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Homework Help: NMOS and CMOS

  1. May 12, 2012 #1
    1. The problem statement, all variables and given/known data

    I saw this image in the internet. I want to learn how to recognize the gate, source and drain for NMOS and PMOS.


    3. The attempt at a solution

    I don't know if I am correct or not.
    M8 ( with an arrow coming out ) is NMOS. M7B (with an arrow going in) is a PMOS.
    I know where the gate of a NMOS and PMOS. But I really have no idea how to recognize the source and drain for PMOS and NMOS.

    I found that there are many version in the internet. Like in LTSpice, Source is always next to the gate. But I am not sure for the picture above.

    Thank you
  2. jcsd
  3. May 13, 2012 #2

    rude man

    User Avatar
    Homework Helper
    Gold Member

    Basically, the arrow is the source.

    This assumes the drain (D) is more + than the source (S) for an N channel, and vice-versa for a P channel.

    HOWEVER: MOS devices can be run in the inverted mode. So for example an N channel might have the drain more - than the source. In which case the device behaves as though it's got the S and D terminals reversed. But still, the convention is to call the arrowed terminal the source.

    (There is often a built-in diode between the S and D so that operation in the inverted mode is limited to saturation conditions. The inverted mode is very seldom used. I once ran across it at work & it turned out to be an inadvertent connection!)
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