ENGR325 Electronics II
Lab #8 – Current Mirror Design
· To experience the operation of a current mirror using MOSFETs.
· To compare an NMOS and a PMOS current mirror
· To compare the results with theory.
1. Begin by designing a current mirror like that of Figure 1 below. Choose a value of R1 that will give you a current through Q1 of 5mA. Start with R2 = 0. Use the BS170 transistors and assume that Kn = 20mA/V2 and that VTN = 1.7V for both transistors. Note that the current through Q1 we called IREF and through Q2 we called IO. If the two transistors are matched, IREF should be the same as IO.
Figure 1. An NMOS Current Mirror Circuit
2. Build your circuit and compare the actual current reading for IO with the current value that you calculated. Also, measure IREF to show that the two currents are equal. Discuss your results.
3. Increase R2 now while monitoring IO to find the largest value of R2 that will not affect the value of IO. Present and explain your results.
4. Can you suggest a modification to your circuit so that the current IO will be two times IREF? If you have an idea, try it and report your results.
5. Sketch the schematic for a circuit equivalent to that in Figure 1 except that it uses PMOS transistors. Assume that the transistor parameters given above also apply to the BS250 transistor,
and choose a value of R1 that will again produce a current IO of 5mA.
6. Build the circuit and again compare the actual current reading for IO with the current value you expected. Also, measure IREF to show that the two currents are equal. Discuss your results.