Miller compensation with a series RC network? - diyAudio
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Old 12th November 2004, 11:09 PM   #1
Bricolo is offline Bricolo  France
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Default Miller compensation with a series RC network?

Hi,


Usually, the Miller compensation is done with a cap. I won't do the maths here, but in the case of an opamp (among others), this compensation creates the non dominating pole, and a zero. The problem with this zero is that it has a positive real part, that makes an instable zero and so reduce the phase margin, degrade the impulse response.... That's basical opamp theory.

This week, one of my teachers told me rapidly about something: using a series resistor with this cap (a series RC network). According to him, this tip has many advantages: the series RC network creates a zero with a negative real part, so stable, so this would add 90 to the phase margin (instead of substracting 90)

A starting R value would be 1k, and the RC time should be equal to the non dominant pole's frequency, in order to compensate it and have a truely 1st order rollof.

But we only talked rapidly about that, and he didn't have the time to show me the maths.
I tried to show this with the math myself, but wasn't able to obtain something with a stable zero. And my simulations with this RC don't show a stable zero, nor more phase margin. But only a gain like this: ˉ\_ and a V shaped phase.


Does anyone know this kind of miller compensation? Can you tell me more about it?

Thanks
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Old 13th November 2004, 08:14 AM   #2
Bricolo is offline Bricolo  France
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no one?
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Old 13th November 2004, 09:54 AM   #3
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Default It's used.

Hi Bob,
You will find that this or its variations are used in many amps .
I haven't worked out the maths nor simulated a circuit with it. If it takes time to draw it up , its a disincentive.
However if you post your schematic and want others to check it out , it might be a faster way to get a response.
Cheers,
Ashok.
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Old 13th November 2004, 10:31 AM   #4
Bricolo is offline Bricolo  France
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I'm not Bob, but thank you for thinking this


No, I don't have a specific schematic I'm working on? I was just wondering about this RC compensation.
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Old 13th November 2004, 02:53 PM   #5
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Default Hmmmm........ I goofed !

Hi Bricolo,
I did wonder about the name because I know that another enlightened one has the same name.
Only after your reply did I realise that you were just quoting him.
Well you did feel good for some time !

You should generally feel good about everything else also and that would normally be above all the rest !

Life is great ! ...........But only when it's going your way !
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Old 27th February 2010, 01:19 PM   #6
GEirin is offline GEirin  Argentina
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Default VAS with Miller compensation zero

Hi all.
Here a schematic.
This configuration is good?
Why hardly any use this configuration in amplifiers?

GEirin

JENSEN TRANSFORMERS, INC. - APPLICATION PAPERS AND SCHEMATICS.
Some tips on stabilizing operational amplifier
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File Type: jpg Miller compensation zero.JPG (20.4 KB, 91 views)
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