Real electronics

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

Real op-amps

Amplifiers using op-amps unfortunately do not follow the golden rules. 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 V_OS, 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 I_B for each input. Normally these are well matched in real op-amps and the average is specified on the datasheet. However they are not perfectly matched and the difference is called the input offset current I_OS. 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.