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Easy transistor question: NPN--the corresponding change?

  1. Oct 15, 2016 #1
    1. The problem statement, all variables and given/known data
    Given: an npn transistor having VBE = 0.76V for a collector current of 10mA at T=350 K (n=1) Find the corresponding collector current for VBE = 0.70V

    2. Relevant questions
    a) Wouldn't that temperature be way to high for the transistor to act normally?? I mean 350K is 170F.
    b) What equations are relevant? Do I need to use diode equations? Which one, please. I have a very large book, and there are equations everywhere.

    3. The attempt at a solution
    I don't really understand the load line base-emitter junction characteristics...
    I know this has something to do with it:
    https://wiki.analog.com/_detail/uni...university:courses:electronics:text:chapter-9
     
  2. jcsd
  3. Oct 17, 2016 #2

    rude man

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    Gold Member

    No.
    b) What equations are relevant? Do I need to use diode equations? Which one, please. I have a very large book, and there are equations everywhere.

    3. The attempt at a solution
    I don't really understand the load line base-emitter junction characteristics...
    I know this has something to do with it:
    https://wiki.analog.com/_detail/university/courses/electronics/text/chptr9-f4.png?id=university:courses:electronics:text:chapter-9[/QUOTE]
    No it doesn't.
    There is a simple equation("simplified Ebers-Moll equation) relating emitter current to Vbe, saturation current, and T. It assumes Vce > Vbe but not excessively so (not in Early region). Saturation current is the base-emitter reverse-voltage leakage current. Limit this equation in comparing the same transistor in differing bias conditions, which is what you're doing here. The saturation current varies very widely from unit to unit and over temperature, but this can be ignored if all you're doing is varying the bias parameters (Vbe and iE etc.) You can also assume alpha = 1.
     
  4. Oct 18, 2016 #3

    mfb

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    2016 Award

    Staff: Mentor

    That is a typical computer CPU temperature, and some transistors are rated for much higher temperatures.
     
  5. Oct 18, 2016 #4

    LvW

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    It is rather simple.
    Based on the knowledge how Ic depends on VBE (exponential law) you simply can create two equations for Ic1=f(VBE1) and Ic2=f(VBE2)
    As a next step, find the ratio Ic2/Ic1 and use your knowledge for dividing two exponential expressions. That`s all.
     
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