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Re: Linearity improvement due to source degeneration
The effective gm of the transistor is roughly gm/(1+gm*R), R is the degeneration resistance and gm is the trans-conductance of the diff pair. Thus, by making gm*R >> 1, the effective gm is ~ 1/R. Since R is more linear compared to gm, one gets better linearity.
For dc bias for CS
Think about an overshot noise happened at VG increasing VGS by delta_vgs. This will cause current ID to increase also by delta_ID.
But this delta_ID will pass through RS whch will increase VS (source voltage) to delta_ID * RS
so this increase will cancell the increased shot at the input maintaing VGS to back to its stabel designed voltage
A MOSFET has a non-linear relation between the gate-source voltage and the drain-source current which generates non-linearity in the input voltage versus the output voltage (across the drain load resistor).
A source resistor reduces the effect of this non-linearity (at the expense of lower gain) since it tends to make the drain-source current more a linear function of the input voltage.
Essentially, the much larger AC voltage across the source resistor due to the collector-emitter current swamps out much of the non-linear effect of the small gate-source voltage change.
The disadvantage is that this resistor voltage at the source also subtracts from the effective gate-source input voltage, reducing the circuit gain.
So you trade off gain for better linearity.
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