Class D amplifier

[russlk]

A number of years ago, (I don't like to think how many) I designed a switching amplifier for a Sonar transmitter application. Another engineer told me that it would not work because the intermodulation products would be too high. He was right, the sum or difference frequencies were only 6dB below the two tones. So, the question is, how do class D audio amplifiers work? Is the distortion low enough that it is not noticable?

[L. Daniel Rosa]

I'm no authority, but I'd put the switching frequency at least one decade higher than the highest frequency I wanted to amplify. The output would then go through a passive low pass filter to reduce the switching artifacts. By having the switching rate that high, you also reduce the aliasing.

[viveguy]

Texas instruments has a number of devices that are class D for audio use. You can get theory and application info at their site for free.Also try Linfinity.
As for as operation, in the case of the TI amps,the output comes out as a triangle. It is then run through a 2 pole L-C filter to shape the waveform into something more sinusoidal.
I don't know if this information relates well to RF but, it's the best i have.

[russlk]

The Texas Instruments class D devices claim low (.01% or better) distortion, but I bet that is with one tone. When there is multiple tones, like music, they are going to mix and produce sum and difference frequencies that are in the audio band and cannot be filtered out. I have not listened to one of these but can't believe it is as good as a linear amp.

[L. Daniel Rosa]

Some amplifiers have a multistage scheme with a class D supplying the power to a class AB with just a few volts to spare. The class AB stage cleans the signal. Result? A compromise between class AB quality and class D efficiency.<p>Low THD? Unless stated otherwise, THD is rated at 1kHz. The proof is in the listening. With extremely low THD ratings (assuming they're true), you probably won't notice outside of a sound room. You'll also need speakers that cost more than your car.

[Joseph]

You can get some information on an interesting Class D design at this site.<p>http://listen.to/audioexperiment

[russlk]

Thanks, Joseph, I will explore that site in detail.

[Joseph]

Your'e welcome, Russ. I have been tinkering with a design similar to the main one on that site.