Audio Power Amplifiers

Choosing an Audio Amplifier for Your Design

- By Stuart Ball

We don't think of audio amplifiers as part of embedded systems, but if you need an audio output, whether for an MP3 player, a warning annunciator, a technician-to-patient voice system in a medical device, or a verbal output for a GPS device, selecting the correct audio components is important.

An audio output consists of several components, starting with something that generates an audio signal, waveshaping and amplifying the signal, and then drives the output device, whether it's a speaker, headphone jack, or other transducer. In this article I'm only going to look at the final output stage: the audio power amplifier.

The output transducer for an audio system is typically a speaker or similar low impedance device. Audio transducers are typically speakers and headphones (which are tiny speakers) with an impedance ranging typically from 2Ω to 32Ω but going up to 600Ω. Due to this, producing usable (that is, audible) output requires an amplifier capable of driving more current into the transducer than is needed for normal signals. For example, to produce 1W of power into an 8Ω speaker requires applying Ohm's power law like this:

P = I2R = V2/R. 1W = I2 x 8Ω, Ι = 350mA. Similarly, V = 2.8V

Specifically, producing 1W into an 8Ω speaker requires 2.8V at 350mA. I calculated these as DC values, but the same principle applies for an AC audio signal. Although generating 2.8V from a 5V or 3.3V embedded system isn't too hard, the 350mA requirement demands more current capability than an MCU output can provide. The audio power amplifier translates a low-level audio signal into sufficient power to drive the speaker (or other transducer).

AMPLIFIER BASICS

Power amplifiers are categorized into five classes: A, B, AB, C, and D. The differences are in how the output power is generated, how the output drive devices are biased, and the efficiency.