Remove the feedback and Lesson 1's first golden rule vanishes with it. The op-amp reverts to what it physically is — an enormous-gain difference detector — and its output slams to whichever rail the sign of demands. That is a comparator: not a broken amplifier but a deliberate one-bit analogue-to-digital converter, answering the question "is this input above that reference?"
The reference divider sets the threshold; the input crosses it; the output flips. Two cautions, both common in practice. First, a comparator built from a general-purpose op-amp is slow — its output must traverse the entire supply range, and it was never designed to be driven into saturation. Dedicated comparator ICs are far faster, and some have open-collector outputs suited to logic. Second, with a slowly-moving input and any noise at all, the output will chatter around the threshold — the cure is hysteresis (a little positive feedback), which is the Schmitt trigger you will meet in ME-5.
Now the ideal assumptions' bill. OUTPUT SATURATION: the output cannot reach its supply rails, typically stopping one to two volts short on older parts, or within millivolts on modern rail-to-rail types. Ask for more and the output clips.
FINITE GAIN-BANDWIDTH: an op-amp's open-loop gain falls with frequency, and the product of closed-loop gain and bandwidth is roughly constant — the gain-bandwidth product, quoted on every datasheet. Gain is not free at speed.
SLEW RATE: the maximum rate at which the output can move, in volts per microsecond, set by internal currents charging an internal capacitance — CA-5's once more, now as a datasheet limit. A large fast signal can be slew-limited even when it is well within the bandwidth, and the symptom is a sine wave turning into a triangle. INPUT OFFSET VOLTAGE: the real input difference required for zero output, typically a few millivolts, which every DC-coupled circuit amplifies along with the signal. SINGLE SUPPLY: with the negative rail at ground, the output cannot go below zero, so a signal that swings both ways needs a mid-rail reference — which is why ME-2's V appeared, and why sensor circuits on a Pico so often build a half-rail node.
An op-amp with GBW MHz is used at a closed-loop gain of . What bandwidth do you get?
A comparator's reference divider uses kΩ (top) and kΩ from a V rail. At what input voltage does the output flip?
An op-amp with mV of input offset voltage is used in a non-inverting stage with a gain of . What output error does the offset alone produce?
Two conceptual checks: (a) why does a comparator have no feedback, and what breaks if you add some? (b) a circuit's output is a triangle when a sine was expected, yet the frequency is well inside the part's bandwidth. What is the likely limit?