VAS with Miller compensation zero

GEirin · 27th February 2010, 01:28 PM

Hi all.

A Resistor with a Capacitor from collector to base of the second stage to create a unity gain stable amplifier.
This configuration VAS is good?
Why hardly any use RC compensation in VAS?

GEirin

JENSEN TRANSFORMERS, INC. - APPLICATION PAPERS AND SCHEMATICS
Some tips on stabilizing operational amplifier.

jcx · 05:47 PM

you should read:

http://web.mit.edu/klund/www/papers/ACC04_opcomp.pdf

basically you need to know where to put the zero - which depends on output circuit, minor pole locations and output loading

power amps usually have slower output Q relative to the front end devices compared to monolithic ops so some relations aren't as clearcut

output loading is usually unspecified and varies between setups, other poles can move because of bias/output current changes

few diy amp designer/builders will have the equipment or knowhow to measure effectively at or above the loop gain intercept frequency

so simpler compensation that errs on the "safe" side with some reduced performance is usually preferred

GEirin · 27th February 2010, 07:13 PM

Hi JCX
Thanks. Very very good information.
I understand you explanation.
GEirin

nigelwright7557 · 27th February 2010, 09:15 PM

The capacitor is a compromise.

Not so small that the amp oscilates.
Not so large that you start to lose bandwidth.

I tend to end up with around 220pF.

Bob Cordell · 28th February 2010, 12:23 AM

Quote:

Originally Posted by GEirin

Hi all.

A Resistor with a Capacitor from collector to base of the second stage to create a unity gain stable amplifier.
This configuration VAS is good?
Why hardly any use RC compensation in VAS?

GEirin

JENSEN TRANSFORMERS, INC. - APPLICATION PAPERS AND SCHEMATICS
Some tips on stabilizing operational amplifier.

Using the resistor in series with the Miller capacitor is something I have virtually always done. Where you put the zero is a matter of degree and good judgment. It should be done conservatively because the excess poles and excess phase you are using it to counter do move around with operating points, loads, etc.

I usually put the zero at least two octaves above the gain crossover frequency.

Getting the zero this way is, in my opinion, a much better way to get a zero than by placing a lead capacitor across the feedback resistor, since that can allow EMI from the speaker port to get back to the input stage.

Cheers,
Bob

GEirin · 28th February 2010, 01:40 AM

nigelwrigh77557 and Bob Cordll
Thanks very much for your replies.

Bigun · 28th February 2010, 02:01 AM

Even the regular approach with a simple Cdom seems not so simple for the beginner (me).

I had fun with Cdom on the TGM amplifier, changing the value and listening to the sound. It had an interesting affect on the sound. Smaller Cdom around 15pF (still stable) produced something in the treble I didn't like, something a bit bright. Larger Cdom around 100pF took the life out of the music. But the goldilocks value, somewhere around 40'ish pF in my case was just the ticket.

GEirin · 28th February 2010, 02:02 AM

Mr. Bob Cordell
What is your opinion about to use R+Miller cap with phase lead for compensation in VAS?
GEirin

GEirin · 28th February 2010, 12:12 PM

Hello Bigun.
You has experienced in the TGM1 a single miller cap (44pF) in VAS. For you, good sound.
My question: a RC compensation in VAS with phase lead, will be the best sound?
GEirin

GEirin · 28th February 2010, 12:36 PM

Hi all
In short
A RC compensation in the VAS, improvement the VAS?
A RC compensation is better than single Miller cap in the VAS?
Thanks
GEirin

27th February 2010, 05:23 PM	#2
jcx diyAudio Member Join Date: Feb 2003 Location: ..	you should read: http://web.mit.edu/klund/www/papers/ACC04_opcomp.pdf basically you need to know where to put the zero - which depends on output circuit, minor pole locations and output loading power amps usually have slower output Q relative to the front end devices compared to monolithic ops so some relations aren't as clearcut output loading is usually unspecified and varies between setups, other poles can move because of bias/output current changes few diy amp designer/builders will have the equipment or knowhow to measure effectively at or above the loop gain intercept frequency so simpler compensation that errs on the "safe" side with some reduced performance is usually preferred Last edited by jcx; 27th February 2010 at 05:47 PM.

27th February 2010, 09:15 PM	#4
nigelwright7557 diyAudio Member Join Date: Apr 2008 Location: Carlisle, England	The capacitor is a compromise. Not so small that the amp oscilates. Not so large that you start to lose bandwidth. I tend to end up with around 220pF. __________________ http://www.murtonpikesystems.co.uk PCBCAD40 pcb design software.

28th February 2010, 02:01 AM	#7
Bigun diyAudio Member Join Date: Jan 2009 Location: K-W Ontario Blog Entries: 2	Even the regular approach with a simple Cdom seems not so simple for the beginner (me). I had fun with Cdom on the TGM amplifier, changing the value and listening to the sound. It had an interesting affect on the sound. Smaller Cdom around 15pF (still stable) produced something in the treble I didn't like, something a bit bright. Larger Cdom around 100pF took the life out of the music. But the goldilocks value, somewhere around 40'ish pF in my case was just the ticket. __________________ "The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.

28th February 2010, 12:36 PM	#10
GEirin diyAudio Member Join Date: Jun 2004 Location: Mendoza	Improving the VAS Hi all In short A RC compensation in the VAS, improvement the VAS? A RC compensation is better than single Miller cap in the VAS? Thanks GEirin

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27th February 2010, 07:13 PM	#3
GEirin diyAudio Member Join Date: Jun 2004 Location: Mendoza	Hi JCX Thanks. Very very good information. I understand you explanation. GEirin

28th February 2010, 01:40 AM	#6
GEirin diyAudio Member Join Date: Jun 2004 Location: Mendoza	nigelwrigh77557 and Bob Cordll Thanks very much for your replies.

28th February 2010, 12:12 PM	#9
GEirin diyAudio Member Join Date: Jun 2004 Location: Mendoza	Hello Bigun. You has experienced in the TGM1 a single miller cap (44pF) in VAS. For you, good sound. My question: a RC compensation in VAS with phase lead, will be the best sound? GEirin

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