Finding the asymptotic gain "G∞" provides insight, and usually can be done by inspection. To find "G∞" we let "g"m → ∞ and find the resulting gain. The drain current, "i"D = "g"m "v"GS, must be finite. Hence, as "g"m approaches infinity, "v"GS also must approach zero. As the source is grounded, "v"GS = 0 implies "v"G = 0 as well. With "v"G = 0 and the fact that all the input current flows through "R"f (as the FET has an infinite input impedance), the output voltage is simply −"i"in "R"f. Hence