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Design >> Analog Design >> Brokaw's Bandgap loop stability
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Message started by aks on Nov 21^st, 2013, 6:23am

Title: Brokaw's Bandgap loop stability
Post by aks on Nov 21^st, 2013, 6:23am

Hi

Need some help in understanding Brokaw's bandgap cell. As far as my understanding in Bandgap circuit there are two feedbacks (+ve and -ve both) but -ve feedback should be the dominant in order to have stable circuit. But by looking at picture below (from brokaw's bandgap paper) it appears that loop formed by vout--> Q2--> -ve i/p of opamp to vout should be -ve feedback as it has larger feedback factor compared to the loop formed by vout--> Q1--> +ve i/p of opamp to vout.
But loop formed by vout->Q2-->-ve i/p opamp->vout suggests it has 360 deg phase shift (base to collector of BJT 180 deg + Opamp =180deg). If so the how is this circuit stable and how does it correct itself to vout=Bandgap ref?
Please enlighten me on this and correct me if I am missing any basic.

Thanks
AKS

Title: Re: Brokaw's Bandgap loop stability
Post by nrk1 on Nov 23^rd, 2013, 8:47pm

The gain from q1 or q2 base to their collectors is negative. And the one without degeneration has a larger gain. So the signs are fine.

Title: Re: Brokaw's Bandgap loop stability
Post by aks on Nov 24^th, 2013, 8:15pm

Thanks a lot nrk1 :) for clarifying my doubt.
I was pondering over it for quite sometime.