The state of the art in class D amplification has progressed rapidly in the past few years, most noticeably for lower-power applications that require less than 50 W per channel. Class D is inherently more efficient than the traditional class AB amplifier because the output stages are always on or off, with no intermediate bias stage necessary.

This efficiency advantage has never held widespread appeal for designers, since the disadvantages of higher parts cost, poor audio performance (compared with class AB), and the need for output filtering often outweighed it. However, two major factors recently reversed this trend:

• Market need: Two rapidly growing end-equipment segments benefit from class D amplification in different ways. The first is cell phones, where speakerphone and push-to-talk (PTT) modes both benefit from higher efficiency, providing longer battery life. Also, the growth of LCD flat-panel displays necessitates "cool-running" electronics because the display color contrast suffers under elevated operating temperatures. Again, class D efficiency means less power has to be dissipated in the drive electronics, hence cooler product operation and better looking pictures.
• Available technology: Driven by market need, improvements in class D technology are now available. Specifically, several manufacturers offer cost equality with class AB, improved audio performance on a par with class AB, and some novel output-modulation schemes that ease the electromagnetic-interference (EMI) burden in many applications.

Some of the newer components, derived from older PWM-style (pulse width modulation) architectures, incorporate sophisticated modulation techniques that achieve "filter-less" operation for lower-power systems. Efficiency claims can be verified on the bench, but some designers suspect that products based on these new techniques will be rife with electromagnetic compatibility/RF interference (EMC/RFI) problems. In reality, effective pc-board layouts and short runs of speaker cable can ensure sufficiently low radiated EMI to pass the applicable FCC or CE standards.

In some applications, the physical layout necessitates long speaker cables. RF emissions must be more tightly controlled in these instances because the speaker cables act as antennas. The longer the speaker cable, the lower the frequency at which it acts efficiently as an antenna. Similarly, some applications have requirements for EMI emissions below that of CE/FCC regulations, perhaps to meet automotive specifications, or where interference with other circuitry at lower frequencies must be avoided.

Employing stereo class D amplifiers in a flat-panel TV is an obvious example. With speakers typically arrayed at the outer edges of the device, long speaker cables are hard to avoid. If analog video signals are present, simply meeting FCC or CE RF emissions may not suffice. (Measurement of these limits is specified from 30 MHz upwards.)

Therefore, suppressing the switching fundamental may be necessary to avoid interference effects with the video signal. If you need to use the traditional LC filters that operate well with older PWM amplifiers, you should analyze them to ensure they're effective in suppressing the high-frequency switching transients produced by the latest amplifiers.

PWM-BASED CLASS D AMPS
Traditional class D amplifiers are usually based on the principle of pulse-width modulation. Their outputs can be configured either as single-ended or as a fully differential bridge-tied load (BTL).

Figure 1 shows the output waveforms typical of a BTL, PWM-based, class D amplifier. The fast switching times and nearly rail-to-rail swings make this type of amplifier very efficient. Conversely, the wide output spectrum implied by those same parameters can lead to high-frequency RF emissions and interference. Output filters typically are included to suppress such unwanted effects.

As Figure 1 shows, the mirror-image waveforms assert very little common-mode (CM) signal on the speaker or cables (lower trace), provided the waveforms of the inverting and noninverting output devices are well matched. Note that a 50% duty cycle represents a zero input signal (idle). You then can design a differential low-pass filter that attenuates high-frequency content in the waveforms (due to the rapid switching) but preserves the low frequencies intended for the loudspeaker.

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The way it is done now, one clicks to see the pic and then has to find your spot back in the text. It is a pain and I don't like it.

If you print out the article, there are no figures printed. You have to click each figure and print it individually. I know you guys can do better than this.