Mosfet Opamp design Pspice help needed

Thread Starter

xsabin

Joined Apr 3, 2013
2
I have a project to design a mosfet amplifier and implement it in pspice. Right now I am just trying to figure out how to use the pmos/nmos in pspice and I made a 2 stage amplifier(basic design from the text) but my output comes out very strange and I am not sure where I am going wrong. Can anyone please help, I attached an image of the ckt In pspice and the output.
 

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WBahn

Joined Mar 31, 2012
26,279
What do you expect the output to be?

If you are at the stage of just trying to figure out how to use NMOS and PMOS devices in PSpice, why not start with a simple single transistor amplifier circuit and then work your way up from there to a differential pair with a resistive load to one with an active load and then to a two stage circuit?
 

Thread Starter

xsabin

Joined Apr 3, 2013
2
I will try a single stage resistor loaded amp.
I was trying to get this to work as actively loaded amplifier is a requirment for the project. For an output i just want an amplified sinusoid. I thought that my gain was to high and the output swing was cutting off my signal but i get the same output no matter what input voltage is.

Thanks for the advice.
 

WBahn

Joined Mar 31, 2012
26,279
I will try a single stage resistor loaded amp.
I was trying to get this to work as actively loaded amplifier is a requirment for the project. For an output i just want an amplified sinusoid. I thought that my gain was to high and the output swing was cutting off my signal but i get the same output no matter what input voltage is.

Thanks for the advice.
You are making one of the classic mistakes, namely throwing the entire design together and then flipping a switch and expecting the output to be what you want. Your problem is probably at an inner point in the design, but since you only looked at the output and didn't walk the design up stage by stage, you are not in a position to even tell if that is a problem or not.
 

crutschow

Joined Mar 14, 2008
25,641
Your circuit (if it's operating properly) has very high gain, so operating it open loop will cause the output to go to one of the rails.
Check the DC output voltage of the amp (to the left of the output capacitor) with both inputs grounded.
Here's a way to measure the open loop gain of your circuit and see if works in Pspice by using very low frequency feedback to give a proper DC bias.
 
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