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In the attached sub 1V bandgap reference circuit, the voltage across R3b should be a PTAT. In simulation I get a CTAT for both the legs. What could be the possible reasons?
It's all about the resistor tempco. "CTAT" is a narrow subset
of PTAT outcomes. That is, where the "proportionality
constant" is zero (-ish).
You can see very different core cell behavior with a zero-TC
thin film resistor, or a high-TC lightly doped silicon one, or
a grain-boundary-dominated high-sheet poly resistor (and
this latter, has terrible "make" tolerance against a roughly
uncontrolled poly grain size & qualities.
Of course there could be other details like a poor choice
of core area ratios at play, or an amplifier which has its own
countervailing Vio drift, or ....
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