I was reading an old copy of Horowitz and Hill's The Art of Electronics and I am trying to wrap my head around operational amplifiers in negative feedback circuits.
As the book explains, as the op-amp sees a positive voltage at its inverting input relative to its non-inverting input, it drives its output strongly negative, which effectively sets up a voltage divider along the resistor between the signal source and the op-amp's inverting input, and the feedback resistor running between the op-amp's output to the inverting input. The gain will be such that the voltage at the inverting input is the same as at the non-inverting input, which is typically ground. Hence the inverting input in a negative feedback circuit is sometimes called a virtual ground.
Now if this is the case, and the inverting and non-inverting inputs are brought to the same potential, won't the op-amp stop amplifying? I mean, there's now no voltage difference, so there's nothing to amplify.
I suspect that this is actually occurring, but as the op-amp stops amplifying, the output voltage moves up toward zero, which raises the voltage at its inverting input, which triggers amplification again until the virtual ground is re-established and the cycle repeats.
Wait, did I just say the cycle repeats? This implies is that the output voltage is actually oscillating at (probably) a very low amplitude and very high frequency, which for all intents and purposes for common circuits is stable. Is my understanding correct?
I bet if I used a capacitor to introduce a delay in the reaction of the op-amp, I could get oscillation at a lower frequency.