Apogee Technology has complained about some of Texas Instruments Inc.'s Class D claims.
In March, TI called itself the first to introduce a single-chip Class D amplifier in the 125-watt/channel range. But Apogee insists it was first, having introduced a 120-W/channel Class D part last November. (Actually, the Apogee parts have been made by STMicroelectronics, TI's biggest competitor for volume shipment of analog ICs. ST introduced its own 200-W amp in January 2004.)
It's no surprise that Class D is becoming a battleground, but the real issue isn't who was first with 120-W parts. Class D has evolved from a whispered development and an audiophile's anathema to a highly fashionable adjective for any current pump-needed for audio or not.
The high-power, high-current power amp sector-occupied by International Rectifier, Zetex and Philips, along with Apogee, TI and ST-is only one segment of power amps, which have a broad spread. Known for their power efficiency, Class D amps are finding their way into handheld systems like press-to-talk phones, where they can drive speakers to a relatively high volume without taxing battery life. These are 1-, 2- and 3-W amps. Another sweet spot is for 5-, 15- and 25-W parts in the cabinets of new-generation flat-panel TVs. These segments are targeted by National Semiconductor, Microsemi Corp., Wolfson Microelectronics and others.
Analog Devices Inc., meanwhile, is targeting the midrange and higher-power segments. Here, 50- and 100-W products serve space-constrained, everything-in-one-box home theater systems-DVD players and receivers with Dolby Digital surround sound. Class D amps can provide high speaker volumes while their efficiency minimizes heat dissipation.
Class D amp technology depends on a switching regulator in which the width and frequency of a digital pulse train create a facsimile of an audio signal. A filter bank-a capacitor and an inductor-smooths out the digital pulses. To simplify design, Class D amp makers have tried heroically to cut the number of ex-ternal components required, but there's always a penalty in sound quality.
The distortion spec on these devices (THD + N) is about 10 percent at their rated power. So, unlike the Class AB technology, Class D amps may be listenable at a fraction of their rated power.
The press release for TI's 125-W integrated amp, for example, cites a "signal-to-noise ratio of up to 110 dB and an impressive total harmonic distortion, typically less than 0.1 percent." But low THD is good for only about the first 40 or 50 W. Data sheets for most of these amps show a distortion-vs.-power graph that looks like an L turned on its side.
With a delta-sigma modulator (rather than a PWM pump handle), ADI's 40-W part is said to have less than 0.005 percent THD-but that's only up to 12 or 15 W. Still, when the audio is an action film's exploding bombs and ricocheting bullets, how much does quality matter?
Since ADI, TI and National have friends among the high-end-receiver makers, they are serious about improving sound quality. Probably moving to promote its DSP line as much as Class D, TI had listening sessions at CES.
And just as Class AB is a power-saving extension of Class A, National is hoping to render Class D as a power-saving extension of Class AB. The company is working on a driver subsystem that can alternate between AB and Class D topologies, depending on power needs.
Stephan Ohr (firstname.lastname@example.org) is technology editor for the EE Times Network and site editor for planetanalog.com.
The distortion specs for many
Class D amplifiers resemble an 'L' turned on its side.