temperature stability in current mirrors
In an NPN current mirror:



\$I_{in}\$ is converted into a voltage by \$Q_1\$ so we get \$V_{BE,Q1}\$.

That \$V_{BE,Q1}\$ is applied to \$Q_2\$ which converts that voltage back into a current \$I_O\$.

This all works by the fact that the voltage &lt;=> current relation:

(\$I_C(V_{BE})\$ and \$V_{BE}(I_C)\$)

of both transistors are absolutely identical.

Unfortunately this relation is quite temperature dependent.

That means that if \$Q_1\$ and \$Q_2\$ do not have the same temperature then their voltage &lt;=> current relations aren't the same and that means that the currents \$I_{in}\$ and \$I_O\$ will not be identical in value.

Possible solutions are:


couple the transistors such that they will be at very similar temperatures.
Use a component where two identical transistors share the same package
use this on an IC where all transistors share the same silicon die
use emitter degeneration to make the voltage &lt;=> current relation less dependent on the transistor: