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Larger the BJT area and lesser is the mismatch.it is always better to go for more wider BJT's and depending on the maximum junction voltage BJT can see,you can select 1.8V/3.3V BJT device
You probably don't care about voltage if the BJTs are diode-
connected. But you can also use the BJTs as the first stage
of gain, in which case you'd need the voltage rating to be
consistent with workng voltage + margin.
Lower-voltage BJTs may have higher gain, which would
be an accuracy / tempco bonus.
Bigger BJT will have better matching but will also see more
low current rolloff of gain. You do not want to be on the slope
(Hfe vs Ic) but rather the "flat top" so that gain is not very
operating-point dependent and startup issues will be minimized.
Look at low temp worst case low hFE model corner, for that.
It wouldn't hurt to try both flavors and see what shows up.
Bearing in mind that CMOS-process BJTs are seldom well
controlled, and often "casually" modeled.
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