Figure 1 : Circuit diagram for MOSFET Wilson Current Source

1. Figure 1 shows the circuit diagram of MOSFET Wilson Current Source.

2. The VDS values of M1 and M2 are not equal. Since λ is not zero, the ratio of Io and Iref is slightly different from aspect ratio.

3. As a result their drain current ID1 and ID2 are also equal. This problem can be solved by adding one diode connected MOSFET, as shown in Figure 2.

4. This modified circuit balances two branches of the mirror and hence avoids the systematic current error that results from the difference in VDS.

5. This ensures that drain to source voltage of M1,M2 and M3 are held constant.

6. This increase output resistance, which further stabilizes the load current.

Figure 2 : Modified Wilson Current Source