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qqic

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op ac simulation help
Jul 06^th, 2007, 3:20pm

Hi all,
I design an opamp, when I double the width and length of input differential pair, the phase margin changed from 60 degree to 40 degree, the structure is folded caccode.
the reason is that when input pair W*L increase, the cds increase quickly!
some of the input differential pair's capacitors is as following according to DC operation point simulation of spectre: cds=72f
cdg=-72.5f
cgd=-10.7f
cgs=-120f

I have 3 questions:
1> why the cdg is minus in DC simulation of spectre ?
2> why cgd is not the same to cdg, and cdg is much high?
3> why cds is much high? in my opinion cds should be very small!

thank you very much!

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ywguo Community Fellow Offline Posts: 943 Shanghai, PRC	Re: op ac simulation help Reply #1 - Jul 7^th, 2007, 12:20am Hi, Assume you are using BSIM 3v3 model, please take a look at the definition using spectre -h bsim3v3. Here, cdg is ΔQd/ΔVg, cgd is ΔQg/ΔVd, cgs is ΔQg/ΔVs. So it is not curious to see minus capacitance. Yawei
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qqic New Member Offline Posts: 2	Re: op ac simulation help Reply #2 - Jul 9^th, 2007, 6:08pm thank you very much! but I still have a problem: why cds is much larger and csd is very small? qqic
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ywguo Community Fellow Offline Posts: 943 Shanghai, PRC	Re: op ac simulation help Reply #3 - Jul 9^th, 2007, 7:59pm Hi, Do you mean cgd and cdg? I think the value of cgd has nothing to do with that of cdg. The specific value depends on your schematic. Specifically, it depends on the parasitic capacitance at each node and the signal amplification from the g to d of the differential pair. Yawei
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vivkr

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Re: op ac simulation help
Reply #4 - Jul 10^th, 2007, 3:21am

ywguo wrote on Jul 9^th, 2007, 7:59pm:

Hi,

Do you mean cgd and cdg?

I think the value of cgd has nothing to do with that of cdg. The specific value depends on your schematic. Specifically, it depends on the parasitic capacitance at each node and the signal amplification from the g to d of the differential pair.

Yawei

Hi Yawei,

Could you elaborate on this point please? Also, how do you actually get the "real" cap information out from the simulator? I use the "captab" option, but it is not always useful as
it tends to print out caps for every single node. Is there a good way of getting the caps out using the operating point info from the calculator?

Regards
Vivek

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