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Simulators >> Circuit Simulators >> How to find a operating region of transistor
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Message started by kiran123 on May 28^th, 2007, 3:38am

Title: How to find a operating region of transistor
Post by kiran123 on May 28^th, 2007, 3:38am

Hi Experts,

I am trying to find the region of operation of transistor, for that i am doing dc operating point analysis and checking the "region" in the printed file , by the same when i check using its terminal voltages i.e

for saturation condition : vgd-vth < 0 and vgs-vth > 0
sometimes even when vgd-vth >0 also the printed information in OP file is showing "saturation"

exactly i don't understand based on what condition does spectre OP, results this region of operation information?

can anybody share their knowledge on this to help me in understanding

Thanks
Kiran

Title: Re: How to find a operating region of transistor
Post by Geoffrey_Coram on May 29^th, 2007, 5:52am

kiran123 wrote on May 28^th, 2007, 3:38am:

for saturation condition : vgd-vth < 0 and vgs-vth > 0
sometimes even when vgd-vth >0 also the printed information in OP file is showing "saturation"

I assume it's not that vds < 0 (and vgs-vth<0 rather than vgd-vth<0)?

What are you using for vth? The parameter or the OP file value? There's the parameter vth0, the zero-bias vth that includes STI and WPE, as well as the bias-dependent one accounting for body effect, etc.

Title: Re: How to find a operating region of transistor
Post by kiran123 on May 29^th, 2007, 6:28am

HI Geoffrey,

For saturation the Text book or condition says Vgs-vth> 0 (for establishment of channel) next to be in saturation while in conduction it should also satisfy Vds>= Vgs-vth but rewriting for somplicity sake

0>= Vgs-vth-Vds
0>= (Vg-Vs)-vth-(Vd-Vs)
0>= (Vg-Vd)-vth [after cancellation of Vs effect ]
0>= Vgd-vth

vth is OPfile value but not the MODEL file parameter (vth0)

so to ensure the transistor in saturation based on MOS terminal voltages i feel
it should satisfy Vgs-vth >=0 & Vgd-vth <= 0
but this is not becoming true always with what is displayed by simulator under "region" in OPfile

let me knwo based on what simulator shows the "region"

thanks for your support on this
Kiran Kumar

Title: Re: How to find a operating region of transistor
Post by mosman on May 30^th, 2007, 6:57pm

hi the region is based on the Vdsat(Saturation Voltage):

when Vgs>Vth it is strong inversion

so if Vds>=Vdsat device stay in sat;
if Vds<Vdsat device stay in linear;

but the Vdsat isnot simply the Vgs-Vth. it is somewhat complex, for example, carrier velocity, substrate level, and so on.

maybe help you.

Title: Re: How to find a operating region of transistor
Post by kiran123 on May 31^st, 2007, 3:40am

hi mosman,

Thanks for your help now i am able to capture the region of operaions without using "region", what is the difference between vds and vds_sat values from spectre perspective?
from your reply what i understood is vds_sat is depending upon many parameters as you said includeing ids vds =vd-vs i.e Voltage between Drain and Source terminals where which inherently i feel this also depends on ids rite? :-/

Title: Re: How to find a operating region of transistor
Post by Andrew Beckett on Jun 4^th, 2007, 1:11pm

The definition of Vdsat is dependent on the transistor model being used.

In the documentation for spectre, there's a whole manual devoted to the device equations for each model - you'll find it in there.

Alternatively, read a book (pretty much any book) on CMOS Analog circuit design. This is a rather important parameter and it's hard to imagine how you'd manage to design any analog circuit without understanding what the saturation voltage is. The Books section of this site should give you a starting point.

I don't know what you meant by "ids rite?" in your post, unless you were breaking the forum rule Do not use instant messaging short hand and meant "right?" (assuming that is IM shorthand - I don't know).

Regards,

Andrew.

Title: Re: How to find a operating region of transistor
Post by Geoffrey_Coram on Jun 5^th, 2007, 8:36am

Vdsat does not depend on Ids; in every MOS model I've seen, the voltages are used to compute intermediate quantities (like Vdsat) and these are then used to calculate Ids, so it's the wrong perspective to think that Vdsat depends on Ids.

When you say that the operating region reported didn't match your expectation based on Vgs,Vgs,Vth, etc., how far off was it? If it's in the microvolts, then it's not worth tracking down.