Can anyone explain this question?

[ee484]

This question is from Razavi's book 7.8 (a).
The question is asking to solve input referred voltage and current, but the answer has only input referred voltage only.

I would like to know the input referred current noise.

I got the input referred voltage noise as follows.
Vn,in^2 = 4KT( 2/3gm1 + 1/(gm1^2*RD) + RG )

and the input referred current noise is In,in^2 = IRD^2 = 4KT/RD.

Are these correct?

 

[pixel]

This question is from Razavi's book 7.8 (a).
and the input referred current noise is In,in^2 = IRD^2 = 4KT/RD.
Are these correct?

I think that you should add also the effect of Rg. It is true that input reffered curent is always equal to current that come into the drain, but consider that beside Rd noise component, trough drain flows part of the mos current In,in(Rg)=ro/(ro+Rd)gm*√(4kTRg) ≈ gm*√(4kTRg), for ro->∞

 

[jlee]

I agreed the input referred current noise is In,in^2 = IRD^2 = 4KT/RD, and Rg should NOT contribute to any current noise.

It is because when we are finding the current noise, we let the input (source in this case) open circuit. As the source is floating in small signal, whatever change on the gate Vg doesn't affect the current at drain. So Rg doesnt have effect on the output current noise, and output current noise = input current noise.

 

[ee484]

Thanks....

I solved a couple of different ways...and I now comfortable with the fact that RG doesn't affect input referred current noise.

Thank you guys...

Good explanation!