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).