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Design >> Analog Design >> Help with a stability problem
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Message started by refugee on Jan 4^th, 2010, 5:37pm

Title: Help with a stability problem
Post by refugee on Jan 4^th, 2010, 5:37pm

I want to ask if the buffer in the loop will affect the close loop phase margin.
If yes, how does it affect the close loop phase margin. (by 90 degree?)

Thanks for your kindness help

Title: Re: Help with a stability problem
Post by Mayank on Jan 4^th, 2010, 8:42pm

Hi,
Anything in the signal path that has poles and zeroes will affect the phase margin...Why dont you just replace the buffer with it's Transfer Function and compute Phase margin from Overall Open Loop Transfer Function...which in your case is dependent only on Opamp Gain Response.

--
Mayank.

Title: Re: Help with a stability problem
Post by buddypoor on Jan 5^th, 2010, 1:19am

Hi,

sorry, but the whole circuit looks a bit "uncommon".
If you apply 100% negative feedback to a stage with a gain of "1" you get an overall gain of 0.5. Is this your intention???

Title: Re: Help with a stability problem
Post by rajdeep on Jan 5^th, 2010, 2:48am

Hi,

If I consider an ideal unity gain buffer (having infinite BW), then it can be said that loop gain of the overall ckt is always 0 DB, and the phase margin (of the loop gain) is 180deg!!!

If I consider a real unity gain buffer then the above statements remain true upto the BW of the unity gain buffer. Isnt it??

Also, why are you concerned abt the closed loop phase margin, when in the feedback path you have a direct connection i.e. no extra poles and zeros in the feedback path?

cheers!
Rajdeep

Title: Re: Help with a stability problem
Post by refugee on Jan 7^th, 2010, 5:55am

Mayank wrote on Jan 4^th, 2010, 8:42pm:

thank you, mayank. I think I got your idea

Title: Re: Help with a stability problem
Post by refugee on Jan 7^th, 2010, 6:02am

buddypoor wrote on Jan 5^th, 2010, 1:19am:

Hi,

sorry, but the whole circuit looks a bit "uncommon".
If you apply 100% negative feedback to a stage with a gain of "1" you get an overall gain of 0.5. Is this your intention???

sorry for the simplified diagram. there are some extra gm block in the forward path. in fact, the output is a large power device. so i need a buffer to drive it

Title: Re: Help with a stability problem
Post by buddypoor on Jan 7^th, 2010, 6:02am

refugee wrote on Jan 7^th, 2010, 5:55am:

Mayank wrote on Jan 4^th, 2010, 8:42pm:

thank you, mayank. I think I got your idea

Yes, everything which was said about the stability of the circuit is OK.
But, REFUGEE, did you realize that the overall gain is - 6dB ??
Is this really your intention?

Title: Re: Help with a stability problem
Post by loose-electron on Jan 7^th, 2010, 7:58am

Its a simplified conceptual drawing.

Loosen up!

Title: Re: Help with a stability problem
Post by buddypoor on Jan 7^th, 2010, 8:33am

loose-electron wrote on Jan 7^th, 2010, 7:58am:

Its a simplified conceptual drawing.

Loosen up!

In this case, I would say: It is dangerous and of no value to speak about stability of a system which is given only as a "simplified conceptual" drawing. Don`t you agree?

Title: Re: Help with a stability problem
Post by loose-electron on Jan 8^th, 2010, 1:28pm

How about - "Could you please provide a drawing that shows some more detail of what you are trying to do?"

buddypoor wrote on Jan 7^th, 2010, 8:33am:

loose-electron wrote on Jan 7^th, 2010, 7:58am:

Its a simplified conceptual drawing.

Loosen up!

In this case, I would say: It is dangerous and of no value to speak about stability of a system which is given only as a "simplified conceptual" drawing. Don`t you agree?