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Using the incremental low frequency small-signal model of the $\text{MOS}$ device, the Norton equivalent resistance of the following circuit is

1. $r_{ds}+R+g_{m}r_{ds}R \\$
2. $\dfrac{r_{ds}+R}{1+g_{m}r_{ds}} \\$
3. $r_{ds}+\dfrac{1}{g_{m}}+R \\$
4. $r_{ds}+R$