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 Class D Switching Power Amplifiers and Power D/A conversion

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 29th April 2010, 12:35 AM #11 diyAudio Member     Join Date: Oct 2003 Location: Near the sea You will need op-amps for implementing active poles, the trick is to wire them in such a way that the circuit does not rely on them much above 20khz. __________________ I use to feel like the small child in The Emperor's New Clothes tale
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Join Date: Jan 2006
Location: Kiel
Quote:
 Originally Posted by Eva You will need op-amps for implementing active poles, the trick is to wire them in such a way that the circuit does not rely on them much above 20khz.
May I rephrase your sentence? I would say "the trick is to wire them in such a way that the circuit does not rely on them near Fo of the output LC filter". I am right?

Also, is the usual integrator an example of the active pole filter? I have tried to read about it somewhere in Internet/Wikipedia, but it is usually connected with too complex mathematics for me...

 29th April 2010, 09:45 AM #13 diyAudio Member   Join Date: Apr 2010 @EVA: Why would I need active poles?
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Join Date: Jan 2006
Location: Kiel
Quote:
 Originally Posted by ViennaTom @EVA: Why would I need active poles?
Explanation in attached image...
Attached Images
 pole.GIF (45.1 KB, 703 views)

 29th April 2010, 01:53 PM #15 diyAudio Member   Join Date: Apr 2010 Oh I see, the idea of introducing an active pole is to have loop gain rising towards lower frequencies, right? Interesting idea...But what is this good for? Output impedance at low frequencies? Isn't it better to have constant loop gain throughout the audio band?
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Join Date: Jan 2006
Location: Kiel
Quote:
 Originally Posted by ViennaTom But what is this good for? Output impedance at low frequencies? Isn't it better to have constant loop gain throughout the audio band?
I would say, in this case each decides on its own, what is better...
But again, 25dB loop gain is too low for me...

 29th April 2010, 06:58 PM #17 diyAudio Member   Join Date: Apr 2010 @81bas: After some calculation and simulation, I came to the conclusion that the loop gain in my circuit is about 20 (equals 26dB) So you are quite right in assessing my circuit! You must be quite an expert! How did you calculate? I divided output pk pk amplitude by (comparator input peak peak ripple times audio frequency voltage gain). e.g. 360 / (0.32 x 56) = 20
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Join Date: Jan 2006
Location: Kiel
Quote:
 Originally Posted by ViennaTom @81bas: After some calculation and simulation, I came to the conclusion that the loop gain in my circuit is about 20 (equals 26dB) So you are quite right in assessing my circuit! You must be quite an expert! How did you calculate? I divided output pk pk amplitude by (comparator input peak peak ripple times audio frequency voltage gain). e.g. 360 / (0.32 x 56) = 20
oh, I am not an expert at all I have seen this value in previously attached image, and supposed that most of UCD designs will have the same loop gain too...
However, I completely agree with your way to calculate the loop gain in UCD. The key point here is the amount of switching ripple at the comparator input, as you mentioned... I think also, that increasing the switching frequency will increase the loop gain of UCD, since amount of the residual ripple will be lower!

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Join Date: Apr 2010
SOCD - implementing the pole...

So I implemented a pole @ ~20kHz (C4, R6). Of course w/o any opamp!!
Now the calculated loop gain is ~ 52dB from DC to ~1kHz, then falling linearly to ~ 26dB @20k. I think that is quite ok for ultra-high bass and lower-mid-area damping factor.
Step response: You can see from the small signal response: it is - of course - nearly perfect. Big signal step response looks like slight instability for the first glance, but it is not: If higher slew rate is demanded than output filter can deliver, the regulator loop is "overloaded" and the high loop gain "pole loaded" regulator will "remember" missed volt-second product during ascent or descent of output voltage and then answer with the overshoot. This is NOT instability. It will occur at high slew rates and can be more or less avoided by placing a bandwith limiting input filter in front of the amplifier (like 5Hz - 25kHz 2nd order, which I recommend for PA amplifier....)
Attached Images
 SOCD pole.jpg (159.0 KB, 647 views) SOCD pole step response 4 ohm 1.jpg (80.1 KB, 599 views) SOCD pole step response 4 ohm 2.jpg (164.9 KB, 543 views) SOCD pole step response 4 ohm 3.jpg (110.8 KB, 511 views) SOCD pole step response 4 ohm 4.jpg (114.2 KB, 116 views)

diyAudio Member

Join Date: Jan 2006
Location: Kiel
Quote:
 Originally Posted by ViennaTom So I implemented a pole @ ~20kHz (C4, R6). Of course w/o any opamp!! Now the calculated loop gain is ~ 52dB from DC to ~1kHz, then falling linearly to ~ 26dB @20k. I think that is quite ok for ultra-high bass and lower-mid-area damping factor.
Cool! I am not sure, whether it will work for real world comparator, but anyway...

Could you please say, how big is the difference in switching frequency between "zero" and "near clipped" output state in your simulations? Usually there is a 2:1 frequency drop, as Eva said, but it is for "normal" UCD...

Last edited by 81bas; 30th April 2010 at 03:46 PM.

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