Calculating VC,VB,VE of this transistor

Discussion in 'Homework Help' started by TBayBoy, Oct 10, 2011.

  1. TBayBoy

    Thread Starter Member

    May 25, 2011
    148
    19
    I'm really scratching my head on this one, about all I can getr is Vrb using RB * IB and then BE at 0.7.

    Any suggestions are welcome.

    [​IMG]
     
  2. steveb

    Senior Member

    Jul 3, 2008
    2,433
    469
    It seems you have answered the question almost.

    Isn't VE pretty obvious?

    And, if you know VBE is 0.7, doesn't that tell you what VB is relative to VE, and then you add VB=VE+VBE?

    And, if you know IB*RB, doesn't that tell you what VC is relative to VB, and you then add VC=VB+Vrb?
     
    TBayBoy likes this.
  3. TBayBoy

    Thread Starter Member

    May 25, 2011
    148
    19
    I was leaning in that direction, but maybe the lack of vcc was spooking me.

    So Ve is 0, vb is 0.7 vrb is 3.96V and Vc = 4.66V?
     
  4. steveb

    Senior Member

    Jul 3, 2008
    2,433
    469
    That seems reasonable to me, with the caveat that the Vb voltage is approximate (it could be 0.6 V, for example). But, this is the traditional assumption we make when we can, and it seems to give reasonable accuracy in this case.

    I agree the lack of need for Vcc raises a red flag, and makes one question the method. There is also the issue of beta of the transistor But, I think the assumption you can make is that Vcc and beta is part of what determines the Ib value, and even though you don't know Vcc or beta, these values are constrained once you know Ib. So all is good.

    For example, lets say that Vcc is 12 VDC. In this case we can calculate the collector current as (12 V - 4.66 V)/1.5K=4.9 mA. Then we can calculate the beta as beta=4.9mA/12uA=408.3.

    Tricky question !
     
    TBayBoy likes this.
  5. TBayBoy

    Thread Starter Member

    May 25, 2011
    148
    19
    Ya, my teacher throws curves once and awhile. :)
     
Loading...