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For a MOSFET in saturation, the small-signal drain current is I_D = (μ_n·C_ox/2)·(W/L)·(V_GS - V_TN)². The transconductance gm equals which?

Agm = I_D · V_GS scaling product
Bgm = μ_n·C_ox·(W/L)·(V_GS - V_TN)
Cgm = V_TN / V_GS as a ratio
Dgm = C_ox · (V_GS + V_TN)² scaled
Answer & Solution
Correct answer: B. gm = μ_n·C_ox·(W/L)·(V_GS - V_TN)
Differentiate I_D w.r.t. V_GS: gm = μ_n·C_ox·(W/L)·(V_GS-V_TN). Substituting (V_GS-V_TN) = √(2I_D/(μ_n·C_ox·W/L)) gives gm = √(2·μ_n·C_ox·(W/L)·I_D).
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