The design of current sources in analog circuits is based on ‘copying’ currents from a reference, with the assumption that one precisely defined current source is already available. Current mirror circuit is used to generate copies of a reference current. There are two major types- Basic / Active current mirror and Cascode current mirror.
Basic / Active current mirror:-
From above figure, Iref is drain current of M1. Therefore Iref can be given as,
Note that, here we have neglected channel length modulation effect. Similarly, Iout can be given by,
From (1) and (2)
Thus by precisely adjusting values of (W/L) ratios of both the transistors, we can get the required value of output current.