Solving for Vout of CMOS Inverter with parasitic resistance

Discussion in 'Homework Help' started by Mary Kate, Apr 14, 2015.

  1. Mary Kate

    Thread Starter New Member

    Apr 14, 2015
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    Due to a manufacturing error, a parasitic resistance (Rp=9kohms) has appeared in the CMOS inverter of Figure 1. Find the output voltage when Vin=0V, verifying the region of operation of the device.
    Given: Vdd=1.8V; kn'=100uA/V^2; kp'= 50uA/V^2; lambda=0 for both devices
    Mp: Vt=-0.5; W/L=5/0.18
    Mn: Vt=0.4; W/L=3/0.18


    Attempts: I know NMOS must be in cutoff and that Imp = Imn = 0. PMOS is conducting because Vgs<Vt. I believe that PMOS is in saturation because Vds (which should be equal to zero) > Vgs-Vt (which is -1.8-(-0.5)=-1.3). I know that if the resistor was not there, Vout=Vdd, but I am very confused about how the resistance will change Vout. When I simulated the circuit it gave Vout a value of 1.2V, but I do not know how to solve for that.
    Any help would be very appreciated!
     
  2. WBahn

    Moderator

    Mar 31, 2012
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    Hint: Voltage divider.
     
  3. Mary Kate

    Thread Starter New Member

    Apr 14, 2015
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    Sorry I'm still confused. I only have one resistor so I'm not sure how to do voltage division.
     
  4. WBahn

    Moderator

    Mar 31, 2012
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    Don't your FETs have a channel resistance when they are turned on?
     
  5. crutschow

    Expert

    Mar 14, 2008
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    You said the device had a parasitic 9k resistance, so you have that resistor and the output load resistor.
     
  6. WBahn

    Moderator

    Mar 31, 2012
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    The output load resistor is unspecified. I thought it was the resistor shown in the schematic, too. But it turns out that that IS the parasitic resistance, Rp, that the problem was talking about.
     
  7. crutschow

    Expert

    Mar 14, 2008
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    You're right, I missed that. :oops:
    So the output ON resistance has to be calculated from the given transistor parameters.
     
  8. WBahn

    Moderator

    Mar 31, 2012
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    That's my take, too.

    @Mary Kate : Do you have what you need to go forth?
     
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