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Design >> Analog Design >> help on second order effect!
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Message started by kibby on Oct 23^rd, 2009, 8:22am

Title: help on second order effect!
Post by kibby on Oct 23^rd, 2009, 8:22am

if the function of mos fet in saturation region is used:
i=1/2*ucw/l*(vgs-vth)^2
which ignore the second order effect;
to analyse the character of gm''(the second derivative of gm) of a simple full differential amplifier,then is the result helpful for design? or it is unreasonable?
why?
thx~

Title: Re: help on second order effect!
Post by rajdeep on Oct 23^rd, 2009, 8:29am

2nd derivative of Ids you mean i.e. the 1st derivative of gm?

If yes, then I wud say theoretically yes. Because, usually the input pair of MOSes will have small L as they are in the signal path, and hence channel length modulation effect will be there.

cheers!
Rajdeep

Title: Re: help on second order effect!
Post by raja.cedt on Oct 23^rd, 2009, 8:32am

hi,
sorry i didn't understand man..could you please explain clearly?

Thanks,
Rajasekhar.

Title: Re: help on second order effect!
Post by Mayank on Oct 23^rd, 2009, 11:22am

Hi,
Yeah same here kibby...please explain your question so that we can comprehend better...

But as far as this basic eqn is considered, please include (1 + lambda*vds) factor also in your calculations. Vds effects are important, mostly in low supplies.

But please explain as to what n why do want second derivative of gm ??

regards,
Mayank.

Title: Re: help on second order effect!
Post by kibby on Oct 24^th, 2009, 4:38am

sorry, may be i did not say clearly before;
if i=1/2*ucw/l*(vgs-vth)^2 is used
then the gm of full differential amplifier is
gm=1/2*ucw/l*(4*Iss/(ucw/l)-2*deltav^2)/sqrt(4*Iss/(ucw/l)-deltav^2) where deltav refers to the input differential voltage;
which appears in razavi's book <design of analog cmos integrated circuit> formula 4.10
and gm'' can be calculated though the expression of ids do not include second order effect!
but, if the channel length effect is included, do you know how long the expression can be? it is beyond hand calculation!
so , i hope that the earlier result is useful, but i am not sure
thank u again for your help!

Title: Re: help on second order effect!
Post by Berti on Oct 26^th, 2009, 12:42am

Kibby, I think a strongly depends on the technology your using whether this simple calculation is reasonable or not. In my experience, for 0.35μm square-law model is ok for 130nm and below it gives bad results. For advanced technology I prefer to use a gm/Id methodology for hand calculations.

Cheers

Title: Re: help on second order effect!
Post by Mayank on Oct 26^th, 2009, 1:20am

Hi kibby,
As Berti suggested, Why don't you use Gm/Id methodology for ckt design rather than hand calculations (taking into acc only 1st order effects ) ??

Gm/Id methodology employs technology plots which you can simulate from spectre/spice on only 1 MOSFET....say you can store FOMs for different vgs,vds,vbs,lengths etc...Now you while building a ckt, you can directly use these curves which include all the secondary effects. It's pretty simple too.

From personal experience, i feel using Gm/Id is the best methodology & gives pretty accurate results when used wisely....

regards,
Mayank.

Title: Re: help on second order effect!
Post by kibby on Oct 27^th, 2009, 5:39am

sorry, i am a new learner here...could you introduce the gm/Id methodology or recommend some readings or papers?
really appreciate your help~ ^_^