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PSPICE - simulation of CMOS astable circuit

  1. Mar 9, 2013 #1
    1. The problem statement, all variables and given/known data

    I'm trying to simulate the following circuit in OrCAD PSPICE.

    attachment.php?attachmentid=56542&stc=1&d=1362875259.png

    2. Relevant equations



    3. The attempt at a solution

    * 8.20

    Vdd 1 0 5V

    MP1 3 2 1 1 PMOD
    MN1 3 2 0 0 NMOD

    MP2 4 3 1 1 PMOD
    MN2 4 3 0 0 NMOD

    R1 3 2 1000
    C1 2 4 1uF

    .MODEL NMOD NMOS (VTO=2)
    .MODEL PMOD PMOS (VTO=-2.0)

    .ic v(2)=5

    .tran 0.5us 10ms 0 1u uic

    .PROBE
    .END

    Resistor and capacitor values were chosen for some arbitrary time constant. I just wanted to see the waveforms at Va, Vo and Vi.

    V(2) corresponds to the gates of the first CMOS and I set that to 5V to "start" the circuit. However, I'm not seeing any oscillations when I simulate the circuit.
     

    Attached Files:

  2. jcsd
  3. Mar 10, 2013 #2

    rude man

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    Give us the voltages at input and output of both inverters. Also your Vcc and Vdd.
     
  4. Mar 11, 2013 #3
    The output plots? Alright. I'm assuming you mean the capacitor voltage when you say Vcc.

    V(1) is VDD = 5 V, V(2) is Vi and Vcc (same node), V(3) is the output of the first inverter, V(4) is the output of the second inverter.

    attachment.php?attachmentid=56606&stc=1&d=1363027605.jpg
     

    Attached Files:

  5. Mar 11, 2013 #4

    rude man

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    Run your simulation longer! It's clear that V3 and V4 are changing.
     
  6. Mar 11, 2013 #5

    rude man

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    How can Vi and Va both be +4.4V?? Are you sure you're running an inverter? What's the part number? Oh, I bet it's your model.

    Vi and Va look like the same voltage.

    R looks like no current thru it.

    I seriously question you model of the device. Pick an actual part number from the list of devices in your PSPICE library. Example: 74HC04.
     
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