Mostfets/BJTS

Discussion in 'Homework Help' started by amano001, Nov 17, 2012.

  1. amano001

    Thread Starter Member

    Sep 7, 2012
    35
    0
    I have been trying to fully understand these reading through the book but I am still a little confused about somethings. Especially the topologies for mostfet common gate common source common drain, what do they mean by common? Each one looks completely different and I can't seem to figure out what the differences exactly are. Common source and common gate both end up being grounded but common drain still has common source as a ground which is what confuses me.

    Also if anyone has good tutorials or example problems that I can go through please leave a link!

    Thanks.
     
  2. BillO

    Well-Known Member

    Nov 24, 2008
    985
    136
    A terminal would be termed 'common' by being attached directly to either supply rail.

    A common drain is also called a source follower and is similar to the emitter follower or, common collector circuit in the land of the BJT.

    There is a ton of stuff on the Internet about this.
     
    Last edited: Nov 17, 2012
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    A common drain circuit will always have some sort of load between the source and AC ground. However, the drain will be AC grounded.
    In general, in a common (X) circuit, terminal (X) will be connected to AC ground. AC ground may be a power supply (grounded on the other terminal), which, ideally, has zero impedance, and therefore is an AC ground.
    The word "common" comes from the fact that the AC grounded terminal is common to the input signal ground and the load (output) ground.
     
  4. amano001

    Thread Starter Member

    Sep 7, 2012
    35
    0
    Hey thanks for the responses, so basically whichever leg is not connected to either Vin or Vout is the "common"?

    And yes I know there is lots of information out there and I am reading it but sometimes there is great places that explain it much better then others.

    Thanks.
     
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