Current Sources and Voltage References

Cascode current mirrors

Using BJTs to Create Current Sources

91

For more than two decades, manufacturers have used laser-trimming techniques

(most notably for adjusting op amp offset voltages) to precisely trim the values of the

bias resistors. Precision Monolithics Inc. (now a part of Analog Devices) was a pio-

neer in this technique known as

zener zapping

. It automatically trims V

OS

at the

wafer level, by using a computer-controlled laser to effectively short one or more of a

string of zener diodes, and thereby reduces the offset voltage from the millivolt level

to less than 100 µV. By using computer control, this process takes less than 1 sec-

ond ...