Class D amplifiers have been around for years. But they've only recently found their way into really high-volume applications, supplant- ing traditional Class A and Class AB amps, said analyst Will Strauss of Forward Concepts (Phoenix).

In addition to lackluster audio quality, the amplifier switching process had also been criticized for generating electromagnetic interference (EMI), which usually requires bulky output inductor/capacitor (LC) filters. To reduce output noise and total harmonic distortion (THD), many companies are now turning to a variety of modulation techniques, including delta-sigma modulation, as opposed to using conventional pulse-width modulation (PWM) in Class D amplifiers. In January, for example, both Analog Devices Inc. (Norwood, Mass.) and Texas Instruments Inc. (Dallas) said their latest Class D amps featured reduced EMI and improved sound quality. Both companies are using modulation schemes to achieve those results.

Analog Devices' AD199x Class D audio amplifiers, for instance, have less than 0.005 percent THD plus noise (N), and up to 2 to 40 watts of output power, while producing 30 percent less heat. This performance is achieved via ADI's closed-loop, mixed-signal integration of seventh-order sigma-delta modulator technology with high-power output drive and bridge circuitry.

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TI's high-performance filter-free stereo Class D audio power amp enables longer battery life and clearer audio quality in a 2 x 2-mm package. The TPA2012D2 draws 6 milliamps of supply current and has an A-weighted noise floor of 27 microvolts rms. In TI's newest Class D amps, the modulation scheme has been modified so that only a very short differential power pulse occurs to prevent "shoot-through" when there is no input signal, the company said.

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In September, National Semiconductor Corp. (Santa Clara, Calif.) expanded its audio Class D portfolio with two high-efficiency Boomer audio amplifiers. The LM4666 for portable handheld devices, including cellular phones, is a fully integrated single-supply switching audio amp. It has an innovative delta-sigma modulator that eliminates the LC output filters used with typical switching amps. The LM4668, aimed at flat-panel monitors, televisions and computer sound cards, uses a uniquely balanced PWM that makes it more immune to noise, according to National Semi.

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Designated the LX1701 line, Class D amplifiers from Microsemi Corp. (Irvine, Calif.) can drive up to 3 watts of mono output into 2-ohm speakers, or 2 watts of output into 4-ohm speakers, providing full 20-Hz to 20-kHz high audio fidelity, with a THD of less than 1 percent. Applications include LCD TVs, desktop monitors, cell phones, PDAs and other handheld or portable audio applications. A low quiescent current consumption of 2 mA results from a proprietary output modulation design.

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