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Homework Help: Solvethis question

  1. Jun 18, 2010 #1
    solvethis question plz

    hello to all....

    i m having some doubts in solution of this question....


    what i know about this ckt is that this is pnp transistor. +9 volt is connected to emitter (where arrow made) and -9 volt is connected to collector.

    i think voltage emitter Voltage Ve should be equal to +9 volt , because current source is not dropping any voltage . but in solution part its found something else i.e 1.68 ....

    why is it ???? plz clear my doubt
     

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  2. jcsd
  3. Jun 18, 2010 #2

    Redbelly98

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    Re: solvethis question plz

    The current source does have a voltage drop.

    Voltages at the base and collector are calculated using Ohm's Law and the known currents through the resistors. The emitter voltage is assumed to be 0.7V above the base.

    Here's a more obvious, convincing example of a current source that has a voltage drop:

    LEC3_2.GIF

    (from http://www.labsanywhere.net/circuit/lectures/lect3/lecture3.php )
     
    Last edited: Jun 18, 2010
  4. Jun 18, 2010 #3
    Re: solvethis question plz

    thanks for help fnd.

    but how to find voltage drop across current source. would it be like 9-Ve ????

    i m unable to understand how is your diagram showing voltage drop across current source ????

    plz clarify
     
  5. Jun 18, 2010 #4

    Redbelly98

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    Re: solvethis question plz

    You don't need the voltage drop across the current source to solve this problem. But if you wish to know it, use the fact that the voltage drops across the current source, the 50k resistor, and Veb must sum to 9V.

    The current source has the same voltage drop as the resistor. And the resistor has a current, therefore the voltage drop is not zero.
     
  6. Jun 18, 2010 #5
    Re: solvethis question plz

    thanks for the reply ....if we remove the resistance from the circuit shown by you , then will there be a voltage drop across current source ????
     
  7. Jun 19, 2010 #6

    Redbelly98

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    Re: solvethis question plz

    That becomes a problem. Similar to asking what is the current when you connect the terminals of a voltage source directly together.

    For a theoretically ideal current supply, the voltage drop becomes infinite since the current is being driven through an infinite resistance (V=IR, and R=∞ for an open circuit).

    But in a real current supply, the voltage drop would be whatever the maximum voltage of the current supply is.
     
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