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Old 8th January 2012, 10:08 PM   #1
RCruz is offline RCruz  Switzerland
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Default JFET Basics

I am opening this new thread hoping to get some better insight regarding jfet implementations.

From reading Horowitz & Hill, I understand that for a given Vds, if we set
Vgs = 0, Ids is fixed at Idss.

If Vgs is reduced, Ids is also reduced until it stops (pinchof)

In this book, I found a picture with jfet characteristic curves where I can see Id = Idss for Vgs=0 but if Vgs >0 than Id is higher than Idss.

I also read elsewhere that one should never allow a positive Vgs because that could let the current flow from Drain to Gate with disastrous results.


So my first question is:

Can a jfet operate at Id higher than Idss ?
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Old 8th January 2012, 10:34 PM   #2
juma is offline juma  Germany
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http://www.tkhifi.com/div/Erno_Borbe..._articel_1.pdf

http://www.tkhifi.com/div/Erno_Borbe..._articel_2.pdf
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Old 8th January 2012, 10:48 PM   #3
Cobra2 is offline Cobra2  Norway
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The Borbely articles are good, but they are written by a man that have lived with FETs for a lifetime, not easy to grasp all for a beginner...

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Old 8th January 2012, 11:17 PM   #4
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Quote:
Originally Posted by RCruz View Post

Can a jfet operate at Id higher than Idss ?
No. Read Borberly articles as Juma suggested. Also there is AN102 and AN103. Google it.
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Old 8th January 2012, 11:29 PM   #5
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yes - it can ;

but - not for long

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Old 9th January 2012, 08:14 AM   #6
juma is offline juma  Germany
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Operating a JFET at Id > Idss is not a problem per se, as long as you keep the Pd under dangerous level. The problem is that at Id > Idss significant current starts to flow through the gate. That way you lose the important property of JFET - the high input impedance and you are out of linear working regime.
That's not a big problem in switching mode but that's not what we here usually use the JFETS for.
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Old 9th January 2012, 11:13 AM   #7
RCruz is offline RCruz  Switzerland
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Thank you ... now I understand I must never let Vgs become positive otherwise I will have leaking gate current.

I will reread EB articles now
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Old 9th January 2012, 12:16 PM   #8
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Marshal Leach's class notes are a good resource.

http://users.ece.gatech.edu/mleach/e...et/thejfet.pdf
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Old 9th January 2012, 01:10 PM   #9
RCruz is offline RCruz  Switzerland
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Thank you Scott

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Old 9th January 2012, 01:15 PM   #10
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Ricardo you can ask directly to Erno Borbely in his Facebook.
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