As the technology became more refined and the sound quality improved, they expanded into low-cost consumer audio equipment as well as portable equipment, which required low power drain to maximize battery life. By the second decade of the 21st century, class-D amplifiers had been refined to the point that they began to be used in professional audio applications as well as in very high-end, audiophile-grade stereo equipment.