Miller compensation with a series RC network?

Bricolo · 12th November 2004, 11:09 PM

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

Bricolo · 13th November 2004, 08:14 AM

no one?

ashok · 13th November 2004, 09:54 AM

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.

Bricolo · 13th November 2004, 10:31 AM

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.

ashok · 13th November 2004, 02:53 PM

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 !

GEirin · 27th February 2010, 01:19 PM

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

12th November 2004, 11:09 PM	#1
Bricolo diyAudio Member Join Date: Nov 2002 Location: Grenoble, FR	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 __________________ Just remember: in theory there's no difference between theory and practice. But in practice it usually is quite a bit difference... Bob Pease

13th November 2004, 08:14 AM	#2
Bricolo diyAudio Member Join Date: Nov 2002 Location: Grenoble, FR	no one? __________________ Just remember: in theory there's no difference between theory and practice. But in practice it usually is quite a bit difference... Bob Pease

13th November 2004, 09:54 AM	#3
ashok diyAudio Member Join Date: Jun 2002 Location: 3RS	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. __________________ AM

13th November 2004, 10:31 AM	#4
Bricolo diyAudio Member Join Date: Nov 2002 Location: Grenoble, FR	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. __________________ Just remember: in theory there's no difference between theory and practice. But in practice it usually is quite a bit difference... Bob Pease

13th November 2004, 02:53 PM	#5
ashok diyAudio Member Join Date: Jun 2002 Location: 3RS	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 ! __________________ AM


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