DC Analysis - determining collector currents.

Discussion in 'Homework Help' started by CircuitZord, Oct 8, 2012.

  1. CircuitZord

    Thread Starter Member

    Oct 8, 2012
    59
    2
    [​IMG]

    Determine exact DC collector currents of Q1 and Q2.

    Q1 is an NPN transistor and Q2 is a PNP transistor.

    R4 and R5 were 8Mohms and 6Mohms respectively, so I assumed that the current into the base of Q1 was negligible. Hence doing voltage division, I have a voltage at the base of Q1.

    As indicated on the diagram, I have several other voltages found through the standard voltage drops on the transistors. Since ib1 = 0, ic1 = ie1

    In determining the collector currents, I am confused on how to proceed. I tried doing KVL but since there are no resistors directly connected to emitters and collectors with the exception of collector Q2 I find it confusing since I can't get a voltage drop.

    Thanks.
     
  2. WTP Pepper

    New Member

    Aug 1, 2012
    21
    6
    Far too little information. How are you measuring the volts etc. Even a good DVM will load such a circuit. Also ib1=0 does not mean ic1=ie1. ic will include Hfe in the ib in the equation.
     
  3. CircuitZord

    Thread Starter Member

    Oct 8, 2012
    59
    2
    Here is the exact question and the schematic as given.

    [​IMG]

    [​IMG]

    I'm not physically measuring voltages, it's all by circuit analysis, any reasonable assumptions are OK as long as it's justified. If ib1 = 0, why can't ic1 = ie1? I thought the KCL for a transistor in general was ie = ib + ic?
     
  4. WBahn

    Moderator

    Mar 31, 2012
    17,788
    4,808
    It's a bit contradictory to ask for an exact solution but then say that any reasonable assumptions are okay. Reasonable assumptions lead to reasonable approximations, not exact solutions.

    With that in mind, the 'reasonable assumptions' are probably only 'reasonable' if their effect on the answer is beyond the third or fourth sig fig. Ignoring the base current probably doesn't satisfy this constraint.

    When I look at the figures, I don't see a question anywhere in there.

    The parameters for the transistors include the Early voltage, which means that it is probably not reasonable to ignore that, either (at least not if the question truly is asking for a 'exact' answer).
     
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