analyzing JFET (Idss and Vp values)

Discussion in 'Homework Help' started by justtrying, Sep 10, 2011.

  1. justtrying

    Thread Starter Senior Member

    Mar 9, 2011
    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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    Last edited: Sep 10, 2011
  2. justtrying

    Thread Starter Senior Member

    Mar 9, 2011
    I am thinking now that I need to consider both and the actual value (i.e. multisim) will fall in between.
  3. Jony130

    AAC Fanatic!

    Feb 17, 2009
    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 (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).
    Last edited: Sep 11, 2011
    justtrying likes this.
  4. justtrying

    Thread Starter Senior Member

    Mar 9, 2011
    Thank you for the datasheet link. The graphs really clarify it.