Current Sources and Voltage References

Wilson current mirrors

Using BJTs to Create Current Sources

81

I

out

at 25˚C = 252 µA

I

out

at 0˚C = 248 µA

I

out

at 85˚C = 267 µA

Using the equation 4.48 would yield:

We can also express the tempco as a value in

parts per million per degree Celsius

(ppm/˚C), by using the formula in equation 4.47:

In this example, the output current’s tempco could also be expressed as 887 ppm/˚C—

only a mediocre current source design over the temperature range. This would sug-

gest that some more work may be needed to bring its tempco down to a few hundred

ppm or lower.

In summary, the lower the tempco, the less the current source will drift over the

intended temperature range ...