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Old 1st June 2010, 02:09 PM   #1
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Default Another post-filter NFB topology

Since yesterday I have the official permission to publish this paper wherever I want. So I decided to do it here as well.

As soon as time permits I will also post info on:

- A short "how to" for designing this loop.

- How we get rid of the first op-amp. This might be intersting for those who want to use it for a self-oscillating amp (hint: the modulator -> power-stage chain has to be non-inverting and the impedance relationships of the loop-filter have to be chosen accordingly).

- How to make the loop an integrating loop or one with a lower corner frequency (hint: make one of the OP-AMP stages a PI controller or LAG filter with a zero at fL).

- How to lay another loop around the whole thing (hint: LAG filter).

Since this is 1.) not patented and has 2.) already been officially released it can be used for commercial or private purposes by whover wants to do so (maybe the first Chinese production runs will even be released within the next 24 hours ).
But it would be nice if the ones who use it would give some feedback about what they use it for and if they are satisfied with it.

Its use is of course not restricted to audio. It might also be applied to motor controllers and power supplies and other switching power converters. It might in fact as well be used for feedback control of ANY topology that includes a 2nd order lowpass function (whether electrical or mechnaical or whatever).

Regards

Charles
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Old 1st June 2010, 04:39 PM   #2
81bas is offline 81bas  Germany
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figures 7a, 7b and 8 are almost unreadable

How deep is the loop gain in the example design in this paper for audio frequencies?
Also, as I understand, output stage gain must be constant, right?
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Old 1st June 2010, 07:25 PM   #3
Andy F is offline Andy F  Spain
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Hi Charles:

Very interesting paper and very good approach. My congratulations for your effort and dedication.

Andy.
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Old 1st June 2010, 08:01 PM   #4
Baldin is offline Baldin  Denmark
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Very interesting. Need to read it through in detail
Thanks for just sharing this information ..... a rare thing these days
Best regards Baldin
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Old 1st June 2010, 10:48 PM   #5
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Default Impedance Ratios

Hi Phase-Accurate
You mention the need to watch the ratio of impedances to keep the RC(H) from being loaded by the input impedance of the differentiator. What do you think is good for the RH to R2 ratio 1:10 or 1:5? I guess I could just simulate it but if you have a rule of thumb? Also, the zero from RC(z) do you just place this where you need a little bump in the phase?

Thanks.
Eric

Last edited by ericbrooking; 1st June 2010 at 11:15 PM.
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Old 2nd June 2010, 09:33 AM   #6
IVX is offline IVX  Russian Federation
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Hi Charles, I'm working in China 3nd year, trust me, man, class D is not hot topic anymore, unlike 10 years ago.
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Old 2nd June 2010, 06:15 PM   #7
ArthurG is offline ArthurG  France
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Quote:
Originally Posted by IVX View Post
Hi Charles, I'm working in China 3nd year, trust me, man, class D is not hot topic anymore, unlike 10 years ago.
What do you mean by "class D is not hot topic anymore" ?
I also live in China and in my field (pro audio) I still have not seen any good and reliable high power class D amp coming from here... All I have seen is low power integrated ICs for hifi/DIY and bad copycats of Lab.Gruppen and Powersoft.
Class D is so hot topic that we have orders for the next 2 years as nothing is available at a cost sensitive price...
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Old 2nd June 2010, 06:43 PM   #8
AP2 is offline AP2  Italy
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Quote:
Originally Posted by ArthurG View Post
What do you mean by "class D is not hot topic anymore" ?
I also live in China and in my field (pro audio) I still have not seen any good and reliable high power class D amp coming from here... All I have seen is low power integrated ICs for hifi/DIY and bad copycats of Lab.Gruppen and Powersoft.
Class D is so hot topic that we have orders for the next 2 years as nothing is available at a cost sensitive price...
hi ArthurG,
which uses amplifiers in your field?
or you build them? (High-power I mean)

Regards
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Old 3rd June 2010, 12:50 PM   #9
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To 81bas:

I will post the original graphics as soon as I have undug them. The problem was that I had to compress the paper in order to fit it to the size restrictions of this forum.

Yes, a constant gain of the the modulator -> power-stage chain is assumed which is usually the case (within practical limits of course). This is of couse depandant on PSU voltage for a classic PWM amp with a triangular carrier. So you'd have to use a reasonably stiff PSU or use compensation. If you are doing a self-osciallting amp then the compensation is intrinsic to the working principle !


To ericbrooking:

1:10 would be nice from the frequency-response accuracy point-of-view. 1:5 would often be better regarding the loading of the first op-amp. The calculated example uses 1:5 as well. Even smaller ratios could be used if: 1.) someone either derives the exact formulae (maybe I'll do it when I am really bored !) or 2.) plays around with a simulator.

The 2nd order loop is intended to achieve additional NFB and not to compensate for any poles. I.e. the open-loop gain will start to roll off at 6dB/octave and then turn to a 12 dB /octave rolloff when going higher up the frequency axis and then return back to 6dB / octave at a frequency that is far enough below the unity-gain point.

Carrier-based amps are prone to distortion that is CAUSED BY the feedback loop (by ffeding back the carrier residual). Self oscillating topologies don't have this. There are several solutions for the problem like compensation schemes that rely on accurate dimensioning. Another solution is as so called MAE loop that doesn't fully compensate for the problem but reduces it. There is a specific dimensioning of such a 2nd order loop that has this MAE characteristic. I will post this dimanesioning a soon as I have undug it.

Regards

Charles

Last edited by phase_accurate; 3rd June 2010 at 12:52 PM.
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Old 3rd June 2010, 01:25 PM   #10
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cancelling one pole of output filter
wow
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