- Joined Mar 19, 2019
The Simpson 260 VOM requires a 50uA constant current source for calibration. So, I've been using that as a target for my circuits output. Here are a couple of JFET circuits I've been working with for the last few days. Adding the 2nd JFET "flattens" the output "slope" across varying loads but also adds instability to the output. I do realize that I am talking about being down in the nanoamp realm. Any tricks as to adding stability to these circuits? The circuits slowly (hours) drift ±1uA and rapidly (minutes) drift ±1-10nA. Touching/pinching the JFETs had no effect on drift so I don't think it is thermal drift in the transistors. I am using a decade resistance box as the "potentiometer" to set the current so I suspect the several resistors in the box may be the culprit. Or, is it simply "noise" from outside effects?