Operational amplifier or Op-amp is basically an amplifier and the basic job of an amplifier is to amplify the input signal. In early days when digital computers were not evolved, at that time the different mathematical functions like addition, subtraction, integration, and differentiation were performed using Op-amp. Just by connecting few resistors and capacitors, it is possible to perform the different mathematical operation and that is why this amplifier is known as operational amplifier.
Op-amp consists of two inputs and one output and most of the amplifier consist of two power supplies, positive and negative. But there are many op-amp IC’s which runs on the single power supply. The input terminal that is marked by positive sign is known as the non-inverting input terminal and another input terminal which is marked as negative sign is known as the inverting terminal. Op-amp in the above figure is one kind of differential amplifier with the single output. It means that this amplifier amplifies the difference between the two input signals.
So, let’s say V1 and V2 are the input signals which is being applied to this operational amplifier and let’s say the gain of this op-amp is A, then the output will be equal to A(V1-V2). If we applied the single input to this operational amplifier and we have grounded another input terminal then at the output will be AV1where A is the open loop gain of this operational amplifier. The reason it is being known as the open loop gain is that it is the gain of the operational amplifier when there is no feedback from the output to the input side. Suppose if you are applying the sinusoidal signal over, then at the output that sinusoidal signal should be get multiplied by the factor of this gain and at the output, you should get the amplified sinusoidal signal. The phase of this output voltage will be the same as the input voltage. Likewise, whenever we are applying input to this negative terminal, and we are grounding another terminal then the output of this amplifier will be equal to -AV2 because the difference between these two input terminals will be equal to 0-V2, that is equal to -V2 . That is why this input terminal is known as inverting terminal.
When we apply the input signal between these two positive and the negative terminals then at output we will get A times differential input signal where A represents the open-loop gain of operational amplifier.