Why does a gain-assisted current mirror need a left transistor?

[0xDEADBEEF]

What is the left transistor for? Just for the voltage drop?
http://en.wikipedia.org/wiki/File:Gain-assisted_current_mirror.PNG

 

[waht]

That's basically a current source that gives a reference current.

 

[dlgoff]

It's called a "diode-connected" transistor and is used, as what says, a current source. You will find them used this way in integrated circuits where small currents sources are required.