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Thermal noise reduction technique

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Hello

Can some one tell me that fr low thermal noise do we have to increase the transconductance or decrease the transconductance of an amplifier. I have read in different research article that increasing transconductance reduces thermal noise but according to thermal noise formula 4KT2/3 gm ro2, it shows reducing transconductance reduces the thermal noise. Which one is correct reducing transconductance or increasing transconductance?
 

For an amplifier, you want to calculate input referred noise.
 

Yes, but for reduced noise we have to reduce flicker and thermal noise both. Flicker noise can be reduced by increasing the sizes. But what about thermal noise?
 

That is going to be reduced with increased size too.

Flicker ~ 1 / (W * L) => Maximize W and L

Thermal ~ 1 / gm ~ 1 / ( sqrt( W/L ) ~ sqrt( L / W ) => Maximize W. Minimize L.

Dependent a bit on if you "voltage-bias" or "current-bias" your transistor.

gm ~ sqrt(W L I) or ~ I / Veff so increasing current is an option too.
 

jjx thanks for your reply but according to the formula of thermal noise i.e. 4KT(2/3) gm ro2 . gm should be low to decrease thermal noise. If we increase current then gm is going to increase
 

Yes,

But as FvM indicates you have to compare apples with apples. If you look at your formula an increase in gm will also increase the gain of the amplifier.

Vout = Vin * (gm rout)

Snr at output for a given input level is

Pout / Nout ~ Vout^2 / (gm rout) ~ Pin (gm rout)^2 / ( gm rout) ~ Pin gm Rout

An increase of gm will improve Snr at the output even though the noise per se increases.

However referring to input enables you to compare translated internal circuit noise with say a specified input signal level that the simplifier should "detect". It simplifies the argument a bit.
 

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