Real op amps are not ideal. They have limitations. To understand and discuss the origins of these limitations, see the simplified op amp circuit diagram shown in Figure 4-1.
Although simplified, this circuit contains the three basic elements normally found in op amps:
The function of the input stage is to amplify the input difference, Vp - Vn, and convert it to a single-ended signal. The second stage further amplifies the signal and provides frequency compensation. The output stage provides output drive capability.