Mayank wrote on Nov 17th, 2009, 11:38pm:I am still not able to comprehend how it doesnt affect noise.....Could you pls. explain a bit further ?
I don't say no affect. It does affect.
But you have to consider degree of affect to output according to your various conditions.
If L1 is resistor, noise contribution to output will be greatly large for larger L1 resistor.
However if input impedance of MOSFET is high and source impedance is low, noise contribution of R1 to output will not be so large even for larger R1 resistor.
This is usual for this circuit configulation.
Which is more effective, series resistor or parallel resistor is dependent on input impedance of amplifier and source impedance of amplifier.
Mayank wrote on Nov 17th, 2009, 11:38pm:& at a MOS i/p why should we model noise as current source and not as a voltage source ??
If you calculate correcty, there is no difference between voltage noise and current noise.
But current noise add no extra node.
More one comment.
If R1 is small, gain of this stage will be small.
So total cascade system NF including this stage as top will be fairly worse.
This will come to nothing even if you make noise contribution from R1 small by decreasing R1.