Overview

Since the first edition of this book, Class-D amplifiers have increased enormously in popularity. This is because Class-D gives the highest efficiency of any of the amplifier classes, although the performance, particularly in terms of linearity, is not so good. The rapid rate of innovation means that this section of the book is much more of a snapshot of a fast-moving scene than the rest of the material. I do not want to keep repeating 'At the time of writing' as each example is introduced, so I hope you will take that as read.

The fields of application for Class-D amplifiers can be broadly divided into two areas; low and high power outputs. The low power field reaches from a few milliwatts (for digital hearing aids) to around 5 W, while the high-power applications go from 80 W to 1400 W. At present there seems to be something of a gap in the middle, for reasons that will emerge.

The low-power area includes applications such as mobile phones, personal stereos, and laptop computer audio. These products are portable, and battery driven, so power economy is very important. A major application of Class-D is the production of useful amounts of audio power from a single low-voltage supply rail. A good example is the National Semiconductor LM4671, a single-channel amplifier IC that gives 2.1 W into a 4 ? speaker from a 5 V supply rail, using a 300 kHz switching frequency. This is a very low voltage by conventional...