PMOS pass transistors

Discussion in 'General Electronics Chat' started by Engineer-it-all, Jan 22, 2010.

  1. Engineer-it-all

    Thread Starter New Member

    Sep 17, 2008
    Hi Peeps,
    i have a question about Pass transistors.

    assume the MOSFETs are allways biased so they are conducting.
    input on drain, output from source and bulk is tied to source.

    an NMOS can pass 0 well. ok
    an NMOS can Pass (1 - Vth). ok
    a Pmos can pass 1 well. ok


    the PMOS trying to pass a 0... my notes say the output is Vth, i can see why it should be this ( if Vgs = 0, Vds = 0 the channel is high resistance and only at (Vgs = 0 - Vth) does low resistance in the channel form)

    but as far as i can tell, this mosfet is generating voltages that come from nowhere. could anyone explain the 'magic Vth on the output of a conducting PMOS pass transistor with a 0 on the input'.


    p.s also a completely unrelated observation. why is it ok to say
    "an NMOS" but is not ok to say "an PMOS"?
  2. SgtWookie


    Jul 17, 2007
    Why don't you post schematic examples of the circuits that you have questions about? That will make it much easier on people who would like to help you.

    Use the "Go Advanced" button below the reply text box, then "Manage Attachments" to upload images from your computer. .png format files are preferred; they are not "lossy" like .jpg images, but are very compact for line drawings
  3. kdillinger

    Active Member

    Jul 26, 2009
    The voltages are coming from the circuit. There are no voltages if it is just lying there on the table. Am I reading your question correctly?

    The rule of thumb is 'an' is used when it precedes a word that begins with a vowel or the sound of a vowel. With nMOS, 'n' is a consonant but 'n' is pronounced as 'en'.
    For pMOS the 'p' is a consonant and the pronunciation also sounds like a consonant.

    "An hour"
    "A minute"
  4. Engineer-it-all

    Thread Starter New Member

    Sep 17, 2008
    sorry about the poor description, hopefully this pic will help.

    is this what you would expect from the 2 scenarios?

    A has the Vth on the Output as the gate is not lower than the signal being passed.

    B fully conducts the zero as gate is below this voltage by the appropriate amout.

    if this is correct, then in A, it appear's like the voltage is generated by the MOS itself.

    thanks again
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
    You can't have a voltage on the source unless there is an external supply to provide it, like this:
  6. Engineer-it-all

    Thread Starter New Member

    Sep 17, 2008
    Safe, cheers for the help.