Two stage op amp: input swing

[tobben24]

Hi,

I am designing an two stage op amp, and are currently running an ac analysis. How should the input swing affect the output in the analysis? The reason I ask is that my circuit gain drops 15 db if I lower my input swing with about 20 %. I thought that if the input swing drops, the op amp performance should increase?

 

[checkmate]

What do you mean by input swing? It would be good if you showed us your testbench schematic.

 

[tobben24]

**broken link removed**. By input swing, I mean that i change the size of Vswing, which can be seen in the testbench.

What do you mean by input swing? It would be good if you showed us your testbench schematic.

 

[checkmate]

Are you trying to find the open loop gain of a voltage follower? If so, you have got your testbench wrong.
The ac voltage source should be injected into the -ve input via the 1GF capacitor, and should have the other terminal connected to gnd.
The dc voltage source (VCM) should be connected to the +ve terminal.

Some connect the VCM and VSWING on the +ve input, and the 1GF cap on the -ve input. That will also work as well.

 

[tobben24]

I am so sorry. The schematic I gave was wrong, here is the correct one: **broken link removed**. My testbench is as you describe at your last section. And as I said, by input swing, I mean changing the size of Vswing.

Are you trying to find the open loop gain of a voltage follower? If so, you have got your testbench wrong.
The ac voltage source should be injected into the -ve input via the 1GF capacitor, and should have the other terminal connected to gnd.
The dc voltage source (VCM) should be connected to the +ve terminal.

Some connect the VCM and VSWING on the +ve input, and the 1GF cap on the -ve input. That will also work as well.

 

[checkmate]

An ac analysis linearizes the transistors (and hence the opamp) around its operating point.
It shouldn't have any performance difference be it vswing is 1nV or 1GV.
Because of this, we usually choose 1V for convenience sake.
For your problem, are you sure you are using the correct voltage source for VSWING?

 

[tobben24]

I found out the issue: I plotted the output and not output/input.

Thanks for your help checkmate! Much appreciated

An ac analysis linearizes the transistors (and hence the opamp) around its operating point.
It shouldn't have any performance difference be it vswing is 1nV or 1GV.
Because of this, we usually choose 1V for convenience sake.
For your problem, are you sure you are using the correct voltage source for VSWING?