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Transistors: NPN, PNP, which is preferred, how do they work?

  1. May 4, 2010 #1
    Hello, I am a High School senior that is going to major in EE next year and I am a having some serious trouble with transistors. I understand them at the most rudimentary level (they function like switches, right... please tell me I am right?) but the nuances that come into play when actually designing a circuit continue to elude me.

    It goes something like this. I draw the circuit up on paper, I go to throw it on a bread board (because I am poor and can't afford SPICE) and then I spend 4 hours struggling with it trying to get it to work. Eventually I get frustrated and bring the schematic over to somebody I know that is in the field, they look at it, say it should work fine and I leave slightly less discouraged but nonetheless frustrated.

    So, here is the crux of the problem; what is the difference between an NPN and a PNP BJT, aside from the obvious, and what are their prospective roles to an emerging EE student. How should I use them, when, and why?

    Thank you so much,

  2. jcsd
  3. May 4, 2010 #2
    At your level there is no difference except for polarity. I have been told that everything else being equal, NPNs are slightly faster and have a slightly lower noise figure because electrons flow more directly to the anode than holes flow to the cathode.

    Do a search for PSPICE download. They have a good student version which is free though it has limitations on the number of components in the circuit. Linear Technology also has a free version of SPICE without such limits.

  4. May 4, 2010 #3
    Thanks a bunch skeptic2, in my searches I also found this neat little Java applet that some guy coded up.


    I like it because it shows electron flow.
  5. May 5, 2010 #4
    Wait, so, just to be sure I have a firm grasp on this.

    This is is assuming that the NPN transistor is forward active.
    In an NPN transistor the Collector must be positive with respect to the Emitter, and the current being controlled by the device is dependent upon the positive voltage bias that is applied to the Base terminal.

    This is also assuming that the PNP transistor is forward active.
    With a PNP transistor, the Emitter must be positive with respect to the Collector, and the current being controlled by the device is dependent upon the negative voltage bias that is applied to the Base terminal.

    Is this a correct analysis?
  6. May 5, 2010 #5
    Yes, that's about it.
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