If an operational amplifier (op-amp) produces a very large output voltage when given a small difference between its inputs, what could be the underlying cause of this behavior?

If an op‑amp’s internal open‑loop gain is very high, even a millivolt difference at its inputs can be amplified to a voltage that reaches the supply rails, so the output appears “very large. ” This usually happens when the amplifier is not in a closed‑loop configuration, the feedback network is broken or damaged, or the device is saturated because the supply voltage is too low or the input is outside its common‑mode range. In such a case the op‑amp’s output will swing to the nearest rail, giving the impression of a runaway voltage. For example, with a 10 V supply and an open‑loop gain of 100 000, a 1 mV input difference would theoretically produce a 100 V output, which the device clamps to the 10 V rail. Thus, the underlying cause is the op‑amp’s high open‑loop gain combined with a missing or faulty feedback path.