Current Sources and Voltage References

86

Wilson current mirrors

cell, as shown in Figure 4.27, along with the relevant equations. Again, the NPN cir-

cuit functions as a mirror-sink, while the PNP circuit is a mirror-source. Of the two

types, the PNP version is more commonly used, because it is used primarily for pre-

cision biasing and as a high-impedance active load. Adding emitter degeneration

(equal emitter resistors, with values typically of between 100 Ω and 1.5 KΩ), as

shown in Figure 4.27, compensates for any slight mismatches in the transistors’

V

BE

s. This helps improve overall accuracy and fully optimizes the current source for

best overall performance.

While the calculations and ...