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Homework Help: NPN Transistor

  1. Sep 26, 2010 #1
    1. The problem statement, all variables and given/known data
    The base-emitter voltage, V_be, typically never exceeds .6V. Because of this low value, the base-emitter junction must be protected against too high a reverse voltage. A diode is usually used to protect the junction as shown in the figure for a npn transistor.

    a. Explain the function of the diode and the conditions under which protection is given.
    b. Draw the equivalent circuit for a pnp transistor.

    2. Relevant equations

    3. The attempt at a solution

    Sorry about the drawing. I am having trouble drawing straight lines with the circuit drawing software I am currently using.

    I have no idea what a transistor does and why. I read various online material about the transistor before our lecture on the circuit element (I didn't have my text book with me). The lecture seemed easy to understand, but I still have no insight as to exactly what a transistor does in the circuit configuration shown in the picture. I have also had 1 lab session dealing with the transistor in an emitter follower circuit. I asked the TA many questions about the circuit, but I could not get a clear understanding as to what the transistor does, or why it behaves the way it does. After the lab, I read the chapter on the transistor, but found it to be vague.

    What is a transistor? What does the transistor do? What is a "collector"? What is an "emitter"? What is the "base"?

    I realize that this is an extremely important circuit element, but from the little I know, it seems to have no function. I know that it certainly does have a function, but I am so lost in trying to understand it.

    I am not worried about answering the above question. I want to first understand the transistor. I apologize if this is posted in the incorrect section.

    If someone could either explain, or point me in the direction of resources that explain the transistor, it would be greatly appreciated.

    Attached Files:

    Last edited: Sep 27, 2010
  2. jcsd
  3. Sep 28, 2010 #2
    If there were a reverse voltage applied to the circuit, would the diode need to be there in order to prevent the forward bias diode in the transistor from breaking down?
    Last edited: Sep 28, 2010
  4. Sep 28, 2010 #3
    The more I think about it, the more it seems obvious. The diode prevents the forward bias diode in the p-n junction (emitter base junction) from taking damage. Protection is given as soon as the reverse voltage is greater than the voltage drop of the added diode (.6V). If that is true, then of course the pnp transistor will have the same properties. The difference being that the added diode would be reflected across base lead axis.
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