# Input impedance of differential amplifier

Can someone please explain to me, how I should look at the input impedance of the opamps, seen through the voltage $$\V_1 V_2\$$?

I have two questions on the differential amplifier:

• Why doesn't the input impedance depend on the feedback resistor $$\R_2\$$ of the first circuit seen by the voltage $$\V_1\$$?

• Why is the input impedance of the second circuit $$\2*R_1\$$ when you put a single source on both terminal and why doesn't it depend on the $$\R_2\$$ that is connected to the ground?