Figure (a) is Wilson current source.

Note that the Vds values of M1 and M2 are not equal. Since $\lambda $ is not zero, the ratio $I_o / I_{ref} $ is slightly different from the aspect ratios. This problem is solved in the modified Wilson current source as shown in the figure (b) which includes transistor M4. For a constant reference current, the drain to source voltages of M1, M2 ans M4 are held constant. The primary advantage of these circuits is the increase in output resistance, which further stabilizes the load current.