# Low Temperature Drift jfet Constant Current Circuit

Low Temperature Drift jfet Constant Current Circuit

You can almost see by looking at the transfer curves of a typical jfet that all the curves for various temperatures cross at a point. My point here is that they really do cross at a point.

Any particular part number for a jfet will have a range of Idss, and each piece will have a slightly different gain. The 2N4416 jfet in your hand might have an idle current of 5 ma to 15 ma, and they will not have the same zero temperature drift current as each other, but each of them will have a zero temperature drift current somewhere. It is wrong to sort some jfets for the same Idss and assume they will have the same zero temperature drift current, but you can adjust any individual jfet to a very low temperature drift. (That current will typically be less than 50% of the initial Idss.)

You can set up a circuit with a variable resistor and adjust it until the current change is very small when you heat the jfet. One good method is to use a heat gun to quickly change the temperature of the jfet and watch which way the current changes. This method includes the temperature drift of the rheostat, but you can replace it with low drift fixed resistors when you find something close to the set point you are looking for. This is a tedious process, and there are IC's that can do the same thing, but it is a cheap way to get a stable current for a personal project.

After you get a stable current, you can use it in all the traditional ways. You can make a stable voltage through a resistor or a constant voltage change on a capacitor. You can connect the constant current circuit to the Vcc rail and use it as a supply or connect it to the Vdd rail and use it as a drain. You can build these with n-channel or p-channel jfets. If you don't have an n-channel jfet, you can just build the circuit upside down with a p-channel jfet. It's a very simple and small circuit that can be treated as a 2 terminal device and installed almost anywhere you want it. It suffers no damage from having its output connected directly to ground or from any applied voltage less than the breakdown voltage of the jfet. Jfets are available with Idss from 5 ua to 150 ma (according to www.mouser.com). http://www.mouser.com/Semiconductors/Discrete-Semiconductors/Transistors/JFET/_/N-ax1rs/

You do need a few volts to keep the jfet in its operating range, and the usual limitations about applying too much power (and melting the transistor) apply.

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