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Old 27th February 2010, 02:28 PM   #1
GEirin is offline GEirin  Argentina
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Default VAS with Miller compensation zero

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.
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Old 27th February 2010, 06:23 PM   #2
jcx is offline jcx  United States
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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 06:47 PM.
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Old 27th February 2010, 08:13 PM   #3
GEirin is offline GEirin  Argentina
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Hi JCX
Thanks. Very very good information.
I understand you explanation.
GEirin
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Old 27th February 2010, 10:15 PM   #4
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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.
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Old 28th February 2010, 01:23 AM   #5
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Quote:
Originally Posted by GEirin View Post
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
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Old 28th February 2010, 02:40 AM   #6
GEirin is offline GEirin  Argentina
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nigelwrigh77557 and Bob Cordll
Thanks very much for your replies.
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Old 28th February 2010, 03:01 AM   #7
Bigun is offline Bigun  Canada
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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.
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Old 28th February 2010, 03:02 AM   #8
GEirin is offline GEirin  Argentina
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Mr. Bob Cordell
What is your opinion about to use R+Miller cap with phase lead for compensation in VAS?
GEirin
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Old 28th February 2010, 01:12 PM   #9
GEirin is offline GEirin  Argentina
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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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Old 28th February 2010, 01:36 PM   #10
GEirin is offline GEirin  Argentina
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Default 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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