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Design >> Analog Design >> Stability in Positive feedback
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Message started by georgejor on Jan 9^th, 2006, 12:57am

Title: Stability in Positive feedback
Post by georgejor on Jan 9^th, 2006, 12:57am

Dear all,
I would like to ask the stability requirement of OpAmp in positive feedback.
I do not know is the requirement same as negative feedback one? ie. phase is prefer > 45 degree?

For example:
Eopamp in+ in- out+ out- gain=1000
R1 in+ vss 1k
R2 in+ out+ 1k
Input: in+ & Output: out+

What is the specification of real Opamp is necessary?
If there is any good material teach positive feedback, please let me know.
Thank you very much!

Best regards,
George

Title: Re: Stability in Positive feedback
Post by vivkr on Jan 9^th, 2006, 5:07am

Hi George,

The fundamental condition for stability (the Nyquist criterion) does not depend on whether you are considering -ve or +ve
feedback. As long as the closed-loop poles of the circuit neevr reach the right-half s-plane, you will have a stable circuit.

I recommend that you read Chapter 14 of Thomas H. Lee's text "The design of CMOS RF Integrated Circuits". This chapter
deals with feedback and it should help answer your questions.

Regards
Vivek