The current mirror circuit is one very important to the understanding of the Op. It enables you to generate a circuit part has a current proportional to input current. This is a current mirror a current-controlled current source.
The current mirror is usually used in input stages of Op, in some high-quality Verstaerkerkonzepten find him up discreetly, usually with matched transistors in an enclosure to minimize temperature dependence.
Here links to current mirror schematics

Here you may find Circuit, Basics, Schematic Diagram and DIY Kit components for Current mirror Current Mirror