BJT help again

Discussion in 'Homework Help' started by hitmen, Oct 22, 2008.

  1. hitmen

    Thread Starter Active Member

    Sep 21, 2008
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    Here is a real elementary question regarding BJT. What happens to Ic when Ib saturates and when it doesnt saturate. How do you calculate the voltage Vce?
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    OK, at the risk of giving too much information.

    Transistors (all transistors) have two states that their most efficient at, cutoff and saturation. This is because they are all the way on or all the way off, and disapate minimum wattage in either condition. I usually treat a transistor in saturation as a closed switch, but it seems that for your purposes figure a drop of .2V between collector and emitter, and use it for wattage and current calculations. This is what digital circuits use.

    The in between state, between saturation and cutoff, is where analog gain is used. The voltage CE is somewhat dependent on the current and Rc. Many circuits, such as this one, have the extra resistors to nail everything down, they compensate for beta, temperature, you name it.

    [​IMG]

    If the circuit does not have R2 and Re, then it is entirely at the mercy of the β of the transistor, and temperature. This is where the formula Ic = β Ib comes in. Calculate the Ib from R1, Vcc, and Vbe drop, then use it to calculate the Ic. Whatever Ic is, it sets the collector voltage, which is also Vce in this case.
     
  3. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    Ib does not saturate, Ic is the one which does. Post your schematic as we can give you an appropriate equation.
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    I missed that, saturation only refers to Ice.
     
  5. steveb

    Senior Member

    Jul 3, 2008
    2,433
    469
    As far as the voltage Vce, when the transistor is in the analog regime (not cutoff or satruated), Vce is simply whatever voltage is left over. I like to make an analogy with a mechanical spring. The Vce is like a "voltage spring" that stretches or shrinks to use up the remaining voltage. For example, if the collector circuit has Rc tied to Vcc, and the emitter has Re tied to ground. Then

    Vce=Vcc-Ic*Rc-Ie*Re

    This is a property of a current source. Voltage sources provide the current required to maintain the specified voltage, while current sources provide the voltage required to maintain the specified current.
     
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