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Two identical NMOS transistors $\text{M1}$ and $\text{M2}$ are connected as shown below. $\mathrm{V}_{\text {bias }}$ is chosen so that both transistors are in saturation. The equivalent $g_{m}$ of the pair is defined to be $\frac{\partial I_{\text {out }}}{\partial V_{i}}$ at constant $V_{\text {oul }}$.

The equivalent $g_{m}$ of the pair is

1. the sum of individual $g_{m}$ 's of the transistors
2. the product of individual $g_{m}$ 's of the transistors
3. nearly equal to the $g_{m}$ of $\mathrm{Ml}$
4. nearly equal to $g_{m} / g_{0}$ of $\mathrm{M} 2$