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Homework Help: Transistor analysis: find currents and voltages

  1. Nov 25, 2012 #1
    1. The problem statement, all variables and given/known data

    I’m having trouble with analysing the transistor circuit attached to find VC, VB, VE, IC, IB and IE.

    2. Relevant equations

    These equations are taken from a previous example

    VE = VBB – VBE
    IE = (VBB – VBE)/RE
    IB = IE/β+1
    IC = β/(β+1) x IE
    VC = VCC – ICRC

    VBE ≈ 0.7V

    3. The attempt at a solution

    VE = 5 - 0.7 = 4.3V

    My attempt falls down at this stage as there is no resistor on the emmiter side to work out IE. I'm sure that I am missing something obvious.....

    Attached Files:

  2. jcsd
  3. Nov 25, 2012 #2
    VE will be zero since it is connected directly to ground :)

    As you've mentioned, VBE ≈ 0.7 which means you should be able to calculate the base current.
  4. Nov 26, 2012 #3

    rude man

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    Homework Helper
    Gold Member

    See above comments
  5. Nov 26, 2012 #4
    I took the relevant equations from a previous example where there was a resistor on the emitter side, RE.

    The previous example was from my first and only lecture on this, so far!, and I am trying to use this previous example to work through the above problem. I have attached the previous example in this thread so that you guys can see the extent of my knowledge.

    The help is much appreciated as I would like to get my head around this before the subject becomes confusing.

    Attached Files:

    Last edited: Nov 26, 2012
  6. Nov 26, 2012 #5

    rude man

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    OK, but here RE = 0 so you don't want to divide by zero ..

    You need to understand the equations rather than just go by a previous example. The equations I gave you, plus the ones you got right, should be fully understood, and they are all you need. Then you can tackle more complex circuits too.
  7. Nov 26, 2012 #6


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    Staff: Mentor

    Don't concern yourself with finding any of these, first off, except for IB.

    Set aside your previous example, it is misleading you. The first thing you need to find here is IB. You don't need to know any other parameters before determining IB. (Though you can assume you are dealing with a typical Si junction at B-E.)
  8. Nov 27, 2012 #7
    Is this correct?

    IB = (VBB - VBE)/RB = (5 - 0.7)/100000 = 43μA

    IE = IB(β+ 1) = 43x10-6 x 101 = 4.343mA

    IC = β/(β + 1) x IE = 100/101 x 4.343x10-3 = 4.3mA

    VE = 0V

    VC = VCC - ICRC = 10 - 4.3x10-3 x 2000 = 1.4V

    VB = VBB - VBE = 5 - 0.7 = 4.3V
  9. Nov 27, 2012 #8


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    Staff: Mentor

    It's looking better, :smile: though I won't check that your calculator is correctly performing its arithmetic.

    But I don't follow this line: VB = VBB - VBE
    What is VB if it isn't VBE? VB is usually the voltage difference between base and earth, and here it's 0.7V.
  10. Nov 27, 2012 #9
    So VB = VBE + VE

    Makes sense, cheers.
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