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Design >> Analog Design >> op ac simulation help
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Message started by qqic on Jul 6^th, 2007, 3:20pm

Title: op ac simulation help
Post by qqic on Jul 6^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!

Title: Re: op ac simulation help
Post by ywguo on 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

Title: Re: op ac simulation help
Post by qqic on 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

Title: Re: op ac simulation help
Post by ywguo 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

Title: Re: op ac simulation help
Post by vivkr on Jul 10^th, 2007, 3:21am

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

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