BJT trans. in Satuation Mode

Discussion in 'Homework Help' started by mo2015mo, Sep 13, 2013.

  1. mo2015mo

    Thread Starter Member

    May 9, 2013
    157
    1
    Hi guys,

    i have been solved the problems as attached photos
    problem 1 as attached photo #1
    What is the value of EE-voltage to make the transistor in Saturation mode??
    btw I solved this problem and found IE = 0.36mA , Ic= 0.342mA and Vcb = 7.8 volts

    Problem 2 as attached photo #2
    What is the value of Rb-resistor to make the transistor in Saturation mode??
    btw I solved this problem and found IB = 0.23mA , Ic= 9.2mA and Vce = 21.2 volts
     
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  2. WBahn

    Moderator

    Mar 31, 2012
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    What is the value you are using for Vce when the transistor is in saturation? What value of Vee makes this happen? Note that, in saturation, the beta is not the same as it is in the active region. For practical purposes -- as well as a large fraction of datasheets -- you often assume a beta of 10 in saturation.

    Did you sanity check your answers?

    What is the voltage across the 10kΩ resistor is the collector current is 342μA? What does this make the voltage at the collector?

    Why are you finding Vcb and not Vce?
     
  3. mo2015mo

    Thread Starter Member

    May 9, 2013
    157
    1
    Vce = -Vcb - Vb = -8.5 volts:confused:
     
  4. WBahn

    Moderator

    Mar 31, 2012
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    Vce is Vc - Vb.

    How can you calculate the collector-emitter voltage using only the voltage from the collector to the base and the voltage at the base? Doesn't the voltage at the collector have to be in there somewhere?

    In your circuit, Vb=0V.

    Did you mean Vbe instead of Vb?

    Using subscript arithmetic, you would need:

    Vce = Vcb - Vbe

    Or you could just find Vc and Ve and take the difference.

    Vc = -12V + 342μA*10kΩmV = -8.58V
    Ve = Vb + 0.7V = 0.7V
    Vce = Vc - Ve = -8.58V - 0.7V = -9.28V
     
    Last edited: Sep 14, 2013
  5. WBahn

    Moderator

    Mar 31, 2012
    17,716
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    In the first circuit, what does the collector voltage need to be in order for it to be in saturation?

    What does the collector current need to be to produce that collector voltage?

    What does the emitter current need to be in order to produce that collector current?

    What does VEE need to be in order to produce that emitter current?
     
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