Real electronics

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20.309: Biological Instrumentation and Measurement

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Real electronics sometimes behave significantly different than the idealized models presented in lecture.

Real op-amps

A model of an op-amp highlighting its behavior in practice.

Amplifiers using op-amps unfortunately do not follow the golden rules to the letter. The open-loop gain is not infinite. Therefore the difference between the input voltages is not zero and the current into or out of the inputs is not zero. A realistic model of an op-amp must incorporate a specification for the input offset voltage VOS, an internal voltage that adds to whatever voltage is externally applied to the input terminals V- and V+. The model also incorporates a bias current IB for each input. These are close in real op-amps so the average is specified on the datasheet. However they are not perfectly matched and the difference is specified as the input offset current IOS. All of these non-idealities have the net effect of causing a non-zero voltage, sometimes large, at the output in a negative feedback implementation even when the input is zero. Luckily there are corrective actions available.