Transistor Equation Solving

Discussion in 'Homework Help' started by Xoteo, Aug 13, 2010.

  1. Xoteo

    Thread Starter New Member

    Aug 13, 2010
    Hello everyone :)

    The most difficult area in electronics for me so far into my first semesters of college has been working with transistors. I don't know if it's because I'm just thick, or if the instructor just can't explain this stuff in a way that I understand. Maybe he uses terms that I can't research and make sense of. Either way, here is a situation that comes up regularly that stumps me every single time.

    Q. A transistor has a Beta DC of 75, VCC = 15 Volts, RC = 1000 ohms, VCE is 7.5 Volts VBB is 5 Volts. What is IB and RB?

    For the life of me, I don't have the slightest clue how to calculate IB and RB without at least already knowing one or the other. Nor can I get IC without IB, so I can't invert that equation to get IB because I know of no other way to calculate IC.

    If I felt it necessary I'd take a picture of my desk right now... it is littered with scrap calculations that arrive nowhere. If anyone could help me with this I would greatly appreciate it.

  2. Xoteo

    Thread Starter New Member

    Aug 13, 2010
    I am an idiot.

    I guess I was just over working my brain trying to complete my assignment. Instead of removing my post. I'll provide my answer in case anyone else has a similar issue.

    To find IB I had to discover VRC:
    VRC = VCC - VCE
    VRC = 7.5

    Then discover IC:
    IC = VRC / RC (Ohms Law)
    IC = 7.5 / 1000
    IC = 7.5 mA

    Using that I can determine IB:
    IB = IC / βDC
    IB = 0.0075 / 75
    IB = 0.0001 A OR 0.1 mA

    Then find RB:
    RB = (VBB - VBE) / IB
    RB = (5 - 0.7) / 0.0001 (VBE is 0.7 in a transistor, which causes the change in voltage drop between collector and emitter)
    RB = 4.3 / 0.0001
    RB = 43 000 Ω
    RB = 43 kΩ

    And so ends my constant frustration. I guess typing it down and forcing myself to wait for help allowed me to clear my head and get a better focus on it. I was likely over complicating it. Something I do to life regularly. Good luck guys!

  3. someonesdad

    Senior Member

    Jul 7, 2009
    No you're not. You're a learning, thinking human being. Welcome to the club. :p When you get out of school, you'll work on problems where there's no textbook, no instructor, and for sure no answers to the problem set. Thus, you have to think for yourself. Your schooling is training you to do that.
  4. hobbyist

    Distinguished Member

    Aug 10, 2008

    Just looking at your steps in calculating the values you required shows you have a very good handle on this subjuect, betteer than what your giving yourself credit for, you deduced the problem to very logical steps and applying your knowledge to using the proper equations, and how to use those equations, shows your learning this stuff real well.

    Keep up the good work.