JFET Matching/Sorting

[jackinnj]

The setup I show allows you to test at 70% of Idss by changing Rd. The Quan-Tech 5173 semiconductor analyzer uses something a bit more sophisticated, but you get the point.

[bear]

where was the 17.6 found?

[partialresponse]

Originally Posted by Nelson Pass

...Why do you need to measure the pinch off voltage?

I bought a few JFETs from C-J to replace some that were damaged in my preamp from ESD. Since C-J supplied supposedly graded parts (for which they have to charge much more than for nominal parts), and since I am anticipating future damage to those parts again (it's very dry in the house and ESD events are frequent) I bought a large batch of nominal (ungraded) parts off the Web and want to make sure I can select the ones that are consistent with those C-J sent me. To do this, I thought it best to characterize both Vp and Idss, i.e., do it for the graded C-J parts and then select from the nominal parts those that have similar Vp and Idss values to the graded parts.

I have used Loudthud's method but with a 1M ohm resistor from source to ground (for the N-channel devices)

-Kofi

[jackinnj]

Originally Posted by bear

where was the 17.6 found?

It's the gain of the JFET inverting amplifier -- but in Multisim the source is pk-pk yet the meter reads RMS, so the gain should be 1.76*1.414. (My error)

[bear]

partialresponse, what type of jfets did you buy and from where? MANY of the jfets being sold on the web now are bogus re-badged parts that do not meet specs because they are not the parts that they say they are.

[partialresponse]

Originally Posted by bear

partialresponse, what type of jfets did you buy and from where? MANY of the jfets being sold on the web now are bogus re-badged parts that do not meet specs because they are not the parts that they say they are.

They are J177 (Calogic) and J210 (Fairchild). Bought them from a seller on eBay. They measure around the values of the ones C-J sent me so they're probably alright.

-Kofi