ENGR325 Electronics II

**Lab #8 – Current Mirror Design**

**Objectives:**

· 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.

**Procedures:**

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 K_{n} = 20mA/V_{2}
and that V_{TN} = 1.7V for both transistors. Note that the current through Q1 we called I_{REF}
and through Q2 we called I_{O}.
If the two transistors are matched, I_{REF} should be the same
as I_{O}.

Figure 1. An NMOS Current Mirror Circuit

2. Build your circuit
and compare the actual current reading for I_{O} with the current value
that you calculated. Also, measure I_{REF}
to show that the two currents are equal.
Discuss your results.

3. Increase R2 now
while monitoring I_{O} to find the largest value of R2 that will not
affect the value of I_{O}.
Present and explain your results.

4. Can you suggest a
modification to your circuit so that the current I_{O} will be two
times I_{REF}? 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 I_{O} of 5mA.

6. Build the circuit
and again compare the actual current reading for I_{O} with the current
value you expected. Also, measure I_{REF}
to show that the two currents are equal.
Discuss your results.