Single MOSFET current source

• 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.

Cascode current mirrors:-

• In the analysis of basic current mirrors, we have neglected channel length modulation. In practice this effect results in significant error in copying currents, especially if minimum length transistors are used so as to minimize the width.

• For basic current mirror, we can modify the equations by considering channel length modulation. We can write,

  

• While VDS1 = VGS1 = VGS2, VDS2 may not be equal to VGS2 because of the circuitry fed by M2.

• In order to suppress the effect of channel length modulation, a cascode current mirror can be used.

• As shown in figure below, one more transistor M3 is added. In the circuit of basic current mirror and Vb is chosen such that Vx = Vy, then Iout closely tracks Iref.

  

• Now, to generate Vb in above circuit (figure a), since the objective is to ensure Vx = Vy, we must guarantee Vb – VGS3 = Vx or Vb = VGS3 + Vx.

• This result suggests that if a gate source voltage is added to Vx, the required value of Vb can be obtained.

• As shown in figure b, the idea is to place another diode connected device M0, in series with M1 and thereby generating a voltage VN = VGS0 + Vx.

  

• The proper choice of the dimensions of M0 with respect to those of M3 yields VGS0 = VGS3. Thus if

  

MOSFET Wilson current source:-

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