It follows that the voltage at the inverting input terminal (u1) is given by u1=u2. That is, because the gain A approaches infinity, the voltage u1 approaches u2 We speak of this as the two input terminals “tracking each other in potential.” We also speak of a “virtual short circuit” that exists between the two input terminals. Here, the word virtual should be em- phasized, and one should not make the mistake of physically shoring terminals 1 and 2 together while analyzing a circuit. A virtual short circuit means that whatever voltage is at 2 will automatically appear at 1 because of the infinite gain A. but terminal 1 as being a virtual ground–the is, having zero voltage but not physically connected to ground.
Now that we have determined u1 we are in a position to apply Ohm’s law and fine the current i1 through R1 (see Fig. 2.5) as follows: