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4 – Op Amp: Ideal and real characteristics

In this lesson we will see what are the main characteristics of an Operational Amplifier.

We will try to understand what are the values that should characterize the ideal case.

And, finally, we will see how much the real case is close to the ideal case.

An ideal operational amplifier should have the following four characteristics:

  1. Infinite Input Impedance. This implies that no current can flow in the input pins. It ensures a perfect voltage coupling from previous stages. We will talk about voltage coupling when we will see a voltage buffer built with an Op Amp.
  2. Zero Output Impedance. This implies that output voltage doesn't vary with output current. It also ensures a perfect voltage coupling with following stages.
  3. Infinite Open-Loop Gain. We will see why an infinite value is desirable when we will talk about negative feedback and virtual short circuit.
  4. Infinite Bandwidth. It means that Operational Amplifier can handle every kind of voltage signal regardless of its spectral content.
  5. Infinite CMRR. We expect that the output voltage depends only on the difference between the input voltages. Unfortunately a part of the output voltage depends on the finite value of the input voltages (specifically, it depends on their average); that average is called common mode input voltage. Common Mode Rejection Ratio measures how much the Op Amp rejects this average voltage value. The higher the CMRR, the less the impact of common mode. An infinite CMRR ensures that the common mode is completely rejected.
  6. Zero Noise. I'm sure it is not necessary to explain why the absence of noise is desirable.
  7. Zero Current OffSet and Zero Voltage Offsets. We will talk about OffSet in the next lesson. By now let consider it is an undesirable phenomenon that would be better if it were not present.