are jfet drain and source interchangeable?

[vixo]

I use 2n3819s sometimes but there are two pinouts - fairchild is different to ON semi, with the drain and the source being changed round. Does this matter? I always thought No, but I was very confused in the past when i bought a few which didnt work. I put this down to the fact they were the wrong pin out, by I have since learnt that there are some fakes of these floating around...

should it make a difference?

[Yansi]

Shouldnt make a difference as far as I know.

[madires]

Not really, it's fine in many cases. But there are also some JFETs where it matters. You can find the real source by measuring the gate-source and gate-drain capacitance. The gate-source capacitance will be much lower.

[Kleinstein]

With many JFETs there is no real difference between drain and source. So a circuit should still work if "reversed", as the variations between units are larger than from reversion the chip.

Especially JFETs for high frequency use can have intentional asymmetry and better performance when used the right way around. AFAIK the gate to drain capacitance should be the smaller one in most applications - so it could depends on the circuit which way is better.

The 2N3819 seems to be asymmetric with a low Crss (gate - drain).

[SiliconWizard]

As said above, not all discrete JFETs are symmetrical (from experience it's even slightly uncommon, but YMMV.) In any case, check the datasheet, although it's not always easy to derive that information from some datasheets.