The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> Analog Design >> Stability in design gain-boosting OP
https://designers-guide.org/forum/YaBB.pl?num=1188804130

Message started by Monkeybad on Sep 3^rd, 2007, 12:22am

Title: Stability in design gain-boosting OP
Post by Monkeybad on Sep 3^rd, 2007, 12:22am

Hi! everybody!
In designing the gain-boosting OP, we add another OP to boost the total gain.
How to design this added OP, how much power should I use to save power and keep the stability?
This means where should I place the -3db frequency of the added OP?

I find a paper discuss the gain boost OP
"A Fast-Settling CMOS Op-Amp for SC circuit with 90-dB DC Gain"

It mentioned the stability condition and I don't understand why.
For stability, W3<W4<W6

Could someone explain it? Thanks very much!

BEST REGARD

Title: Re: Stability in design gain-boosting OP
Post by Monkeybad on Sep 3^rd, 2007, 12:41am

The paper

The paper was removed. You can find it in the IEEE Journal of Solid-State Circuits, December 1990 edition. It is titled "A fast-settling CMOS op amp for sc circuits with 90 dB DC gain", and is by Klaas Bult and Govert J. G. M. Geelen.

-Forum Administrator

Title: Re: Stability in design gain-boosting OP
Post by Monkeybad on Sep 3^rd, 2007, 12:43am

gain boost pole
For stability, W3<W4<W6
W2 is the -3db frequency of the Aadd,
W3 is the -3db frequency of the Aorig,
W4 is the unit gain BW of Aadd,
W5 is the unit gain BW of Aorig,
W6 is the second dominant pole of Aorig,

Title: Re: Stability in design gain-boosting OP
Post by vivkr on Sep 3^rd, 2007, 1:30am

Hi,

You probably should not be posting a paper which you have not written so publicly. However, to answer your question,
there are 2 loops here, one formed by the main amplifier and the other by the gain boosting amplifier. Both have their
nondominant pole at W6, but have their dominant poles at W3 and W4 respectively. Hence, W6 > W3 & W6 > W4 follow
automatically.

As for W3 < W4, if the gain boosting amplifier has its dominant pole below the main amplifier, then you will have no useful
gain boosting, especially in case you are using it in a switched-capacitor amplifier. You will actually have a slow settling component
due to the pole-zero doublet formed at the lower of the 2 frequencies (W3 or W4).

I am surprised that you should ask this question, because the paper happens to be one of the best written ones with a great deal of explanation
about this very point.

Anyway, I hope that answers your question.

Regards
Vivek

Title: Re: Stability in design gain-boosting OP
Post by Monkeybad on Sep 11^th, 2007, 7:35pm

Thanks, vivkr
What you mean "the main amplifier"?
Did you mean the total amplifier(after gain boosting) or the original amplifier?

Title: Re: Stability in design gain-boosting OP
Post by ywguo on Sep 12^th, 2007, 4:52am

Hi Monkeybad,

The main amplifier means the original amplifier in Klass Bult's paper.

Yawei