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bandgap reference layout consideration

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hayaloo

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bandgap reference layout

hi all

im designing a bandgap reference with two serial bipolars but im stuck at layout. i need to know a technique for implimenting this circuit , i red that diode ratio must be 8:1 but i dont know the reason. if anyone can help me with designing layout for:
a)the cascoded current mirror(which consist of pmos)
b)the opamp
c)diode
d)resistors

for this bandgap circuit i realy appreciate it
thank you
 

dick_freebird

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The diode ratio does not have to be 8:1, the larger the
better in terms of mismatch-tolerance and the signal
amplitude for temperature. But 8:1 wants a 3x3 bed
with the "1" surrounded by the "8".

Resistors, try to use matched-geometry "slugs" as far
as possible and put dummies on the ends of rows.

Diodes, that's something you want a closer look at.
A diode-connected BJT is liable to be a lot more ideal
than a plain P-N diode. You would like to verify that
the diode you pick has -some- region where the I-V
curve is dead log-linear over at least two decades of
current. I work in an SOI technology where we only
have gated diodes and you can't find a straight line
to save your life. Which makes everything not come
out as simulated, needs veriloga custom models and
fun stuff like that. Point is that diodes get very little
respect from modeling departments, and what you
are depending on for your tweaking may all be a
bunch of "good enough" garbage. Take your own
data if you can, and see.
 

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