The term operational amplifier was coined in 1947 to describe differential amplifiers of high gain capable of amplifying DC signals as well as AC signals. The differential nature of these amplifiers simply means that we're amplifying the difference between two input voltages. We will show a generic operational amplifier (op-amp) as in Figure 3.
Figure 3: An Operational Amplifier
In our analysis of this amplifier, we will make the following simplifying approximations to the model: