How
would one implement a current mirror with a non-integer ratio and good
matching?
Very good question. The
inexperienced designer might think that simply by adjusting the
width and/or length of
the MOSFET a non-integer ratio current mirror can be designed
with good matching.
Unfortunately, as you clearly know, the effects of oxide encroachment
and lateral diffusion coupled with,
in nano-CMOS processes, the variation in the MOSFET’s
electrical
characteristics, such as the threshold voltage, with differing device sizes
make
good
matching challenging.
So,
to answer your question, I would use the topology seen in Fig. 30.20 and sum
the
segments to get the
ratio you need. The same size device is used for the entire circuit.
The
2W/L device is implemented using two W/L devices in
parallel…see comments on
page 1043 and
elsewhere in the book (e.g. Fig. 20.35) concerning matching.
Note
that some MOSFET models have issues accurately simulating this W-2W
circuit.
See
the comments here from
the first edition of the mixed-signal book (see the bottom of
the mixed-signal book’s errata page here).