Circuit Idea/Slowing Circuit Processes

The cause is delayed, albeit for a moment, relative to the effect - first the cause (input signal) appears, and then the effect (output signal) changes. Therefore, in order to introduce causality into inertialess, proportional electronic devices, they must be deliberately made slow, inertial, integral. By slowing down the action of the studied devices many times over, it becomes commensurable with the speed of human thinking and allows to "peek" into the transition process, where there is a hidden valuable information about the circuit operation.

This heuristic approach can be applied to op-amp circuits with negative feedback. How op-amp works is a key point to understanding op-amp circuits. Reading classical formsl explanations, we could not understand what and how an op-amp really does in op-amp circuits with negative feedback. We gradually realize a paradoxical fact: although an op-amp is really a proportional and, as they consider, an almost "non-inertial" device, in order to understand how op-amp circuits work, we have to think of it as of an integrating, inertial device what it actually is. Figuratively speaking, we have to think of the op-amp as of a "being" that "observes" continuously its input voltage and changes its output voltage until it manages to zero the input voltage. If we think of the op-amp in terms of Vout = A.Vin (i.e., Vin and Uout change simultaneously), we fall into a vicious circle traveling the feedback loop and we can never understand circuit operation. We have to assume that the output voltage stays late regarding the input voltage. More philosophically speaking, we have to see the causality in the op-amp operation; to think of the input voltage as of a cause and of the output voltage as of a result.

We can find this concept in the H&H The Art of Electronics, "The golden rules" section (p. 177): What the op-amp does is "look" at its input terminals and swing its output terminal around so that the external feedback network brings the input differential to zero (if possible). As you can see, Horowitz & Hill have also animated the inanimate op-amp with the sole purpose of showing what it actually does in negative feedback circuits.

See also

Op amp myths (Integrator Inside section) by Barrie Gilbert