Current mirror biasing circuit
Basic function of current mirror here is to "copy" the Ibias current to main circuit via dividing by 25 (due to transistor sizes 600 vs. 24).

In order to maintain stability of circuit, we generally use transistors in saturation. Very small current may cause transistor not to saturate. 

We can use transistors to obtain Ibias instead of resistors. If you maintain saturation of transistor (see diode connected), you can get stable and constant current.