# Jfet transductance equation question

Discussion in 'General Electronics Chat' started by ricardo7890, Jul 19, 2013.

1. ### ricardo7890 Thread Starter New Member

Aug 13, 2012
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I have a simple question about a depletion type jfet current transduction equation. It's from the book "ARRL handbook of radio communications".
Id=Idss(1-Vsg/Vp) ^2
I can't seem to make sense of it, first I'm not sure why its Vsg instead of Vgs. But most importantly if Vsg is the same voltage as Vp the drain current does not come out to be zero. In fact the equation can be simplified to (1/Vp -Vsg/Vp)^2 *Idss
When Vsg=Vp the drain current doesn't equal zero, in fact the smaller Vsg is the larger he drain current, I've seen other equations that make more sense like Idss(1-Vgs/Vp)^2 which makes much more sense.Could someone please help me understand.

2. ### Jony130 AAC Fanatic!

Feb 17, 2009
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The correct equation should look like this

$\text{I_{\small{D}}=I_{\small{DSS}}$$1- \frac{V_{\small{GS}} }{V_{\small{p}}}$$^2 \$

3. ### ricardo7890 Thread Starter New Member

Aug 13, 2012
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Yeah that's what I thought. Is any one else that has read the ARRL handbook for radio communications able to explain what the equation is the way it is?

4. ### Jony130 AAC Fanatic!

Feb 17, 2009
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The equation show in ARRL has only a small error.
They swap Vsg to Vgs. And I don't see any other error.
Because for Vgs = Vp we have

Idss * (1 - 1/1)^2 = Idss*0 = 0A

5. ### ricardo7890 Thread Starter New Member

Aug 13, 2012
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Actually I didn't write the equation clear enough it actually goes
Idss[(1-Vsg)/Vp]^2

6. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Do you have any link to this ARRL handbook about JFET?

7. ### LvW Active Member

Jun 13, 2013
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Must be wrong. V is a voltage and cannot be subtracted from "1".

8. ### ricardo7890 Thread Starter New Member

Aug 13, 2012
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The equation is in the lower right, I hope the text is large enough to read.

9. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Well clearly they have something printing error. Try to find errata for this book.

10. ### ricardo7890 Thread Starter New Member

Aug 13, 2012
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Ok thank you , you have been very helpful