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Transistor Question

Discussion in 'Electrical Engineering' started by Peter, Apr 30, 2005.

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  1. Peter

    Peter Guest

    If you have say 15volts going through a 1k resistor into the base of a
    transistor, the emitter has an emitter resistor as well... say 500ohms and
    the collector is tied directly to Vcc of 25 volts.

    How do you figure out the max current and voltages through the collector,
    emitter and base???


    The collector can draw as much as it wants because it's tied to Vcc, you
    don't know the voltage at the base because of the Re. Normally you'd have
    the emitter tied to ground, so your base is approx. 0.7v and you figure out
    your base current that way, then multiply it by beta, etc.... But the
    emitter resistor is confusing me.


    Does anyone know???


    Thanks in advance!
     
  2. Dwayne

    Dwayne Guest

    Yes I do.
    I'm going to assume that the base-emmitter drop is 0.7 Volts (can be changed
    at anytime).
    Vb=Ve+0.7
    ib=(15-Vb)/1k
    ie=(Ve-0)/0.5k
    ie=(Beta+1)*ib
    therefore
    (Ve)/0.5k=(Beta+1)*((15-Vb)/1k)
    (Vb-0.7)/0.5k=(Beta)*((15-Vb)/1k)
    Where
    Vb = base voltage
    Ve = emitter voltage
    ib = base current
    ie = emitter current
    Beta = internal gain of Xsistor

    And the rest is trivial.
    np

    Dwayne
     
  3. Dwayne

    Dwayne Guest

    This line should be
    (Vb-0.7)/0.5k=(Beta+1)*((15-Vb)/1k)
     
  4. Dave D

    Dave D Guest

    Not with an emitter resistor it can't. In a standard common emitter
    configuration, if the circuit in question has an emitter resistor then the
    collector current is limited by that. Strictly speaking, this isn't an
    electrical engineering question, it's electronics. You might find a lot of
    interesting and relevent electronics discussion in one of the electronics
    newsgroups.


    Dave
     
  5. Peter

    Peter Guest

    Sorry Dave. I tend to ask circuit questions in the engineering group and
    repair related to regular electronics newsgroups. Not that I am trying to
    offend anyone at all.

    Thanks for the help everyone!
     
  6. J. B. Wood

    J. B. Wood Guest

    Hello, and "strictly" speaking it is an EE question. Unless you think
    that the practice of EE is restricted to power generation and
    distribution. Most universities in the US don't think so. "Electronics"
    or "electronics engineering" is but a subset of EE practice. My .02
    worth. Sincerely,

    John Wood (Code 5550) e-mail:
    Naval Research Laboratory
    4555 Overlook Avenue, SW
    Washington, DC 20375-5337
     
  7. Dave D

    Dave D Guest

    I was merely trying to direct Peter to an electronic-specific newsgroup
    where he could find advice from people specifically trained in electronics,
    I didn't set out to offend anyone.

    Dave
     
  8. Dave D

    Dave D Guest

    Peter, I'm sorry if I gave you the wrong impression! It simply occurred to
    me that an electronics group might be more suited to your question. There's
    nothing wrong with posting such questions here, and I certainly wasn't
    having a go at you, just offering friendly advice :)

    Dave
     
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