Transistor calculations.....

Discussion in 'General Electronics Chat' started by tjwilson, May 7, 2010.

  1. tjwilson

    Thread Starter New Member

    May 7, 2010
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    Iam trying to calculate supply voltage in a simple biasing transistor.

    My values are as follows: Vcc=?, Vrb=83000 ohms, Vrc=500 ohms, Vce= 10volts. I know that Vbe's voltage drop is 0.7 volts.

    How do I find my supply voltage (Vcc) with just these values?
    The Transistor is a NPN, my beta=100

    Can anyone help please and thank you.

    Tim
     
  2. Bosparra

    Member

    Feb 17, 2010
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    You need to add the current to get to an answer, everything else would be a guess.
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    It must be school work because you cannot buy a transistor with beta=100. The beta is different for each transistor of one part number and changes with temperature and current. The beta might be in a range of 50 to 200.

    A real transistor circuit never uses a single base resistor unless it connects to the transistor's collector. It needs two resistors that make a voltage divider plus an emitter resistor.

    But the supply voltage cannot be calculated for the school work because the schematic was not shown for us to see if the base resistor connects to the power supply voltage or if it connects to the transistor's collector.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    A circuit like this:

    [​IMG]

    Of course it's homework.
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    Everybody says that is a horrible way to bias a transistor. The beta of the transistor must be perfect (it never is perfect) and the supply voltage must never change. The transistor in that extremely simple circuit is just a thermometer.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    Of course it's horrible.

    It's a homework assignment. They're supposed to be horrible. :)

    Wait until our OP starts asking questions on 741 opamp circuits. Hope they have a tomato umbrella.
     
  7. retched

    AAC Fanatic!

    Dec 5, 2009
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    It only took me 4 hours to think of what a tomato umbrella was.

    @tjwilson:

    Any luck?
     
  8. mik3

    Senior Member

    Feb 4, 2008
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    Can you explain the meaning of the tomato umbrella? :p
     
  9. retched

    AAC Fanatic!

    Dec 5, 2009
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    When someone says something unpopular (usually on stage, like a bad joke) the audience throws rotten tomatoes at them. The victim usually knows they are in for it, so they bring an umbrella to block the tomatoes.
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    I throw verbal rotten tomatoes at every school kid's lousy old 741 opamp.
     
    SgtWookie likes this.
  11. SgtWookie

    Expert

    Jul 17, 2007
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    This thread has become much more funny than I expected it to be, but I'm afraid it has also violated some of the forum rules.

    My reference to the "tomato umbrella" was of my own making. It was a reference to days of old when a poor performance on stage would result in the actors being pelted with rotten vegetables. At that point in time, it would be very handy to have an umbrella to deflect such rotten vegetables.

    I suggest that the OP's question be re-located to the "Homework Help" forum, and for much of the rest of the nonsense to be stripped out, and assigned to a different thread.

    Really, our OP did ask a valid question. I've found the answer via simulation, but can't provide the math to back it up.
     
  12. Ghar

    Active Member

    Mar 8, 2010
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    Work from what you know.

    Vbe = 0.7.
    Vce = 10

    I think Sgt's schematic is right, and if so you can easily find the collector current.
    Vce gives you the voltage across Rc:

    Ic = (Vcc - Vce) / Rc

    Vbe gives you the voltage across Rb:

    Ib = (Vcc - Vbe) / Rb

    Then you know that:
    Ic = beta * Ib

    You've got a set of linear equations... shouldn't be hard to solve.
     
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