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.