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Old 1st October 2013, 02:36 AM   #1
Ripcord is offline Ripcord  United States
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Default Jfet Idss Tester

Is this the correct method? I followed Nelson Pass's description, or at least I'm hoping I did. ( "I connect the Source and Gate to ground (for N) and measure the current.
With an 11 volt supply and assuming a typical value of 10 mA, use 100 ohms
in series with the Drain (the voltage drop across it will give you the Idss) for
a Vds of about 10V, and connect the other side of the resistor to +11V.

Reverse the supply polarity for P channel." )
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Old 1st October 2013, 03:00 AM   #2
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Watch the power dissipation.

BAD is when they are too hot to touch.

Otherwise, yes this will work fine.
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Old 1st October 2013, 04:26 AM   #3
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Just to follow up, this is the first time I have matched Jfet's. Out of twenty fets, here's the results in mA's:
2 x 5.00
2 x 6.16
2 x 4.82
then:
6.59
6.37
5.85
5.86
5.71
6.19
6.44
6.07
6.11
5.09
4.86
4.13
4.92
4.40
Is this pretty typical? The Jfet is a 2N5952. I needed four matched pairs.
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Old 1st October 2013, 09:16 AM   #4
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Your load resistor is the wrong value. Should read 1k for 10mA otherwise 100R will draw 100mA.
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Old 1st October 2013, 10:18 AM   #5
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Actually the transistor will draw Idss, but the majority of the supply voltage will be Vds. 6.59mA X 100R= 0.659V. 10.341V X 6.59mA= 68.2mW. Should be OK for even a SOT-23 J-fet. A higher goss J-fet like J111 or 2N4091 might begin to get warm. It is best to try to have relatively the same Vds for each DUT so you don't want it too large, but large enough to get an accurate measurement. 100R is fine here.

The self bias with or without a source resistor is why J-fets make such great current sources, especially when cascoded.
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Old 1st October 2013, 05:44 PM   #6
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One last question, what is a reasonable difference for considering being matched? For instance, is 6.11, 6.16 and 6.19 close enough?
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Old 1st October 2013, 08:35 PM   #7
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Quote:
Originally Posted by tom42107 View Post
One last question, what is a reasonable difference for considering being matched? For instance, is 6.11, 6.16 and 6.19 close enough?
That 1% low and 1.5% approx high . Given 1% resistors I say that a good match.
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