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Class D audio-power amplifiers

Class D audio-power amplifiers

Class D amplifiers are much more efficient than other classical amplifiers, but their high efficiency comes at the expense of increased noise and distortion. You can assess the frequency- and time-domain characteristics of a Class D amplifier, including the output filter, using online simulations.
Unless you’ve been stuck on Survivor Island, you know growth in battery-operated electronic devices has exploded in the last few years. And, of course, one of the prime requirements for any battery-operated device is low power consumption. Every device in a signal chain must be as power efficient as possible to achieve a long battery life. This requirement applies to such ubiquitous components as amplifiers. For high efficiency, a Class D amplifier is the best type to use. A Class D type is much more efficient than other classical amplifiers but tends to achieve increased efficiency at the expense of increased noise and distortion.

With the advent of IC versions of Class D amplifiers, you can easily design an efficient amplifier suitable for battery-operated devices. The key to using an IC version of a Class D amplifier is designing an appropriate output filter. You can use any of the classic filters depending on your needs: Bessel for flat phase at the expense of stopband attenuation, Chebyshev for high-stopband attenuation at the expense of passband ripple, and Butterworth for no ripple. However, Butterworth is not a good choice for flat phase or for achieving the best stopband attenuation. The advent of online tools, such as WebSim, makes it easy to simulate various filter scenarios and examine frequency- and time-domain trade-offs. Just make sure you know what the performance objectives of the output filter need to be to optimize your design so you won’t be voted off the island.

Traditionally, power amplifiers rely on a constantly biased output stage to produce low distortion. Low distortion results when you bias a transistor or MOSFET within its linear range so that signal excursions do not drive the output device near the saturation or cutoff condition. Power amplifiers must also be able to source and sink current so that the output can swing positive and negative with respect to ground.

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