This FAQ is intended to capture a variety of knowledge about the selection, application, and measurement of Class D amplifiers. The emphasis of this FAQ is towards Class D amplifiers in the 0.5W to 2W range intended for portable media devices such as mobile phones, portable DVD players, and portable navigation systems. However, the vast majority of this information can be applied to Class D amplifiers from milliwatts to kilowatts of output power.
What is a Class D amplifier?
A Class D amplifier uses pulse-width modulation circuitry to keep its output transistors operating either all the way on or all the way off. That is, at any time the instantaneous output voltage is either at one supply voltage or the other, ignoring a brief period of transition during the switching. Therefore, the output current is conducted from a device with no significant voltage drop.
Ohm's law states that power is equal to voltage multiplied by current. A Class D amplifier keeps the voltage component of this equation near zero, removing most of the power dissipation from the output stage. Class D amplifiers have higher efficiency than all other topologies; they have typical efficiencies as high as 95%, although most are in the mid 80% range. Class D amplifiers switch at a frequency much higher than the audio band. Most Class D amplifiers switch from about 300kHz to 2MHz.
Why should I use Class D?
Because Class D amplifiers are very efficient, they provide the best use of the finite power available from a battery or other limited power source. Furthermore, this high efficiency eliminates heat-sinking requirements for most amplifiers below 10W output power. Class D amplifiers do not heat their neighboring components as much as other topologies, reducing the ambient temperature in the enclosure. Additionally, the thermal efficiency of Class D amplifiers allow them to use standard IC packages with no special consideration for thermal dissipation.
When should I not use Class D amplifiers?
The most important reason why Class D amplifiers are not used in all applications is that the switching of the outputs causes EMI . In most applications this EMI can be contained such that the device meets EMI certification, but this is an additional concern causing some designers to avoid Class D amplifiers.
A secondary concern for Class D amplifiers is that their sound quality in general is not as good as Class AB and other topologies. While comparing the two topologies on paper can lead to this conclusion, in the final application it is often not an issue because the loudspeaker dominates the distortion of the system.
What is a half-bridge/single-ended Class D amplifier?
A half-bridge Class D amplifier has one output per channel. The loudspeaker connection for a half-bridge (also know as a single ended output amplifier) is between this single output and ground in a dual supply system. In a single supply system a large capacitor is used to block the VCC/2 DC voltage from appearing across the loudspeaker load. This capacitor is usually several hundred microfarads or more, depending on the bass requirements of the system and the impedance of the loudspeaker.
The half-bridge amplifier is good for systems that already have a symmetrical dual supply voltage. The expense and size of the required DC blocking capacitor makes them not as desirable for single supply systems.
What is a full-bridge/differential Class D amplifier?
A full-bridge Class D amplifier has two outputs per channel. A full-bridge amplifier is also known as a Bridge-Tied-Load (BTL) amplifier or a differential output amplifier. The loudspeaker connection for a full-bridge Class D amplifier is between the two outputs in either a single or dual supply system. The Class D amplifier has a low output offset, eliminating the need for a DC blocking capacitor.
The full-bridge amplifier provides the smallest solution size in most systems, and is the most common Class D amplifier topology.