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calculating emitter stablizied biasing sat current

Discussion in 'Electronic Basics' started by Telus News, May 25, 2005.

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  1. Telus News

    Telus News Guest

    Hi all:
    I have a simple emitter stablized circuit. with Vcc = 12V, Rc = 2.2k, Re =
    1.2k
    I did Vcc / (Rc + Re) to find the sat current = 3.53mA ( assuming Vsat =
    0V )
    but the answer on my textbook gave me 5.13 mA!!

    Is it my problem or the textbook's problem?
    thanks for any help...
     
  2. Your logic seems okay to me. But I'm not trained in electronics, so
    I'll listen to others on this, too.

    I tend to think about BJTs as having Vce(sat) of about < 0.4V, very
    often in the near .05V-.15V range. But that's barely anything
    different to what you were saying. I can imagine how to increase the
    emitter current, of course, by simply jacking up the base voltage and
    dumping base current like it was going out of style. But the
    collector current would only then decline a little as the (Ic+Ib)*Re =
    Ie*Re voltage jacked up the collector voltage, which rides at Vce(sat)
    above it, and thus dropping the collector current somewhat. I assume
    they were talking about "sat current" as collector current, yes?

    At 5.13mA, just through the collector resistor Rc alone, you've about
    used up all your 12V supply -- with only about .7V left over. This
    sounds a lot like someone imagining that the saturation current is the
    entire 12V supply going through the collector resistor and a diode to
    ground. If the wrong polarity BJT were wired up, it might act like
    that, I suppose.... But it really sounds more like you are assuming
    they were talking about a collector resistor when they were really
    talking about a base resistor. Or they assumed that and wrote
    something else.

    Oh, well. I'm stumped.

    Jon
     
  3. Rich Grise

    Rich Grise Guest

    Your arithmetic is correct, assuming Vcesat = 0. Isat will be even less
    with a non-zero Vcesat. It wouldn't be the first time that a textbook
    has a typographical error. (Or maybe the purpose is to find out if
    you're confident enough in your coursework to actually _find_ the
    error(s) in the textbook! ;-) )

    Cheers!
    Rich
     
  4. Jerry R

    Jerry R Guest

    What's the Vee? -5.2V maybe?

    ....jerry
     
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