the operational amplifier

The operational amplifier is the Lego brick of analog electronics. Picture a little chip that watches two input terminals and reacts to even the tiniest difference between them. Whatever voltage difference appears between its two inputs, it multiplies by a huge number and presents at its single output. On its own this is almost uselessly twitchy, but wrap a couple of resistors around it and it becomes the most flexible building block in the whole field.

Precisely, it is a high-gain differential amplifier with two inputs (an inverting input marked minus and a non-inverting input marked plus) and one output, powered from two supply rails. Its output equals A times (V+ minus V-), where A is the open-loop gain, typically 100,000 to over 1,000,000. Because A is so enormous, even a 0.1 mV difference between the inputs would demand 10 V out at A of 100,000. So on its own the output simply slams against a supply rail. The genius is to feed part of the output back to the inverting input (negative feedback), which tames that wild gain into a precise, controllable one.

The name comes from analog computers, where these amplifiers performed mathematical operations: add, subtract, integrate, differentiate. Today a usable op-amp costs pennies in an 8-pin package, and with two resistors you set an exact gain. Almost every analog signal, from a microphone to a sensor to an audio output, passes through one. Master the op-amp and you can build real circuits.