# analyzing JFET (Idss and Vp values)

#### justtrying

Joined Mar 9, 2011
439
I have to analyze a BIFET amplifier for a prelab. I have a pretty solid grasp of how to do it (with a help of some notes) however... all questions that I have done up to now provide Idss and Vp (Vgs off) values for you. Now I have to use the spec sheet to figure out what they are. Right now I am stuck on getting through the JFET...

In the specs for BF245 B, the range for Vp is -0.5V to -8V and the range for Idss is 6mA to 15mA. I know (or I think that I do) that these would represent the corresponding transfer functions and there are many more that fall in between (that could be generated using Shockley's eq'n). I still do not know how to pick a value to use in the calculations to find my q-point. It would seem that there are at least two correct answers... Using -8V and 15mA, I got Idq of 3.91 mA and Vgsq of -3.91V. Running this through multisim, the answer is quite a bit off as there Idq is 1.286 mA. My calculated values also give me a negative Vds ??

So, if someone could give a hint on how to pick those pesky values off of a spec sheet, thank you...

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#### justtrying

Joined Mar 9, 2011
439
I am thinking now that I need to consider both and the actual value (i.e. multisim) will fall in between.

#### Jony130

Joined Feb 17, 2009
5,439
Simply, as a designer you must consider the real JFET manufacture spread between the JFET parameters.
For the BF245B we have:
Vp = - 3.7V typical and Idss = 10mA typical.
Vp_min = 1.6V and max = - 3.8V
Idss_min = 6mA and max =15mA
http://elenota.pl/pdf/Philips/bf245a-b-c_2.pdf (figure 5 and 6)

I have two JFET in my bench
First is BF245A and i measure Vp = - 1.9V amd Idss = 5mA
The datasheet show Vp = - 2.2V typical and Idss = 4mA
And my BF245C witch I measure have Vp= - 4.8V and Idss = 15mA (-5.6V/ 17mA typical).

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• justtrying