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.
the mixed-signal book’s errata page here).