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Old 30th January 2013, 10:52 PM   #11
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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.
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Old 1st February 2013, 07:32 PM   #12
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where was the 17.6 found?
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Old 1st February 2013, 08:22 PM   #13
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Quote:
Originally Posted by Nelson Pass View Post

...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
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Old 1st February 2013, 09:25 PM   #14
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Quote:
Originally Posted by bear View Post
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)
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Old 1st February 2013, 09:48 PM   #15
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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.
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Old 2nd February 2013, 06:02 AM   #16
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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
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Old 29th May 2013, 07:59 PM   #17
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Will jfet IDSS measure differently after burn-in, I wonder?
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Old 29th May 2013, 08:14 PM   #18
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if you really burn them , yes

they'll have same Idss as a nail

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Old 29th May 2013, 08:21 PM   #19
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OK, helpful.
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Old 29th May 2013, 08:43 PM   #20
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shortly - no
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