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Electronics => Beginners => Topic started by: sspj9 on February 03, 2016, 06:17:51 pm

Title: JFET approximations (gate-Voltage)
Post by: sspj9 on February 03, 2016, 06:17:51 pm

Hi,
I designed a JFET to output a constant current of 10mA for an assignment. Using 2N3819 with Vdd 20 V, we assumed that Vg is 4V, however we need to explain to why we set it to 4V. I mean we could set it to any value right and just change the resistors. Can anyone help?
Thanks.

Title: Re: JFET approximations (gate-Voltage)
Post by: Simon on February 03, 2016, 06:52:42 pm

Not sure, the other way of asking the question is explain why you chose the resistors, presumably you have cme up with a suitable compromise of resistors and Vgs

Title: Re: JFET approximations (gate-Voltage)
Post by: Gyro on February 03, 2016, 07:21:20 pm

This Vishay App note might be of some help to you:

http://www.vishay.com/docs/70596/70596.pdf (http://www.vishay.com/docs/70596/70596.pdf)

Title: Re: JFET approximations (gate-Voltage)
Post by: T3sl4co1l on February 03, 2016, 10:42:40 pm

2N3819 isn't a very good choice for this... in most cases, Vg would need to be forward biased.

Tim

Title: Re: JFET approximations (gate-Voltage)
Post by: alsetalokin4017 on February 03, 2016, 10:52:28 pm

"...because it works...."    ?

 :-//

No, I guess that answer might be ok in a physics class, but probably not in an engineering class.