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Homework Help: Quick PMOS question

  1. Oct 13, 2006 #1
    I have a PMOS transistor circuit set up.

    These are my parameters:

    Vthreshold = -0.4 V
    Vgs = -3 V

    Vgt = Vgs - Vthreshold = -3 + 0.4 = -2.6 V

    Vds = Vgs - Vthreshold = -2.6V

    I'm trying to figure out what operating region this stupid thing is in. There are so many different ways of figuring this out, that it's getting very confusing -- it seems every source uses a different notation.

    I know it's not in the Cutoff region, because Vgs < Vthreshold. However, since Vds and Vgt are the same, how do I decide if it is in Linear or Saturation region?

    For linear: Vgs < Vthreshold and Vds > Vgt

    For saturation: Vgs < Vthreshold and Vds < Vgt

    This is really starting to bug me.

    If I could figure out how to find Vgd -- that is, voltage from gate to drain, I might be able to figure something out. However, my Vin = 2V and the drain is connected to a variable resistor and then to ground, I think Vgd = 2V. But I don't think this is possible since it's a PMOS, so I'm ignoring that parameter for now.

    Thank you.
  2. jcsd
  3. Oct 31, 2006 #2
    what is Vgt? never have come across this....
    The voltages around MOSFET usually have this relation Vgs = Vgd + Vds. So, I don't think that you can get Vds the way you did.
    To determine the region of operation you need to look at inequality of this sort: Vds ? (Vgs - Vtp)
    However Vds is not given.... so you'd either have to know extra parameters like Id, W/L or assume a value/region and then verify.
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