Editing Current mirror (section)

==== Extensions and complications ====
When Q<sub>2</sub> has ''V''<sub>CB</sub> > 0&nbsp;V, the transistors no longer are matched. In particular, their ''β''-values differ due to the Early effect, with
: <math>\begin{align}
 \beta_1 &= \beta_0 \\
 \beta_2 &= \beta_0 \left(1 + \frac{V_{CB}}{V_A}\right),
\end{align}</math>
where ''V''<sub>A</sub> is the [[Early effect|Early voltage]] and ''β''<sub>0</sub> is the transistor ''β'' for ''V''<sub>CB</sub> = 0&nbsp;V. Besides the difference due to the Early effect, the transistor ''β''-values will differ because the ''β''<sub>0</sub>-values depend on current, and the two transistors now carry different currents (see ''[[Gummel–Poon model]]'').

Further, Q<sub>2</sub> may get substantially hotter than Q<sub>1</sub> due to the associated higher power dissipation. To maintain matching, the temperature of the transistors must be nearly the same. In [[integrated circuit]]s and transistor arrays where both transistors are on the same die, this is easy to achieve. But if the two transistors are widely separated, the precision of the current mirror is compromised.

Additional matched transistors can be connected to the same base and will supply the same collector current. In other words, the right half of the circuit can be duplicated several times. Note, however, that each additional right-half transistor "steals" a bit of collector current from Q<sub>1</sub> due to the non-zero base currents of the right-half transistors. This will result in a small reduction in the programmed current.

See also an [[two-port network#Example: bipolar current mirror with emitter degeneration|example of a mirror with emitter degeneration to increase mirror resistance]].

For the simple mirror shown in the diagram, typical values of <math>\beta</math> will yield a current match of 1% or better.
[[File:Simple MOSFET mirror.PNG|thumbnail|200px|Figure 2: An n-channel MOSFET current mirror with a resistor to set the reference current I<sub>REF</sub>; V<sub>DD</sub> is positive voltage.]]