MOSFET PSPICE Circuit Simulation

Thread Starter


Joined Nov 27, 2009
Hi, I am trying to do a spice simulation of a dynamic circuit using PMOS and NMOS. Basically there is an precharge PMOS in series with the N-BLOCK LOGIC consists of the equation /A+BC, which is two parallel NMOS with one of the leg of the parallel has two NMOS in series, then in series with the evaluation NMOS.

I have a vpulse for the clk on the PMOS and the bottom evaluation NMOS. Then I am just testing to see if the output changes with VDD or GND voltages going to the gates of the N-BLOCK LOGIC NMOS gates.

The simulation looks correct for the case where all the NMOS gate inputs are 0V (means turned off), so the output stays high during the precharge and evaluation stage. But when I try to do different combinations on the gate inputs, it gives me pretty much the same result.

Any suggestions?


Joined Jul 17, 2007
Post your schematic, and the results of your simulations.

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Joined Jul 17, 2007
Good deal. :)

I had a bit of a time getting the "hang" of MOSFETs.

It helps a whole bunch if you remember to keep the source terminal pointed in the direction indicated by what the channel is.

If it's a P-channel, you know that the source terminal should be towards the more positive area of the schematic; if it's an N-channel, you know that the source terminal should be towards the more negative area of the schematic.

A basic guideline for creating schematics is that more positive voltages are near the top, more negative towards the bottom.

Another basic guideline is that inputs should be towards the left side of the schematic, and outputs towards the right side. Since the gate of a MOSFET is the control input, you should try to orient the MOSFET so that the gate is on the left.


Joined Jul 26, 2009
What I hate the most about MOSFETs is that there are so many schematic symbols of them relative to bipolars. What gets me to this day is 3 terminal vs. 4 terminal. I have to stop and think.