Small signal model for PNP vs NPN

Discussion in 'General Electronics Chat' started by jut, Nov 22, 2009.

  1. jut

    Thread Starter AAC Fanatic!

    Aug 25, 2007
    Can anyone explain why the small signal model for the NPN transistor is the same as the PNP? The book's explanation, which I attached as a pdf, is not doing it for me. I thought the current direction for pnp vs npn was oppostie and thus the current source in each model would be opposite, but apparently not.
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
    The clue is in the very first line under 4.6.3.

    This is telling you that the model is an AC one. As such it does not have polarity.

    You normally need to perform two analyses on a transistor circuit, especially a multistage one,
    One for DC - the biasing. Where the capacitors are represented by open circuits.
    and one for AC - the signal. Where the capacitors are represented by short circuits.