BJT layout matching in bandgap ciruits

[saha.123]

In band gap circuits,
in layout if we have bjt 1:8 or 2:16 ratio the matching is done in such a way by keeping one bjt in centre and the remaining 8 surrounds the centre one(3X3) format.Can any explain the reason behind these matching concept??
thanks in advance

 

[dick_freebird]

The one-in-the-center is nicely "centroided". But for
even better matching you ought to consider another
ring of dummies outside the 1:N core, otherwise there
is still a radial litho matching dissimilarity.

 

[erikl]

Both the center cell and the other 8 cells have their own, best symmetrical environment. It doesn't matter so much if the ratio deviates a little bit from 8 (e.g. 7.9): the whole band gap circuit has to be adjusted anyway.

If you need more BJTs anyway, you could surround the 3x3-cell block by another 16 "dummies" to create a 5x5-block, thereby guaranteeing a ratio closer to 8.0 for the inner block, for a nearly 3-fold greater area cost, however. And more difficult routing, of course.