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Simulators >> Circuit Simulators >> pole/zero analysis
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Message started by raja.cedt on Jun 8^th, 2009, 10:45pm

Title: pole/zero analysis
Post by raja.cedt on Jun 8^th, 2009, 10:45pm

hi,
In hspice i am getting list of all poles and zero's, i just want to know in hspice is there any way to find which pole is corresponding to which node. I used to see every node phase shift whichever have more shift that is dominant and remaining also like that only, but what abut complex poles?

thanks for your time

rajasekhar.

Title: Re: pole/zero analysis
Post by buddypoor on Jun 9^th, 2009, 12:31am

Hi Rajasekhar,

Question: Why do you expect that each pole does correspond to a certain node within the circuit ?
Think of an active filter of 2nd degree. The complete circuit may exhibit 2...4 nodes (sometimes even more) - and which node belongs to the pole ? It can´t be answered.

Title: Re: pole/zero analysis
Post by raja.cedt on Jun 9^th, 2009, 1:04am

hi buddypoor,
thanks for your answer and what my intension is lets say hspice gave some 6 poles now i want to move 2nd and 3rd dominate pole to higher value, so i need which node is giving those poles so that i can compensate them. Interestingly in one my opamp 2nd dominate pole is complex i just want from where it is originated?

Thanks
Rajasekhar.

Title: Re: pole/zero analysis
Post by buddypoor on Jun 9^th, 2009, 4:40am

Yes, I understand the problem. However, for my understanding a real pole belongs ton RC combination (not necessarily to be identified as a single R or a single C) and , theoretically, it can be allocated to one specific node. But a complex pole pair belongs either to an LC resonance circuit (not in your case) or to a part which includes some sort of feedback - and that´s a circuitry which certainly has more than one node. However, it should be possible to identify this part of the circuit - at least by inspecting/modifying the capacitances belonging to it. Would this approach work ?
LvW

Title: Re: pole/zero analysis
Post by raja.cedt on Jun 9^th, 2009, 5:17am

hi,
i will try but i am getting complex pole for an telescopic op amp in open loop, up to my knowledge there shouldn't be any complex for open loop. For finding pole if you see particular node wave form that will have more phase advance (means node associated with dominate will give phase shift earlier than 2nd dominate and complex pole means phase shift from 0 180 suddenly.)

thanks,
rajasekhar.

Title: Re: pole/zero analysis
Post by buddypoor on Jun 9^th, 2009, 5:39am

raja.cedt wrote on Jun 9^th, 2009, 5:17am:

hi,
i will try but i am getting complex pole for an telescopic op amp in open loop, up to my knowledge there shouldn't be any complex for open loop................
.............................

In this case, there should be a hidden feedback path within your circuitry (for example CB-capacitances).

Title: Re: pole/zero analysis
Post by raja.cedt on Jun 9^th, 2009, 9:04am

hi buddypoor,
your guess is correct, but it less gain loop i think you can visualize whats the loop (It is a nmos based telescopic opamp and pmos load is diode with wide swing) ,so may be that's the culprit..

thanks,
Rajasekhar.