Current Sources and Voltage References

166

JFET current regulator diodes

P-channel. The N-channel example is adjustable over a 500-µA range. However,

while convenient, the tempco of the potentiometer will degrade the overall perfor-

mance. Once the desired setting has been determined, one should replace the

potentiometer with a good-quality, low-tempco, low-tolerance (0.1%) metal-film resis-

tor. The P-channel JFET provides a fixed current with good performance, using just

two parts, which could both be tiny surface-mount components.

Circuit B shows a fixed 2-mA cascode current source, which can provide better than

300-ppm regulation, with an output impedance of more than 5 MΩ. The circuit shown ...