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Negative power and PNP transistors

  1. Jan 14, 2006 #1

    From what I have learned, when someone refers to negative power, they are refering to a point in a circuit that has lower voltage compared to another area that has more voltage. For example, one test point in a circuit is 8 V while another test point is 4 V. That 4 V point in the circuit is -4 V in comparison to the 8 V point.

    I know there are transistors that work off of negative power (PNP) and positive power (NPN). NPN transistors work by positive power applied to the base lead while PNP transistors work by negative power applied to the base lead.

    This is what I don't get, if PNP transistors want negative power, then why don't you just use a NPN transistor and provide very little current to the base lead.... wouldn't that be the same thing?

    Also please provide an thorough explanation to my question, I really want to understand this.

    Thanks you very much.
  2. jcsd
  3. Jan 14, 2006 #2
    I think you are very confused on something having to do with transistors. You seem to be using power and voltage interchangeably. You can't do that.
  4. Jan 14, 2006 #3
    Yeah, Im really new to this stuff so I may not have my words straight.:redface:
  5. Jan 14, 2006 #4


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    Lets assume you means power as in power supply in which case you are actually is referring to voltage.

    If you really mean power. That is expressed in Watts and is positive in value. (just think of equation [itex]P=i^2r[/itex], no matter which direction current is going, when you square it, the sign becomes positive)

    You are right, when you are measuring a lower voltage potential with respect to a higher reference potential, the difference is negative. Regarding NPN versus PNP biasing, I recommend you first think about how a PN junction (a diode) behaves, both in forward and reverse bias. How would you connect a supply voltage to a PN junction for a forward bias? reverse bias?
  6. Jan 15, 2006 #5


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    This site (howstuffworks) might be a good start to understand semiconductors.
  7. Jan 18, 2006 #6
    Wonderful article. Things a much clearer now, thanks.

    Though I do have one more question:

    Lets say we have a PNP transistor, so technically the positive in the PNP are the areas with the holes and when you apply N type conduction (access electrons) to the base lead (middle lead), it in a sense pushes the access electrons in the middle of the sandwhich away so P type conduction can take place between the Emitter and Collector? Am I right on this?
    Last edited: Jan 18, 2006
  8. Jan 19, 2006 #7


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    Sounds right to me.
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