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Design >> Analog Design >> opamp feedback confusion
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Message started by summi on Sep 30^th, 2012, 1:51am

Title: opamp feedback confusion
Post by summi on Sep 30^th, 2012, 1:51am

Dear forum,
I would appreciate if any can comment on the opamp polarity selection in the attached file? For me it make sense, however i haven't seen this kind of sign selection.

Br,
Summi.

Title: Re: opamp feedback confusion
Post by aaron_do on Sep 30^th, 2012, 2:52am

What's wrong with it? Just maintains negative feedback for both loops. I wonder what is the point of this kind of amplifier though?

Aaron

Title: Re: opamp feedback confusion
Post by summi on Sep 30^th, 2012, 10:43pm

many thanks to Aaron for the reply. I just want to make sure weather it is correct or not. Many people told me other way around(two op amp sign's are interchanged), Could you please tell is that correct? in my opinion it is not correct...

Br,

Title: Re: opamp feedback confusion
Post by aaron_do on Oct 1^st, 2012, 2:59am

I'm sorry, I looked at it wrongly. Your friends are right. The signs should be switched. The second stage is a non-inverting amplifier and the big loop is inverting.

Aaron

Title: Re: opamp feedback confusion
Post by summi on Oct 1^st, 2012, 3:18am

Dear Aaron,
second opamp also has -ve feedback, it has two feedbacks +ve through R1,R2 but -Ve through R4 and first opamp.
Point me if any thing wrong..

Br,
Summi.

Title: Re: opamp feedback confusion
Post by raja.cedt on Oct 1^st, 2012, 5:01am

Hello summui and Aron,
Your choice is correct, other choice would lead to +ve FB dominate over -ve feedback.

Thanks,
Raj.

Title: Re: opamp feedback confusion
Post by Ken Kundert on Oct 1^st, 2012, 11:09am

If I were you I would be very wary of this circuit and its large signal stability. This issue is that you have two opamps in a tight feedback loop. If the input of this circuit changes abruptly then the first opamp could enter slew rate limiting. During this interval the first opamp will be acting like an integrator and so will exhibit 90 degrees of phase shift. If the second opamp normally contributes the dominant pole, then you will have 180 degrees of phase shift around your loop and you will have created an oscillator.

At the very least, I would run a simulation of this circuit where I apply a large pulse with very rapid transitions to the input with the simulator using its conservative settings to see if the circuit is prone to sustained oscillations.

-Ken