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supermoment

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Pole analysis differ from theory
May 08^th, 2010, 12:16am

I built common source amplifier by using a single nmos transistor with ideal 1k ohm load (Rd), bulk tied to ground (no body effect). Bias Vgs at around 0.7V and do AC simulation & pz analysis

width = 10um x 4 finger ; length=0.18u ; vdd=1.8V

Result from pz analysis
Pole1=-4.582GHz
Zero1=70.72GHz

AC measurement:
Av_DC=17.49dB
Av_DC=7.486 V/V
3dB_BW=4.753GHz

Extracted intrinsic cap of the nmos
gm=8.791m

Cdg=-19.78f
Cdb=-2.23f
Cds=11.28f
Csg=-26.51f
Csb=-3.342f
Csd=-23.11a ~0
Css=29.88f
Cdd=10.73f
Cgg=49.37f
Cgd=-10.72f
Cgs=-33.81f
Cgb=-4.849f

From theory,
Av_DC=gm*Rd=8.791m*1k=8.791 vs simulated 7.486
poles=1/(2*pi*Rd*(Cdg+Cdb)=1/(2*pi*1k*(19.78f+2.23f))=7.23GHz
vs simulated pz analysis=4.582GHz (huge difference)
3dB_BW = 4.753GHz

I try to relate simulation result with theory.

If back calculated from pz analysis
Cload=1/(2*pi*f*Rd)=1/(2*pi*4.582GHz*1k)=34.73fF

which is very near if Cload=Cdg+Cdb+Cds=19.78f+2.23f+11.28f=33.29f
however Cds is different sign with Cdg+Cdb...

Hope someone can clarify it...
it should be simple...but cant reach it..

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supermoment Senior Member Offline Posts: 167	Re: Pole analysis differ from theory Reply #1 - May 8^th, 2010, 12:19am Does overlap capacitance Cgdovl factor into the Cgd and Cgsovl factor into Cgs?
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pancho_hideboo

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Re: Pole analysis differ from theory
Reply #2 - May 8^th, 2010, 3:41am

What simulator do you use ?
Do you use Synopsys HSPICE which is "Golden Standard Simulator" ?

The followings are general notes for you.

- Always describe correct tool's name and vendor's name which you use as tool or simulator.
- Don't do multiple posts which are same content.
- Don't request source code or behavioral model without any efforts.
- There are many simulators which have analyses called as PSS, PAC and Pnoise.
- Describe in detail with using correct terminologies.
- Warnigns are different from Errors.
- ADS is not name of simulator.
- There is no tool which name is Cadence.
- Don't use Direct Plot of Cadence ADE blindly without knowing definition.
- All gains in Direct Plot of Cadence ADE are "right", "true" and "practical" voltage gain.
- Don't mix up Simulation with Post Processing. They are completely different phase.
- MATLAB are different from Simulink.
- Learn measurements using actual instruments. Not "EDA Tool Play

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http://www.youtube.com/watch?v=mjIxGh55bMM&feature=related

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supermoment

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Re: Pole analysis differ from theory
Reply #3 - May 8^th, 2010, 3:56am

using Cadence, spectre simulator.
(do not have synopsys, Yes i know HSPICE is golden standard)

Is the discrepancy because of the PDK accuracy? I am not sure. Because I found weird BW result in other circuit. That's why i constructed a simply testcase, common source amplifier

Av_DC_dB �--> value(dB20((VF("/out") / VF("/vgs"))) 1 ?histoDisplay nil ?noOfHistoBins 1)

Av_DC �--> mag(value((VF("/out") / VF("/vgs")) 1 ?histoDisplay nil ?noOfHistoBins 1))

3dB_BW --> bandwidth((VF("/out") / VF("/vgs")) 3 "low")

where vgs is input with small signal acm=1mV, out is output node

the gm & Cxx are obtained from OP (in dc)

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pancho_hideboo Senior Fellow Offline Posts: 1424 Real Homeless	Re: Pole analysis differ from theory Reply #4 - May 8^th, 2010, 7:20am I think result of Cadence Spectre PZ analysis is reasonable. Maybe there are some issues in interpretation of results of DC operation point information.
Back to top	Kita━━━━━━(ﾟ∀ﾟ)━━━━━━ !!!!! http://www7.plala.or.jp/ungeromeppa/flash/kita.html http://www.youtube.com/watch?v=mjIxGh55bMM&feature=related IP Logged

supermoment Senior Member Offline Posts: 167	Re: Pole analysis differ from theory Reply #5 - May 8^th, 2010, 7:26am pz result is almost same with AC response curve. The thing i doubt is the extracted capacitance by using OP the values seems smaller... could someone compare those extracted cap with other simulator or foundry in 0.18um technology?
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supermoment Senior Member Offline Posts: 167	Re: Pole analysis differ from theory Reply #6 - May 8^th, 2010, 7:34am one more query, why Cds is not counted in freq analysis. source is grounded, so Cds can be part of the load at drain load. I have checked few textbooks, Cds is not counted. Is it because it is too small?
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supermoment Senior Member Offline Posts: 167	Re: Pole analysis differ from theory Reply #7 - May 9^th, 2010, 8:25pm Found out that Cds splits into Cgb, Csb, Cdb... therefore Cds is not counted since Cdb is already taken in.
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raghu_n_ch New Member Offline Posts: 1	Re: Pole analysis differ from theory Reply #8 - May 25^th, 2010, 7:52am Are you accouting for device rds. There will be miller multiplication of the cap Cdg depending on the gain, this also needs to be considered.
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