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How to Substitute 2N3819 with BF256B


With the switch by industry to SMD, JFETs have faced a massive cull in through hole devices. I thought the popular 2N3819 might have survived as still quite a few BJTs of that ilk have, but alas. The only TO-92 packaged substitute I can find is BF256B, which fortunately is a pretty good swap. The Jxxx devices are no good as substitutes when used as current sources as their Idss is way too high. So as a few of my projects use the 2N3819 (always as a current source) I thought I would write a little note to help.


Key Details

The first issue is the pinout is different. The 2N3819 has a pinout of drain, gate, source when looking from above with the flat towards you. The BF256B has a pinout of gate, source, drain.

The second issue is the Idss, whilst similar (actually within the tolerance of a 2N3819, which is pretty wide) it does give a different theoretical current for a given tail resistor.

It is easier to explain the solution to the pinout issue with a picture.

Pinout solution

Step 1 - rotate the BF256B 90 degrees clockwise.

Step 2 - bend the gate lead down.

Step 3 - bend the source lead to the left.

Now to fix the current. If you keep the same value of tail resistor as you had for the 2N3819 the current will be too low, so you need to reduce the value of the tail resistor. I have found that you need to reduce it by approx 34%. So multiply the value by 0.66 to get the value right for the BF256B.


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