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Bandgap reference circuit : Vbg (bandgap voltage) Vs. temperature curve

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student_of_analog

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Hi all,

I was working on a bandgap reference circuit and I wanted to know if the Vbg versus temperature curve,
i.e. the bow shaped curve has to have a centre point (flattest part of the curve) at 27 degree celsius, for Vbg=1.26V.
I was trying to simulate my circuit and I see that my curve is flat at the leftmost extreme rather
than being flat at the centre.

Any thoughts on why this is happening ?
 

The tempco is not optimized. There are two reasons you want
the flat in the center. One, it minimizes overall Vbg span across
the environmental envelope. Two, it positions your region of
least temperature sensitivity where you are likeliest to operate.
Although this latter, might vary by application (hotter or colder).

An arbitrary 1.26V target may not let you get the tempco you
want. The ideal "bandgap" voltage for a process, varies. Get a
good temperature curve and then target the rest of the circuitry
to whatever the bandgap is happiest at. Unless you have to
meet the arbitrary number.
 

sch.pngtempco.png

Thank you dick_freebird for the reply, so looks like the bandgap (tempco pic attached ) has
more negative tempco than positive tempco, so is that why the curve is flat around -200 degree celsius range ?
What should I do to increase the positive tempco and reduce the negative tempco so that I get flat part of the curve around 27 degree celsius ?
Also I need to meet the 1.25V number, so any hints on how that can be achieved along with the proper tempco?

Thanks for your reply again.
 

The curvature is apparent.
You may need to do curvature compensation to improve it.
You can find some good papers about that from IEEE
 

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