In MOSFET current-source circuits, the output resistance is a measure of the stability with respect to changes in the output voltage. This output resistance can be increased by modifying the circuit as shown in figure which is a cascode current mirror. Assuming all transistors are identical, then Id = Iref.

To determine the output resistance at the drain of M4, we use the small signal equivalent circuit. Since Iref is a constant, the gate voltages to M1 and M3, and hence to M2 and M4, are constant. The ac equivalent circuit for calculating the output resistance is shown in figure (a). The small signal equivalent circuit is given in figure (b). The small signal resistance looking into the drain of M2 is ro2.

Writing a KCL equation, in phasor form, at the output node. we have

$I_x=g_mV_{gs4}+\frac{V_t-(-V_{gs4})}{r_{o4}}$ .......(1)

Also, Vgs4 = -Ix.ro2 ........(2)

Substituting equation (2) in (1), we get

$I_x+\frac{r_{02}}{r_{04}}I_x+g_{m1}R_{02}I_x=\frac{V_x}{r_{04}}$

The output resistance is then

$R_0=\frac{V_x}{I_x}=r_{04}+r_{02}(1+g_{m1}r_{04})$

Normally, gm.ro4 >> 1, which implies that the output resistance of this cascode configuration is much larger than that of the basic two transistor current source.