Quote:A low impedance at the common source acts to turn the rectified voltage at this node to a current which will ultimately flow easier into the tank.
What makes up the voltage at this node? it is the accumulation of bias current and the noise associated with it. This just a different way of looking at a well derived phenomenon (well at least my viewpoint).
If we look at it in a more canonical way, considering noise injection is in terms of current which is actually the case. -- i
n2 noise current from the Tail current source (both flicker & thermal , generated due to it's transcoductance gm)
Reason to consider noise injetion in terms of Current not Voltage -- If the node under consideration has an infinite impedance(as is the case at gate of a MOS), we would have considered it as noise voltage.
If we are investigating noise at a node with a finite impedance, then we should always consider noise sources as current & then drop it across appropriate node impedance & convert it to noise voltage.
That being said, If we now consider noise injection in terms of current,
Higher the cap from common source node to GND,
OR Higher the Inductance from drain of tail current source to Common Source of diff pair,
Lesser is the propagation of Noise current into the TANK,
& so is the conversion of Noise current to Noise voltage,
& so lesser is the upconversion.
Meaning, more the parallel Cdrain (or series L), lesser the effect of noise from tail current source.as i have understood & is quoted in many texts.
--
Mayank
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