Operational Amplifiers

An operational amplifier, or op-amp, is a high-gain differential voltage amplifier with two inputs and one output, packaged as an integrated circuit. It amplifies the difference between its non-inverting and inverting inputs by a very large open-loop gain. In an idealised model the op-amp has infinite gain, infinite input impedance and zero output impedance, which makes circuit analysis straightforward and gives results close to those of real devices for most purposes.

The op-amp's usefulness comes from negative feedback. By returning part of the output to the inverting input, a designer fixes the closed-loop gain with a few external resistors instead of relying on the imprecise open-loop gain. The inverting amplifier produces an output proportional to and opposite in sign to its input, while the non-inverting amplifier gives an in-phase output with a gain set by the feedback ratio. With negative feedback the two inputs are held at almost the same voltage, a condition often called the virtual short, which is the key to analysing these circuits.

Without feedback, or with positive feedback, the op-amp acts as a switch rather than a linear amplifier. As a comparator it drives its output to one supply rail or the other depending on which input is higher, useful for detecting threshold crossings. Adding positive feedback gives a Schmitt trigger, whose two switching thresholds provide hysteresis and clean switching in the presence of noise. Diode-based clipping circuits limit signal swing to protect later stages. Animations help by showing how the output responds instant by instant as the input crosses the relevant thresholds.

Frequently asked questions

What is the virtual short in an op-amp circuit?

With negative feedback the very high gain forces the two inputs to nearly the same voltage. Treating them as equal greatly simplifies analysis of the surrounding circuit.

How does a comparator differ from an amplifier?

A comparator uses the op-amp without negative feedback, so the output swings fully to one rail or the other depending on which input is larger, rather than amplifying linearly.

Why add hysteresis with a Schmitt trigger?

Positive feedback creates two separate switching thresholds. This hysteresis prevents rapid, repeated toggling when a slow or noisy signal lingers near the trip point.