Overview of digital power amplifiers

This article describes the need for and the simplified theory behind digital power amplifiers.

The need for amplifiers

Most audio processing circuits work at low signal levels of a couple of Volts or so. When the processing is finished, the signal has to be amplified and applied to the loudspeaker. The amplifier’s job is to put a voltage out to the speaker that is in precise proportion to the input signal. The loudspeaker will then draw current out of the amp according to the speaker’s impedance.

Historically, power amplifiers have been of so-called Class AB design. But there is an alternative design that uses digital techniques called among other things, Class D, PWM or just plain “digital” amplification.

George Biner

Need for high efficiency Amplifiers necessarily use some power themselves in the act of amplifying: we want to minimize this. Most of this power is dissipated in the output transistors. We will need to define a few basic terms: “Volts” and “Amperes”, the quantities that dictate power dissipation in an electrical device, and the resultant “Power” dissipated by a device (in this case, the amplifier’s output devices).

Volts and amps

A ball sitting on a hill has a lot of potential energy: if you nudge it, it will move down the hill. The higher the ball sits on the hill, the more work it can do. The difference in potential at the top and the bottom of the hill is called the “potential difference”.

A ball already rolling down a hill has kinetic energy. The faster it is rolling, the more energy it has.

Electrons have similar two quantities, and they are the main quantities of concern in any circuit. Potential difference is measured in Volts and speed (or how many electrons per second) is measured in Amperes.