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How to bias opamp in bandgap reference circuit?

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hebu

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Hwo to bias opamp which is used to force two nodes voltage eaual?

If we use PTAT current generated by bandgap core itself, another loop is created.
The stability is also a problem?

How to bias this opamp?
 

caosl

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you can bias it with other current reference(eg. a constant gm) or by bandgap itself, both working, no problem if designed carefully.
 

yibinhsieh

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Useing PTAT current from bandgap to bias the OP is OK.

But, it will have both positive and negative feedback.

Therefore, the OP design should be carefully to treat with the stability issue.

But it is almost OK if you use dominant pole compensation.

Yibin.
 

hebu

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yibinhsieh said:
Useing PTAT current from bandgap to bias the OP is OK.

But, it will have both positive and negative feedback.

Therefore, the OP design should be carefully to treat with the stability issue.

But it is almost OK if you use dominant pole compensation.

Yibin.
How to break the loop of positive and negative feedback to check the phase margin?
It's not straight forward to do this. any suggestion?
 

iamxo

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you can add a large inductor to keep dc pass but stop ac current.

then you can add an ac source to simulate the loop.

but this is not very accurately correct (because of the node impedance).
 

elbadry

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It's not a big concern to bias the opamp by the bias cell output. The bias current is a common mode signal that should see good rejection from the opamp. This will most probably reduce the loop gain to negligibly small values ( well below 1 ). You can also simulate the loop for PM and GM as well as perform transient step test to make sure everything is stable
 

caosl

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It's not a big problem, just do it accord to normal opamp design, bias the mosfets in saturation region.
 

hazemafify

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you can bias the OP-AMP by the mirrors in the biasing cell and about the stability you can put a compensation capacitance to ensure the stability and you must make stability analysis.
 

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