What is the importance of IDSS value and matching in amps using jfets - diyAudio
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Old 30th August 2013, 05:49 PM   #1
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Exclamation What is the importance of IDSS value and matching in amps using jfets

I understand the reasons for matching IDSS in symmetric stages of amps using jFETs. Why however is the value of IDSS important (high, med, low)? I just purchased some matched jFETS for a aleph j project and was asked if I preferred matched high, med or low IDSS jFETS. I am not a engineer.
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Old 30th August 2013, 10:31 PM   #2
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The specs for JFETs are very loose for unsorted parts. Idss could vary over a 5 to 1 range but typically it's about half Idss (max). The circuit in which the part is used may not work for a part with too low an Idss. For the Aleph J you should be safe with High and Medium parts. IIRC the Aleph J has a differential input so the offset of the input pair is a concern to minimize DC at the output. Idss matching does not guarantee a low offset, it's just the first step. Vgs(off) and Yfs (transconductance) are also important. Additional testing always costs more.
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Old 30th August 2013, 11:07 PM   #3
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Two things.

What he said, and in some circuits the voltages on the next device may depend on having a certain vdrop across a resistor, which in turn is governed by the current through the device driving it. There are other reasons, but that is a good one.

The second thing is that beware of alleged Toshiba through hole jfets, especially 2SJ74, many are bogus rebadged other parts that are not really suitable subsitutes, even though they are jfets.
http://www.bearlabs.com -- Btw, I don't actually know anything, FYI -- [...2SJ74 Toshiba bogus asian parts - beware! ]
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Old 30th August 2013, 11:57 PM   #4
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I have heard about the fraud issues. I got my matched ones from a reputable dealer. I also ordered a bunch from a military and surplus semiconductor supplier through a industrial and reputable finding service. I had to order a lot of them and have the equipment to check them and match them myself. If they test out ok how can you determine if they are repackaged frauds. What sort of yield can you expect from buying a lot 200+ of a specific model 2sj78bL assuming the parts are the real deal. Ie % duds , low med high IDSS? To match yourself or have someone else sell you matched ones. That is the question
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Old 31st August 2013, 11:11 AM   #5
AndrewT is offline AndrewT  Scotland
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Mystery 'sj74'? jfets
regards Andrew T.
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