Testing a 2N3819 JFET, alternative JFETs?

Thread Starter

hrs

Joined Jun 13, 2014
340
Hi,

Is this a correct way to test a 2N3819?
jfet test circuit.png
In an LTSpice simulation the resistor drops all the voltage as I would expect. On the breadboard all of the supply voltage is across the transistor. I tried multiple parts. Have I been duped?

I bought this a long time ago when I still thought Ebay etc would be good enough for hobby use. This was bought from Utsource. They have their own stock and they do drop shipping. This part comes from drop shipping.

What comparable JFET would you buy today? SOT23 would be ok if it avoids this TO-92 NOS fakery. Or I did it wrong of course and the part is fine ...
 

crutschow

Joined Mar 14, 2008
27,732
You should get at least 2mA through the transistor in your circuit.
Are you sure you have the transistor is connected exactly as shown in the schematic?
Double-check the transistor pinout for the package you have.
 

Thread Starter

hrs

Joined Jun 13, 2014
340
It is the way to check the Idss
Yes, and 0V over the resistor would imply no current I think?
You should get at least 2mA through the transistor in your circuit.
Perhaps you mean 0.2 mA, 6V/22k = 0.27 mA?
Double-check the transistor pinout for the package you have.
:eek:I was using the OnSemi datasheet. The device is marked Fairchild. The Fairchild datasheet has source and drain swapped. Perhaps I destroyed the device* so I took a new one, used it according the Fairchild datasheet and changed R to 2k for ~2 mA. The result is the same. I've attached a picture of the breadboard.

*Though I have read that for some of these small signal JFETs it is ok to swap source and drain.
 

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bertus

Joined Apr 5, 2008
21,534
Hello,

The 22k ressitor is to large.
Try a 1k resistor and retest.
Also check the pinout:
2N3819 pins.png
When there is no voltage accross the resistor, it could also be a bad contact in the breadboard.

Bertus
 

Thread Starter

hrs

Joined Jun 13, 2014
340
After putting in 1k things are as before, not working. I also put my DMM in circuit measuring 0.0 uA from PSU to drain.
 

Thread Starter

hrs

Joined Jun 13, 2014
340
I thought of looking in a stash of components graciously given to me by someone no longer interested in electronics. Lots of old stuff. I found some 2N3819s and a K49 (2SK49?). Both appear to behave like JFETs in the above test circuit (1k resistor) but both drop around 0.7 V between drain and source. This drop increases with supply voltage. I missed this at first, but so does the simulation.

Alec_t wrote somewhere on this forum that a 2N3819 is in fact a BJT (head asplodes). Does the BJTness of this JFET cause it to drop voltage?
 

Thread Starter

hrs

Joined Jun 13, 2014
340
Why would you think there would be no voltage drop?
You are seeing the channel resistance of the FET when it is fully on (below).

View attachment 232721
I don't know. I thought it would be "super low" like with MOSFETs. The datasheets I looked at don't mention rDS(on). They mention admittance, conductance and susceptance in mmhos. Then my eyes glazed over and I continued the experiment.
 

Alec_t

Joined Sep 17, 2013
12,203
Alec_t wrote somewhere on this forum that a 2N3819 is in fact a BJT
I don't remember doing that, but have checked and indeed I did. We don't normally think of it as a bjt, but its internal structure has both n-type and p-type silicon, with a junction between gate and source/drain.
 
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Thread Starter

hrs

Joined Jun 13, 2014
340
I don't remember doing that, but have checked and indeed I did. We don't normally think of it as a bjt, but its internal structure has both n-type and p-type silicon, with a junction between gate and source/drain.
Hi Alec,

I was referring to this post:
https://forum.allaboutcircuits.com/...t-on-peak-atlas-analyser.133788/#post-1113775
But in the context of that thread it's different, like you say. Now that I have a good JFET I can measure around 0.9V between GS and GD.
 

BobTPH

Joined Jun 5, 2013
3,661
Unless I am missing something , your breadboard picture shows no connection to pin 1 of your jfet, and no connection to one of the pins of the resistor.

Can someone else please check out the picture?

Bob
 

sparky 1

Joined Nov 3, 2018
579
The article is formatted with an arrow go to the next page at the bottom page.
https://www.allaboutcircuits.com/te...hpt-5/junction-field-effect-transistors-jfet/
in contrast is an article that is gives detailed examples
https://www.nutsvolts.com/magazine/article/fet_principles_and_circuits_part_2

hrs asked about an old report
https://forum.allaboutcircuits.com/...s-shown-as-bjt-on-peak-atlas-analyser.133788/
No that is not true of a real 2N3819 reads n-jfet which means n-channel junction field effect transistor. Test fine without any problem.
In circuit it acts like a jfet and not at all like an NPN. That report is misleading. At first transistors seem to not work.
Keep trying with an NPN
 
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crutschow

Joined Mar 14, 2008
27,732
We don't normally think of it as a bjt, but its internal structure has both n-type and p-type silicon, with a junction between gate and source/drain.
Yes it may have n and p type silicon but its structure is significantly different than a BJT, so normally or abnormally, it should not be thought of as a BJT.
 
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