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Old 30th August 2013, 05:45 AM   #1
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Default Mystery 'sj74'? jfets

Hi All - I bought a pack of 10 "sj74"'s recently from China, with a limited expectation of getting real parts. I've a Mini Aleph-J I'm halfway through building and getting appropriate jfets is proving a sticking point.

I did a quick test on a few of them (ala Borbely's jets articles - attached) and came up with the following results... Note I used a virtual ground provided by a pair of 1K5 resistors across a 20V supply, as opposed to dual supplies of 10V

#1 Vp= -1.20V, Vgs @2mA = -335mV, Vgs @ 5mA = 106mV
#2 vp= -1.05V, Vgs @2mA = -294mV, Vgs @ 5mA = 158mV
#3 Vp= -1.55V, Vgs @2mA = -398mV, Vgs @ 5mA = 42.7mV

First time I've done this, so if there's any glaring errors, please mention it... other than the fact that as I understand it these aren't sj74's as I understand how to interpret the results.

So, my question to you all - what are they? And can I actually still use them in a mini AJ?

Addendum - I have a pile of J174's headed my way as a potential substitute... pending my learning to use them in that particular circuit!
Attached Files
File Type: pdf Audio_Electronics_JFETs.pdf (487.4 KB, 372 views)
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Old 30th August 2013, 07:30 AM   #2
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That is a nice read, Thanks for the post!!

I just got some LSK170C's that I want to use for an Alpeh X and that will help me more of how to understand what I am getting into with JFET's.

I will probably the the ACA (Amp Camp Amplifier) First with them.

Here is a simple little circuit that I discovered just recently that will help find the parameters of both N and P type JFET's.

Simple circuit lets you characterize JFETs | EDN

Enjoy !!

jer
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Old 30th August 2013, 10:49 AM   #3
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by aspringv View Post
#1 Vp= -1.20V, Vgs @2mA = -335mV, Vgs @ 5mA = 106mV
#2 vp= -1.05V, Vgs @2mA = -294mV, Vgs @ 5mA = 158mV
#3 Vp= -1.55V, Vgs @2mA = -398mV, Vgs @ 5mA = 42.7mV
positive Vgs indicates you have increased the bias too far.
If those results are correct then the Idss is far lower than 5mA.
From there you can then see that for low Idss devices the Vp is far too high for a high gm device.

Measure the Idss and using the Vp compare to a typical 2sj74 to see if you have a high gm device.
You can use Borbely's equation to determine gm.
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Old 30th August 2013, 12:50 PM   #4
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Default Idss's

@AndrewT - I did actually work out the idss's for these parts previously, and they range from 3.9mA to 5.1mA over the set of ten. I'll get the idss for these three and do the math on getting the gm.

@geraldfryjnr - Thank you for the link to that circuit - I'll have a read shortly. I'll attach the other Borbely article to this post for you should you not have found it already.

I guess the next question is "what are they then?"

... just got the idss's #1 is 4.42mA, #2 is 3.96mA, #3 is 4.90mA.
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File Type: pdf JFET's The New Frontier Part 2, by Erno Bordely.pdf (121.7 KB, 136 views)
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Old 30th August 2013, 12:57 PM   #5
AndrewT is offline AndrewT  Scotland
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They are not high gm devices, meaning they are fakes.

There are not many pchannel jFETs, you may be able to see which they could be by comparing your devices to the charts in other datasheets.
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Old 30th August 2013, 01:50 PM   #6
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Just had a quick reread of the Borbely article and he defines Vp as 0.1uA. I believe I had about 11uA during that test so I presume my data might be inaccurate. I reckon I might call it a night and give it another whirl tomorrow morning.

@AndrewT - Thanks for the confirmation. I'm going to redo my rather jury rigged test setup tomorrow and retest the lot I think and then go through the math of it all properly.

Last edited by aspringv; 30th August 2013 at 01:52 PM. Reason: AndrewT response
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Old 30th August 2013, 02:13 PM   #7
AndrewT is offline AndrewT  Scotland
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Did you use Borbely's diagram of the test jig?
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Old 30th August 2013, 03:46 PM   #8
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Imo, not much diff between the voltage at 11ua and 1ua...
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Old 30th August 2013, 04:29 PM   #9
WalterW is offline WalterW  Netherlands
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Quote:
Originally Posted by AndrewT View Post
Did you use Borbely's diagram of the test jig?
I guess he did.

@ Aspringv: I'm curious how you made the 2 mA en 5mA CCS for the test jig?

Are your 2SJ74's GR types?
Can you post a picture? Maybe they are the same fakes as I have...

I ordered 10 and 20 from different Dutch sources and they appear to be from the same fakes distributor.
They also have a gm of about 8 to 9 mS.
Strange enough when I put them in a F5 test amp they seems to work fine....
Still wondering which P-channel JFET they used to produce the fakes.
My BL's range from 3 -16 mA Idss, that says enough. Also when testing for Idss the real JFETS slowly rise in Idss when heating up. These fakes decline when heating up.

Walter
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Old 31st August 2013, 02:21 AM   #10
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There is another thread with pictures of what at least some of the fakes are... I could likely find it, it is a general thread on fake semiconductors... along with those pix identifying the bogus parts (fairchild, fwiw) I put up curve traces of the real and fake.
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