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Homework Help: Air's Bipolar Junction Transistor Q&A Thread

  1. Dec 10, 2009 #1


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    Hi, I'm learning Bipolar Junction Transistor and need help to confusion which arise so I will keep posting questions which I don't understand to this thread. Thank you in advance. So my questions start:

    When using hybrid pi model and t model, we determine the dc operating point first. In one example, I saw that they make vi=0. My question is that should all sources be made zero to find dc currents? Also, determining dc operating point means only finding dc collector current or do we find other currents (such as dc emitter current) or voltages?
  2. jcsd
  3. Dec 10, 2009 #2


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    If you want to refer to voltages on a diagram, you need to include the diagram.
    See the "paper clip" thing at the top of the edit screen. That is for attaching stuff.

    Also, since this is the homework section, you need to show your own efforts at finding this information.
  4. Feb 18, 2010 #3
    1st question: yes, for bias you can/should assume the AC inputs are zero - they are presumed to be small signal in most designs anyway - negligible excursion away from the DC Q-point.

    2nd question: usually you are doing some KVL loops which include both Ib and Ic (and by KCL, Ie) along with Vbe. The exact variables depend on the circuit (base bias, degeneration, etc.) but remember the device parameters typically available for BJTs: Iceo, Vceo & Hfe. These are enough to do pretty all the KVL loops.
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