# Difference opamp input resistance

I don't understand how Sedra/Smith got $R_{id} = 2R_1$. Why doesn't any of the current go through the grounded $R_2$?

• Thanks, but how could $i_1$ flow through the virtual short? Technically there is no return path for $i_1$ either since the opamp terminals should be open-circuited. – hesson Apr 27 '14 at 17:29