JFET Current Source

The JFET (Junction Field Effect Transistor) component is easy to wire as a current source.

JFET N Channel J310 depletion mode current source sink circuit learning electronics lesson 0058
JFET N Channel J310 depletion mode current source sink circuit learning electronics lesson 0058

A current source is any component or power supply that sets a certain amount of current to a conductive load, regardless of how high the voltage is.

Technically speaking though, if the amount of current is being set on the low side of the load (closer to ground/negative supply), then it is actually a current sink.

There needs to be enough power supply voltage to cover the power loss from the current source, plus the power needs of the load.  The current source drops voltage, and provide resistance as needed to maintain a set amount current.

  • Make sure you never exceed the power rating of the current source (or any other component). Check the datasheet(s) for…
    1. Maximum voltage(s).
    2. Maximum current(s).
    3. Maximum device power dissipation (PD or Pd).

When it comes to N channel JFETs, you can automatically get it’s maximum Drain to Source current (Idss) by wiring both the Gate and Source directly to ground. Any load will be on the Drain side.

You will usually want to limit that current by adding a resistor between source to ground. The gate is still connected directly to ground.

The more resistance that is used, will result in less current than Idss. It will never cause more current than the Idss.

You simply start with a JFET that has more Idss than you need. Then add resistance as needed to lower the current to what you want.

A “current regulator diode” is a JFET that already has the Gate and Source wired together in order to always output it’s Idss.  They only give you 2 terminals to work with (since Source and Gate are tied together), so manufactures commonly package them to look more like a diode. They come presorted by  their Idss current.

  • I have S102-T current regulator diodes. They provide a pretty reliable 1mA of current as long there is enough supply voltage.


I got my J310 JFETs and S102-T current regulator diodes from the Joe knows electronics semiconductor kit. I really like having this kit as it provides a quick assortment of commonly known semiconductors. This is an affiliate link that supports this site when you click on it.

Idss:

Connecting the gate directly to the source will automatically set a maximum current that can flow through it. That amount of current is it’s Idss.

  • I is the mathematical symbol for current. Id basically means “current through the device”. “Device current” for short.
    1. A is the mathematical symbol for ampere (amp for short). Ampere is the unit, for current.
  • Double s is commonly used to refer to the Source pin (of an N channel FET) being connected directly to the negative power rail. Sometimes you will come across Vss being used to indicate the negative supply voltage (or ground) for a FET based circuit/IC. You might see it even if there are no FET transistors in the circuitry.

The Idss of a JFET is both variable by part number, and also manufacturing differences. Each part # will have a range of possible Idss.

  • A particular part number will have a certain range of Idss. Any randomly chosen J310 should have an Idss of between 24mA to 60mA. J310 is all I have, and it is a common component, so it is the only example I will be using. I got mine from the Joe Knows Electronics kit advertised below.
    1. If you absolutely need to set 60mA through a J310, then you might find one that can do so, if you have a lot of them to test. But, it’s better to just get a JFET that is already guaranteed to have at least 60mA Idss.
    2. If you will never need to set more than 24mA of current, then any J310 should work just fine.

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