opamps

An operational amplifier (op-amp) is a high-gain differential voltage amplifier with very high input impedance and low output impedance. It has two inputs, the non-inverting (+) and inverting (−), and a single-ended output. In typical use, external feedback and bias networks set the signal gain and determine the overall behavior.

Internally, most op-amps include a differential input stage, one or more gain stages, and an output stage. The vast open-loop gain makes the device highly sensitive to the difference between inputs. When negative feedback enforces a small input difference, the closed-loop gain becomes predictable and stable, allowing the op-amp to implement precise analog functions.

Key specifications describe real devices: open-loop gain, gain-bandwidth product, slew rate, input impedance, output impedance, input bias current, and input offset voltage. Real op-amps also have limits such as finite input common-mode range and limited output swing relative to supply rails. Other important metrics include common-mode rejection ratio (CMRR) and power supply rejection ratio (PSRR).

Common configurations include the inverting amplifier, non-inverting amplifier, and voltage follower (buffer). Other configurations realize summing, differential, integrator, and differentiator circuits. In ideal theory, an op-amp has infinite gain, infinite input impedance, zero output impedance, and infinite bandwidth; in practice, these are approximated with varying degrees of fidelity.

Op-amps are central to analog electronics, used in instrumentation, audio, control systems, sensor conditioning, and active filtering. They are available in many families (BJT, FET, CMOS) and packages, and range from precision, low-noise devices to high-speed, high-power variants. The 1960s saw first widely used IC op-amps such as the μA741, which helped standardize op-amp usage.