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1.1. Characteristics of op-amps (What is the ideal op-amp?)

Generally, amplifiers should neither affect the preceding circuit nor be affected by the subsequent circuit. Therefore, amplifiers should have high input impedance and low output impedance.
Op-amps have characteristics close to these requirements. The following compares the ideal and real op-amps:

Although there is no such thing as an ideal op-amp, you can assume the ideal op-amp early in the design stage. However, you should consider the differences between the ideal and real op-amps when you proceed to the detailed design stage.
For example, if the input impedance of an op-amp is low, its input voltage is derived from the input impedance of that op-amp and the output impedance of the preceding device. The low input impedance of an op-amp also affects its feedback loop. If the output impedance of an op-amp is large, its output voltage is derived from the output impedance of that op-amp and the impedance of its load.
In typical applications, however, the input impedance of an op-amp is negligibly large compared with the output impedance of the preceding circuit, and the output impedance of the op-amp is negligibly small compared with the impedance of the subsequent load. Therefore, these impedances do not normally have a significant impact. The same is true of the other parameters shown above.
It is necessary, however, to check their impact when creating a detailed design.