The current mirror is very important to the understanding of the basic circuit Operationsverstaerkers. It allows you to create a circuit in one part of the input current proportional to current. This is a current mirror a current controlled current source.
The current mirror is usually used in input stages of Operationsverstaerkern in some high-quality Verstaerkerkonzepten find him up discreetly, usually with matched transistors in a housing to minimize the temperature.
Here links to current mirror schematics

Here you will find Schematic Circuit Diagram for DIY and further information on Current mirror Current Mirror
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Basic circuit of the current mirror	www-ebs.e-technik.uni-ulm.de/Tutorium/Grundschaltungen/g007/
Experiments with the current mirror	www.allaboutcircuits.com/vol_6/chpt_5/14.html