Operational amplifiers

An operational amplifier (op-amp) is a high-gain, direct-coupled voltage amplifier with two inputs — an inverting input (marked −) and a non-inverting input (marked +) — and a single output. It amplifies the voltage difference between its two inputs. The open-loop gain is deliberately enormous, typically 100,000 or more, which on its own would be impractical to use directly. The op-amp becomes useful when it is wrapped in negative feedback: a fraction of the output is fed back to the inverting input, so the circuit's behaviour is set by a few external resistors rather than by the device's own large and poorly controlled gain.

Two simplifying rules describe an ideal op-amp working with negative feedback: essentially no current flows into either input (very high input impedance), and the feedback drives the two inputs to almost the same voltage. From these rules follow the classic building blocks: the inverting and non-inverting amplifiers, the unity-gain buffer (voltage follower), the summing amplifier, the difference and instrumentation amplifiers, and the integrator and differentiator. Without feedback the same device acts as a comparator, switching its output fully high or low according to which input is larger.

Real op-amps depart from the ideal in ways that matter in design. Key specifications include the gain-bandwidth product, slew rate (how fast the output can change), input offset voltage, input bias current, common-mode rejection ratio, and the limited output voltage swing. The name itself dates from 1940s analog computers, where op-amps performed mathematical operations such as addition and integration; the monolithic 741, introduced in 1968, made the integrated op-amp inexpensive and ubiquitous. Today op-amps appear throughout signal conditioning, active filters, audio, sensor interfaces, instrumentation and analog control.

Frequently asked questions

Why are op-amps almost always used with negative feedback?

The raw open-loop gain is huge and varies between devices and with temperature. Negative feedback trades that excess gain for a precise, stable gain set by external components, plus wider bandwidth and lower distortion.

What is a “virtual ground”?

In an inverting amplifier the non-inverting input is tied to ground, and feedback forces the inverting input to nearly the same potential. That node sits at about 0 V even though it is not wired directly to ground, so it is called a virtual ground.

What limits how fast an op-amp can respond?

Mainly the slew rate, which caps how quickly the output voltage can change, and the gain-bandwidth product, which sets the usable bandwidth at a given closed-loop gain.