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Old 15th October 2004, 07:26 PM   #11
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Default Re: Coldn't resist: feedback from 4th order lowpass !!

Quote:
Originally posted by phase_accurate
Just some remarks:

1.) I actually know that real life is real life and simulations are just simulations so I don't want any discussions on that subject.

Regards

Charles
What happens when you drive things into clip?

Tah
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Old 15th October 2004, 07:45 PM   #12
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Unfair, possibly but....

There is a difference between small signal and large signal operation.

Clip1 is the amp overdriven with a 'complex feedback loop'
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Old 15th October 2004, 07:48 PM   #13
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Here's 'normal, operation.... the amp is not driven outside its limits
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Old 15th October 2004, 07:51 PM   #14
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Here's what you get if you limit the loop(s)
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Old 17th October 2004, 08:55 AM   #15
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Hi Genometrics

What type of amp did you simulate ? The behaviour looks like expected from a higher order forward-path topology like the one used within higher order ds-modulators. Is it ?
The picture below shows what I got when I made a simulation of "my" amp overdriven by 6 dB. It also shows a slight tendency to "stick to the rails" but not very severe, but precautions would have to be taken anyway since the user expects "normal" behaviour from an amp in every situation.
One would have to use a double- or higher- order loop for getting good THD and IMD with this amp, and then the same behaviour accounts as for your the amp that you simulated.

Regards

Charles
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Old 17th October 2004, 02:00 PM   #16
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Hello Charles,
you did it! Seems that RLC tolerance requirement is pretty hard (28.38uHn; 515nF)?
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Old 17th October 2004, 03:59 PM   #17
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Quote:
Seems that RLC tolerance requirement is pretty hard (28.38uHn; 515nF)?
Hi Ivan

I haven't tried the circuit's susceptibility to tolerances. But I also think that tolerances must be tight and that's where 2nd order output filters definitely have their merits.
But fourth order LPF should be doable. I will do a little more thinking about output filter topologies. Maybe some compromise between 2nd and 4th order will come to my mind. But this wouldn't be a classic inductor-capacitor-inductor 3rd order lowpass because this might react dangerously to capacitive loading.

Regards

Charles
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Old 7th November 2004, 01:28 PM   #18
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Hi Charles,
did you see Bruno's WO 2004/049561 A1 patent? I've today download it, big stuff about PWM noise shaper, and US 6803816 B2 - seems new too.
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Old 11th November 2004, 03:44 PM   #19
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hi.

>Because there are still many out there who think that it is almost impossible to build PWM amps using post-filter NFB takeoff >
no you dont see many but there are some (we have a few....)

rgds - karsten madsen - cadaudio.dk
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Old 11th November 2004, 07:40 PM   #20
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hi km,
and they 4th order too?
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