How to test a JFET transistor

This is the simplest way to test a JFET transistor, there is a more difficult way using a multimeter that I will show you later in this article but I always think it’s best to start with the easy way. If you’ve read any of my other stuff you’ll know I’ve mentioned these component testers before. If you want to know more you can read here.

You will need to have the JFET out of circuit so you will have to de-solder it from the circuit board first. If you don’t know how to do this I have an article you can read here.

This is the easiest way to test a JFET transistor. Just plug the device in throw the lever to hold and connect it up and press the button. I’m pretty sure anybody could do that.

You not only get to know that the device is working but it will show you what the pin out and some other stuff.

As you can see from the picture above, the component is confirmed as a working N channel JFET. You can also see that pin 1 is the source, pin 2 is the gate and pin 3 is the drain.

You don’t need to know anything about the device you are testing and you don’t need to try and hold it while holding multimeter probes on to various legs and take readings. It’s a lot quicker to use the component tester and if you haven’t got one I’d say get one because the time and trouble you’ll save if you’re into electronics over the years is, well, a lot. Oh and they are cheaper than a multimeter, but if you want to take more time, trouble and expense using a multimeter, then,,,,

How to test a JFET transistor using a multimeter

You can’t get a complete test of a JFET just by using a multimeter, to test it fully you would need to build a small circuit to turn the device on and off so you can measure it in both modes. You can perform a simple check though with a multimeter.

You will need to know the pin out diagram for the device you’re testing, that’s which legs are the drain, gate and source. This is easy enough to find on the internet.

Because a JFET can effectively stay turned on even when disconnected from a circuit you have to make sure that it is turned off before you test it. A nice easy way is to push the three legs into a piece of conductive foam, the type that static sensitive devices get delivered in.

This will make sure that the device is turned off. Then set your multimeter to a low ohms range and measure the resistance. It will vary depending in the exact device but something like 100 ohms would be normal, if it’s very high or not conducting at all then it’s probably defective. As you can see from the picture above it’s reading 121.8 Ohms in its conductive state.

It makes the testing a lot easier if you plug the JFET transistor into a breadboard and then plug leads into it for the connections.

What’s the difference between a MOSFET and a JFET transistor

Both are field effect transistors and obviously have some similarities, put basically because a JFET is a simpler device to a MOSFET it can be manufactured easier and is cheaper. A JFET has lower impedance than a MOSFET

What is a JFET transistor and how does it work

A JFET is a junction field effect transistor and only works in depletion mode unlike a MOSFET which can be depletion or enhancement modes, you can read more about MOSFET’s here.

In beginners terms a JFET can be thought of as a voltage controller normally closed switch.

A JFET has three legs or terminals, a drain, gate and source. Like a normally closed switch current can flow between the gate and source or source and gate when the JFET is off, however when you apply a voltage to the gate the JFET turns on restricting the current flow.

JFET’s come as N channel or P channel, an N channel device needs a negative voltage at the gate and a P channel a positive voltage.

What is a JFET transistor used for

As a JFET is basically a voltage controlled switch or voltage controlled amplifier it won’t come as any surprise to know that JFET’s are used in almost as many applications as common bipolar transistors are from oscillators to amplifiers.

What is the difference between a JFET and a bipolar transistor

JFET’s and bipolar transistors are both types of transistors but a JFET is a unipolar transistor.

A bipolar transistor has a base, collector and emitter as its three terminals and a JFET has a drain, gate and source as its three legs or terminals.

A bipolar transistor is controlled by current where as a JFET is controlled by voltage. FET’s have a higher impedance than bipolar transistors and generally consumes less power.

What are the advantages of a JFET

A JFET is less noisy than a bipolar transistor, it has a longer life, a higher impedance and is more efficient than a bipolar transistor, they also have a better temperature stability but they are more susceptible to damage from static electricity. They are also usually cheaper than bipolar transistors.