AC analysis in Ansoft Designer with Nexxim

Hi,

I'm trying to do a simple opamp (with MOSFETs and resistors) spice-styled AC analysis with Nexxim, but cannot get the right result. It seems that I need to use the Linear Network Analysis to do that, but exactly how, and how about the input and output port impedance, and what should I plot in its simulation results (the S21 parameter?)? Is there an old way in Nexxim like in Cadence or other spice simulator to do the AC analysis?


Thanks in advance!

CC

You can not find s-parameters by AC analysis.To find s-parameters or other 4-port parameters like Y ,Z, H and ABCD , you have to use s-parameter simulation.But generally op-amp designs are simulated AC analysis to find AC gain and phase.In that case two nodes ( output and input ) are selected to find gain and other important values either they are not terminated or terminated with any impedances

It should be noted that AC analysis is totally different from s-parameter analysis and doesn't give the same result.
When you do a AC analysis, source shuld have very low voltage( for instance 1pV ) and components are assumed as linear.

But s-parameter analysis assumes always the termination impedances are constant..

There are many other properties that should be noted here but next time..

Thanks for the reply.

In fact I'm not trying to find the s-parameter here. I just want to find the gain and phase response of the opamp like the way in hspice, spectre, or eldo. The problem is, in Ansoft Designer (with Nexxim), it seems that it does not have an analysis called "ac analysis". but a so called "linear network analysis", in which I can specify the frequency domain analysis, and specify the frequency range of the analysis. But after the simulation, the results just give me the s-param, the Z, Y, ABCD, etc, and various gains (in microwave sense, not the usual Vout/Vin), which depend on the input and output port impedances. I just don't know how to, like in spice, just specify the input ac source, then do an ac analysis, then plot out the dB(Vout/Vin) result to extract the dc gain and phase margin those kinds of information.