The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Simulators >> Circuit Simulators >> Question about AC analysis in Cadence (Spectre)
https://designers-guide.org/forum/YaBB.pl?num=1472615392

Message started by tenso on Aug 30^th, 2016, 8:49pm

Title: Question about AC analysis in Cadence (Spectre)
Post by tenso on Aug 30^th, 2016, 8:49pm

This is a 2 part question.

1) Is AC analysis essentially a small signal analysis? I know that we use the "AC magnitude" parameter in Cadence when we are doing simulations for frequency response. But the spice .ac option doesn't seem to have a field for AC magnitude. Does SPICE assume that the magnitude is set to 1?

2) I was simulating the open loop gain of an op amp circuit and I used the following setup measured in Allen's book.
The resistance and cap values are chosen to be large so that the 3dB frequency is low. Sure enough, it gave the expected value of my open loop gain of around 70 dB.
When I get rid of the feedback path and ground the inverting pin, I am getting a gain of around 44dB when I run an AC analysis. Why the difference and shouldn't open loop gain be measured with no feedback?

Title: Re: Question about AC analysis in Cadence (Spectre)
Post by Geoffrey_Coram on Sep 2^nd, 2016, 11:32am

1) Spice ac analysis is a small-signal analysis; the dc operating point is computed, the circuit is linearized, and then the ac magnitude(s) you specified for your sources are used to stimulate the circuit.

You can set the magnitude to whatever you want:

v1 a 0 1 ac 2

Some simulators will warn or error out if you don't have an ac magnitude set for any source.