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Homework Help: NPN Transistor Circuit Analysis

  1. Jan 24, 2012 #1
    1. The problem statement, all variables and given/known data
    Determine the voltage level at V3. For each case also determine which transistors are on and saturated, on and active, and which are off. Let R1=R2=R3=2.2kΩ and let Rc=240Ω and Vcc=5V. Calculate β for each transistor as well.

    We are to assume that if the transistor is on, then it is in the saturated state so Vce=0.2V. If β>20 then the transistor is in active and Vce=2V+. Also, Vbe=0.6V if the transistor is on

    Case #1: V1=V2=0V
    Case #2: V1=0V V2=5V


    2. Relevant equations
    KVL, KCL

    3. The attempt at a solution
    Ok, so here's what I did for the first case where both V1 and V2 are 0V. Since they are 0V, Ib=0A and therefore β=∞ for both Q1 and Q2, right? If so, then I'm basically left with the "upper" circuit containing Q3 and V3.

    I then found Ib=Vcc/(240+2.2k)
    Ib = 0.002A

    I did the same approach for Ic=Vcc/(240)
    Ic = 0.0208A

    β=10.1 so Q3 is then on and in the saturated region. To find V3 I simply subtracted the voltage drop of the 240Ω resistor

    V3 = 5 - (240*.002A) = 4.51V

    I don't know if this is right at all since I've never done any circuit analysis with transistors before. If it's right, I'm still completely lost on how to start Case #2 where V2=5V and V1=0V. Any help is greatly appreciated.
  2. jcsd
  3. Jan 25, 2012 #2


    User Avatar

    Staff: Mentor

    Your method looks right, though I haven't checked the arithmetic. It is unusual to use β in the two ways you have. Writing β=∞ is not right. Just say IB=0 and therefore IC=β*0=0.

    When V2=5v there will be current into the base of Q2, hence collector current, too.
  4. Jan 26, 2012 #3
    excuse me. , where are you find this question. I also need this similar question to finish my homework
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