Editing Current mirror (section)

==== Extensions and reservations ====
A useful feature of this mirror is the linear dependence of ''f'' upon device width ''W'', a proportionality approximately satisfied even for models more accurate than the Shichman–Hodges model. Thus, by adjusting the ratio of widths of the two transistors, multiples of the reference current can be generated.

The Shichman–Hodges model<ref>{{usurped|1=[https://web.archive.org/web/20120617000000/http://www.nanodottek.com/NDT14_08_2007.pdf NanoDotTek Report NDT14-08-2007, 12 August 2007]}}</ref> is accurate only for rather dated{{when|date=March 2013}} technology, although it often is used simply for convenience even today. Any quantitative design based upon new{{when|date=March 2013}} technology uses computer models for the devices that account for the changed current-voltage characteristics. Among the differences that must be accounted for in an accurate design is the failure of the square law in ''V''<sub>gs</sub> for voltage dependence and the very poor modeling of ''V''<sub>ds</sub> drain voltage dependence provided by ''λV''<sub>ds</sub>. Another failure of the equations that proves very significant is the inaccurate dependence upon the channel length ''L''. A significant source of ''L''-dependence stems from λ, as noted by Gray and Meyer, who also note that ''λ'' usually must be taken from experimental data.<ref name=Gray-Meyer3>
{{cite book
 |author=Gray
 |title=p. 44
 |date=27 March 2001
 |publisher=Wiley
 |isbn=0-471-32168-0
 |display-authors=etal
}}</ref>

Due to the wide variation of ''V''<sub>th</sub> even within a particular device number discrete versions are problematic. Although the variation can be somewhat compensated for by using a Source degenerate resistor its value becomes so large that the output resistance suffers (i.e. reduces). This variation relegates the MOSFET version to the IC/monolithic arena.