Virtual short circuit
In the case of an ideal operational amplifier with negative feedback, the term virtual short circuit expresses the fact that both inputs have the same electrical potential regardless of the input signal , which gives the impression that there is a short circuit . However, no current flows between the inputs, hence the “virtual” attribute, but only via the feedback network. Theoretically, it is necessary so that the output voltage becomes finite, since the gain of the difference between the two input voltages in the ideal operational amplifier is infinite.
In the case of the OP circuit as a non-inverting amplifier, a voltage drops across R 1 which is equal in amount to the input voltage. The impression arises that there is a short circuit between the differential inputs of the OP, but no current flows between the inputs. In fact, the ideal OP with negative feedback regulates its output voltage so that there is no voltage difference between the differential inputs. The current flowing in from the OP output via R 2 causes a voltage at R 1 that corresponds to the input voltage U E.
As a result of the virtual short circuit, the OP circuit of inverting amplifiers has another special property: the virtual ground . In this circuit, one differential input is at ground potential and the other is in the middle of the feedback branch. As a result of the virtual short circuit, one end of resistor R 1 is apparently connected to ground.