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Old 29th January 2013, 06:12 AM   #1
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Default JFET Matching/Sorting

Could someone kindly describe a simple circuit for measuring turn-off voltage (Vgs off) for JFETs (both P and N channel, J210, J177)? Something that makes use of a single resistor and, say, a +/- 11V power supply. The turn-off range for the N-channel J210 is -3V ~ -1V, and that for the P-channel J177 is 0.8V~2.3V. I have a batch of parts of both I need to sort according to Vgs(off) and Idss.

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Kofi
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Old 29th January 2013, 04:11 PM   #2
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Vgs(off) is typically measured at a constant Source to Drain voltage, but you can get a good idea with a constant Gate to Drain voltage. For N-channel JFET connect Gate to ground and Drain to +11. Now simply measure the Source voltage to ground with a DVM. Some people suggest using a 1 Meg resisitor from Source to ground, but it is not necessary. For P-channel, simply reverse the voltage polarity.
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Old 29th January 2013, 05:09 PM   #3
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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 29th January 2013, 05:53 PM   #4
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Thank you both (Loudthud for how to measure Vgs(off), and Nelson for how to measure Idss). I was also pointed towards this article by Erno Borbely.

Kofi
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Old 29th January 2013, 08:31 PM   #5
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Sorry, I see that I misread the actual question....

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Old 30th January 2013, 11:10 AM   #6
AndrewT is offline AndrewT  Scotland
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Loud,
where did you find that Vp (Vgs off) measurement method?
I just tried it and it does not work.
2sk170, Idss 6.3mA, Vp -0.511V
Using your "no -ve & no resistor" method I measure +0.845V (+ve not -ve.)
i.e. the error for the example shown is 1.35V
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Last edited by AndrewT; 30th January 2013 at 11:19 AM.
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Old 30th January 2013, 04:17 PM   #7
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This test is easy and cheap, but it is probably not how the manufacturer
measures it.

With no Source to Ground resistance, what you will see is the fet working
into the impedance of the meter. The Source will measure positive with
respect to ground, giving a negative Vgs value.

The Source resistor you want depends on at what value you specify the
Drain current at pinch-off, which will never be zero. The 2SK170 is spec'd at
0.1 uA and -0.2 to -1.5V as I recall. That means a pretty high resistance
value, around a megohm, and for this test to be really accurate you have
to recursively adjust the resistor value until the current is 0.1 uA.

You have to ask yourself whether you want to go to the trouble of doing
that. Why do you need to measure the pinch off voltage?

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Old 30th January 2013, 05:45 PM   #8
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Vgs(off) should be interpeted as the Gate to Source voltage required to reduce Drain current to near zero (actually to the value specified in test conditions) with reference to the Source at the specified Drain to Source voltage. So the black lead of the DVM should be connected to the Source and would read a negative voltage for an N channel JFET. Sorry, I didn't specify a polarity for the DVM.

0.511V at 1Meg would be 0.511 uA
0.845V at 10Meg would be 0.0845 uA

Note that the Toshiba data sheet sepcifies Vgs(off) is tested at at 0.1uA. What they are saying is that at -1.5Vgs the current will be less than 0.1uA. There will be some variation depending on the the exact current the part is tested at as well as temperature and voltage. This little DIY test circuit should not be interpeted as something to measure device parameters exactly, merely as something for matching parts. For instance, you might see a couple of precent difference if you measure Idss at 9V instead of 10V. No biggie for matching as long as you measure them all at the same voltage, hopefully somewhat close to the circuit you intend to use the parts in.
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Old 30th January 2013, 06:51 PM   #9
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Want to measure pinch-off voltage, or gm?
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Old 30th January 2013, 08:19 PM   #10
Blues is offline Blues  United States
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Much like curve tracing to match fets that we know have already been characterized by the manufacturer using guess what? -a curve tracer, I find testing for Vp a waste of time and effort -we're not going to operate it at Ids=0 anyway. A curve trace might be needed if fets are suspected/found to be fakes and the seller insists they're genuine and balks to give a refund.

Find out your DC operating voltage (Vds) and your bias current (Ids) and match...heatsink it if needed and monitor temp if you can as a bonus.
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