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Design >> Analog Design >> basic question on noise equations
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Message started by vamshikrishna on Apr 25^th, 2009, 1:16pm

Title: basic question on noise equations
Post by vamshikrishna on Apr 25^th, 2009, 1:16pm

Hi,

If we consider the thermal noise eqn of mosfet it looks like this : id^2 = 4KT*gamma*gm;

which means noise contribution is directly proportional to gm,

but if I look at the input referred voltage noise of the same its 4KT*gamma/gm which is inv prop to gm, thats why I am a bit confused.

so should we have high gm or low gm for a mosfet to get less noise contribution from it?

Please clarify this.

Thanks
vamshi

Title: Re: basic question on noise equations
Post by thechopper on Apr 25^th, 2009, 6:35pm

Hi vamshi,

If you consider the noise in saturation region then gm is proportional √ID. therefore ID increases with gm^2.
Consequently, since noise increase with gm the SNR ID/id^2 increases with gm. So the larger the gm the better.

In triode region (I think you were referring to that noise equation, but then you should replace gm with the output conductance gdo) the idea is similar since the larger the gm the smaller the RON and therefore (as in any resistor) the smaller the thermal noise.

Hope this helps
Tosei

Title: Re: basic question on noise equations
Post by raja.cedt on Apr 25^th, 2009, 9:46pm

hi,
it depends on how transistor is being used.lets consider common source amp with diode convected load,here main transistor will be giving directly input referred voltage so for this high gm required where are load device is dumping noise current into main transistor so it gm should be as low as possible.Like this we have to analyse for every ckt that whether Transistor is dumping current or directly giving input referred voltage noise.
Thanks,
Rajasekhar.

Title: Re: basic question on noise equations
Post by Asmodeus on Apr 26^th, 2009, 2:40am

use large gm for input transistor and small gm for current loads !!

Don't see thing keeping in view output noise. See things through input referred noise. As gain will increase both noise n input by same amount.
It is the very reason to use the term "input referred noise". :)

Title: Re: basic question on noise equations
Post by vamshikrishna on Apr 27^th, 2009, 10:41am

Thanks for the replies. Now I got some idea of how to take care of noise while designing.

Thanks
vamshi