# Homework Help: PSPICE - simulation of CMOS astable circuit

1. Mar 9, 2013

### stn0091

1. The problem statement, all variables and given/known data

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

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.

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2. Mar 10, 2013

### rude man

Give us the voltages at input and output of both inverters. Also your Vcc and Vdd.

3. Mar 11, 2013

### stn0091

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.

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4. Mar 11, 2013

### rude man

Run your simulation longer! It's clear that V3 and V4 are changing.

5. Mar 11, 2013

### rude man

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.