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Simulators >> Circuit Simulators >> help with analysizing a opamp model
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Message started by urian on Apr 12^th, 2010, 10:14am

Title: help with analysizing a opamp model
Post by urian on Apr 12^th, 2010, 10:14am

hi,there
Recently i find a model of ideal opamp building with ideal components from analogLib in cadence IC5.It is shown below.
I think the net A and B are equal to inp and inn,so what's the effect of the two vcvs connecting them? Is it necessary? So as the net C and D,E and F.I think they are also equal with each other.So what does the second and third vcvs components perform? The gains of vcvs are all set to 1

And with the cmdmprobe component be placed in the end while the CMDM parameter be set to -1,dose it is used for the differential loop gain simulation?Like using a stb analysis to see the gain and phase of the closed-loop(open-loop?)?

BTW,I find there is iprobes in the cmdmprobe.Coz I dont use iprobe before.I guess that iprobe is a replacement of Middlebrook' method for loop stimulation.So I need not to inject a AC source and large res or cap or inductor like the classic method do,is it right?

Thanks in advance for any help!

Best regards
urian

Title: Re: help with analysizing a opamp model
Post by ray_wang on Apr 12^th, 2010, 7:12pm

For the 1st question, vcvs is to provide infinite driving capability and zero capacitance and zero resistance. I think A & B is unnecessary, while C & D, E & F are necessary. After deleting A & B, C & D is to ensure "virtual open" and E & F is to ensure the TF of the opamp will be unchanged with your load.

Title: Re: help with analysizing a opamp model
Post by urian on Apr 13^th, 2010, 9:25pm

Thanks,ray_wang.
If I use probe to test the stability of loop with stb analysis in spectre,need I inject a AC source in the loop,or just place the iprobe?

Title: Re: help with analysizing a opamp model
Post by Andrew Beckett on Apr 14^th, 2010, 4:54am

You do not need an AC source. Simply point the stb analysis at the iprobe (in fact if you use ADE, point at the cmdmprobe instance; it figures out which component within the cmdmprobe to probe automatically).

The iprobe is just a zero-volt source (a current probe) and the stability analysis takes care of injecting the signal at that point and measuring at that point (it identifies the loop to measure the loop gain).

Regards,

Andrew.