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Class D amps reduce EMI, improve sound

Once dismissed as providing only average-quality audio, Class D switching
amplifiers appear now to be turning the tide and gaining acceptance in consumer
applications. Companies like Analog Devices, Microsemi, National Semiconductor
and Texas Instruments recently announced a wave of analog-input Class D
amplifiers.

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.


Cell Phone Application for TI's TPA2012D2 Class D Amp

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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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