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Old 26th October 2004, 12:23 AM   #1
RHosch is offline RHosch  United States
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Default Class D and output impedance

Just curious... is there anything inherent in the design and/or operation of class D amplifiers that would prohibit them from having a high output impedance?
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Old 28th October 2004, 03:02 AM   #2
RHosch is offline RHosch  United States
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Hmm... that's kind of what I was thinking too, but I guess it took a good explanation to really convince myself!

Seriously... this is way out of my field of expertise. Is there a short answer (like "of course, you moron!"), of is this one of those questions having an answer filled with "well, maybe, if..., possibly but..., etc.?"
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Old 28th October 2004, 06:19 AM   #3
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The principle itself prohibts them to have a high output impedance !!!

O.K. That was indeed neither very technical nor entirely correct.

Without any feedback applied the output impedance is given by the filter, the rdson of the FETs and the PSU. That means at low frequencies it is possible to have a low output impedance without any feedback at all, thouhg I wouldn't build any amp without feedback.

If you apply NFB correctly you can achieve low output impedance throughout the whole audio range.

There is another thing that comes into play:
Because of the working principle there is no need for SOA protection like it is used with linear amps. Max-current limit is absolutely sufficient. This gives these amps the ability to drive quite "cruel" loads with ease. The bass reproduction of some class-d amps is really stunning. Not that they are doing megabass compared to linear ones but they reproduce bass in a very clean and effortless way.

Regards

Charles
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Old 29th October 2004, 06:05 AM   #4
RHosch is offline RHosch  United States
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hmm, so no ClassD as transconductance amps then. Bummer.
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Old 29th October 2004, 06:41 AM   #5
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Sorry I think I didn't quite understand what you actually wanted to know.
It is of course possible to make a class-d amp that is acting as a current-source. But it wouldn't be easy. Since you'd have to modify an existing design or make your own. And there are some stability issues as well.

Regards

Charles
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Old 29th October 2004, 07:16 AM   #6
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Hi,

actually I think this is very easy, even easier than voltage source class D design. Basically you take classD amplifier, omit filter capacitor and employ current mode control. I see several metods of various complexity:

-simple hysteretic current control of output inductor current. Drawback may be voltage noise and current ripple at the output.
-two phase hysteretic current mode control with self synchronised controllers like used in hybrid Vienna amplifier
-multiphase average current mode control with separate control of each phase current
-multiphase control with autotransformer output stage power combiners like used in RF amplifiers or more recently in Bruno's patent on mutiphase DSD amplifier.

Advantage of multiphase technique is ripple cancelation and in the last example also lowering of voltage noise at the output. If you interested in design, I can dig out references to the cited examples.

Best regards,

Jaka Racman
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Old 2nd November 2004, 12:57 AM   #7
RHosch is offline RHosch  United States
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I'm very interested in the fact that it should be possible. For now, I'll file that tidbit in the back of my mental warehouse somewhere, marked "take a look at this again soon."

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