which bandgap structure is better

Vamsi Mocherla said:

Remember that there are two loops in the bandgap circuit.

Loop I
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Positive Feedback, which is the bootstrapped diode connected current mirrors which can potentially increase the current.

Loop II
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Negative Feedback, which is the loop formed due to the diode in one branch and doide and resistor in another. The current through the resistor will determine the source voltage of the bottom NMOS transistors. Increase or decrease in the current will determine the increase or decrease in the source voltage of the NMOS FETs.

Implication
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Now for any stable circuit, the feedback is negative or the negative feedback should dominate the positive feedback. Hence both the feedbacks should act on the same node. Importantly the node at which they sum determine the stability of the circuit for them to be additive. The summing node in this case is the source node with transistors N2 and N4.

Hence the circuit on the left should be stable.

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DenisMark said:

Simple problem. Of course you have to use A. Circuit B is compeltely wrong.
Circuit A has both negative and positive feedbacks. Resistor R1 serves to reduce loop gain < 1 and to make it unconditionally stable. R1 can be placed in circuit only by one way which corresponds to circuit A (R1 with N2 form local negative feedback).
Circuit B has only positive feedback. That is why it's not stable.
In every reference you are able to find circuit similar to A (not B).

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