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).