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4 – Op Amp: Ideal and real characteristics

A real Operational Amplifier usually has the following characteristics:

  1. Finite Input Impedance. Its value depends by technology used to build the Op Amp. Usually it is about 10 M \Omega for Operational Amplifier whom first stage use BJT technology. It increases for MOSFET technology. Anyway this value is high enough to be considered, with good approximation and in most cases, infinite.
  2. Non-Zero Output Impedance. Usually its value is between 1 \Omega and 10 \Omega. This value is low enough to be considered, with good approximation and in most cases, equal to zero.
  3. Finite Open-Loop Gain. Its real value is usually equal to 100000 \div 200000 \left( 100 dB \div 106 dB \right) but it can raise to 1000000 \left( 120 dB \right). This value is high enough to be considered, with good approximation and in most cases, infinite.
  4. Finite Bandwidth. Usually Operational Amplifier's cutoff is near 10 Hz. Really bad! It implies that Op Amps can handle only signals with a bandwidth less than 1 MHz. This is a big limit.
  5. Finite CMRR. This value is high enough to be considered, with good approximation and in most cases, infinite.
  6. Non-Zero Noise. Unfortunately noise is always present in every electronic component.Fortunately building technology reduces it and its incidence is usually negligible.
  7. Current OffSet and Voltage Offset does exist and, in most cases, can not be overlooked. Fortunately it can be reduced to negligible values by means of a trick that we will see in the next lesson.

As you can see, in most cases, we can consider the behavior of the Operational Amplifier as if it were ideal.

Biggest problems are created by Bandwidth and OffSet.

 

 

 

 

 

 

 

 

 

In the first case, we just work with signals that have a bandwidth lower than 1 MHz .

In the second case we must operate on the offset correction's pins.

By now we consider these two aspects as negligible.

We will assume that signals have an adequate Bandwidth and that OffSet problems have been adequately corrected.