Finding the biasing resistor for MOSFET

Discussion in 'Homework Help' started by wind_blast942, Sep 12, 2010.

  1. wind_blast942

    Thread Starter New Member

    Sep 7, 2010
    22
    0
    Hi all

    I have to find the Rs of the following P-MOSFET circuit such that my Id = 0.2mA. Vth and Kp are given.

    I can assume that the transistor is operating in the saturation region. However i realised that there are 2 different methods in solving this and not sure which of it is correct.


    Method 1:
    Assume saturation, via Id = kp(Vgs - Vth)^2, find Vgs.
    Setting Vds = Vgs - Vth (assumption for saturation), find Vds
    Finally getting Rs = (Vdd - Vds - IdRd)/Id.

    Method 2:
    Realise at DC, Cin causes no current to flow through R2.
    Assume saturation, find vgs using the formula in method 1.
    Since there is no voltage drop across R2, Rs = (Vdd - Vgs)/Id.

    Both methods sound reasonable to me, but since they both give different values, not sure which one is correct.

    Please help.

    Thanks.

    [​IMG]
     
  2. Ghar

    Active Member

    Mar 8, 2010
    655
    72
    Your assumption of Vds = Vgs - Vth is wrong, saturation is when Vds > Vgs - Vth

    It's only equal at the exact border between saturation and triode.

    You have everything else correct:

    If there's no current in R2 then Vg = 0.
    If Id = 0.2mA then you can find Vgs, and since you know Vg you can find Vs.

    Both of these equations are correct:
    Rs = (Vdd - Vds - IdRd)/Id.
    Rs = (Vdd - Vgs)/Id.
     
  3. wind_blast942

    Thread Starter New Member

    Sep 7, 2010
    22
    0
    Thanks for the clarification. Just one more thing, when u mean triode are u referring to the linear region?
     
  4. Ghar

    Active Member

    Mar 8, 2010
    655
    72
    Yes, it's just another name for it.
    The names do get mixed up quite a lot... saturation is the linear amplification range, triode is the linear resistance region, in a BJT saturation is the resistance region while active is the amplification one... always need to stay on your toes as to what someone is talking about!
     
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