current mirror
Current mirror consists of 2 perfectly matched transistors(either BJT or MOS) with their gate connected.. In one of them,the gate and drain wil be shorted so that both wil be at the same level and since Vds > Vgs - Vt,the transistor wil be in saturation region..
We can see that even the 2nd transistor wil have the same Vgs and Vds values as the first transistor.. Hence the currents in the both transistors wil be same.. This is because,we assume both the transistors to be matched..
Thus we can use the transistor with the gate shorted to the drain as reference current and can use it to control the current to the other transistor to which load is usually connected!