We now consider the circuit function of the op amp. The op amp is designed to sense the difference between the voltage signals applied at its two input terminals (that is, the quantity u₂ – u₁) multiply this by a number A , and cause the resulting voltage A(u₂ – u₁) to appear at output terminal 3 . Here it should be emphasized that when we talk about the voltage at a terminal we mean the voltage between that terminal and ground; thus u₁ means the voltage applied between terminal 1 and ground.