Jfet matching setup

[davada]

Originally Posted by Conrad Hoffman

Long ago National Semiconductor published an app note for building an FET curve tracer to work with a 'scope. It you have a scope it's a worthwhile thing to build, though it only does small signal FETs. It gives a single curve, not a family like the Tek and other tracers do. I don't think the circuit is on-line anywhere, at least I didn't find it, but I can probably dig it out and scan it if anybody's interested.

Hi Conrad,

Please do.

Cheer,

David.

[davada]

Hi Conrad,

Was it anything like this?

Curve Tracers

We used this in school for demonstration a very long time ago.



David.

[davada]

And this

[AndrewT]

Ian Fritz paper was just linked to today here
matching 2N4401
I think this BJT matching method transfers to jFETs.

[Conrad Hoffman]

Here's the National circuit. For educational purposes, so hopefully it falls under fair use. I've built this and it works well.

[AndrewT]

Ian Fritz matching jig works with jFETs.
I changed the Vds diode to an LED for a higher Vdg of ~ 1.8V instead of 0.6Vdg
I reduced the pair of 100k to 10k+-0.05% to allow for much higher drain currents in k170bl.
These source resistors could go much lower yet, maybe down to 1k+-0.05% if you require matching at 5 to 10mA.

DUT takes a bit of time to settle to the same temp as REF.