The modern operational amplifier dates way back to the early days of electronics history. Specifically, the op amp was first created in the 1930s, when Bell Telephone was just starting to run telephone cables throughout the country.

In the early days of the telephone, engineers had a difficult problem with phone lines that ran more than a few thousand feet. Long phone lines needed amplifiers to give their signals a boost, but the amplifiers available at the time were very fidgety — too sensitive to weather (temperature and humidity) and unable to work consistently over the range of voltages used in early phone lines.

A Bell engineer named Harry Black was working on the amplifier problem in 1934 when an idea struck him as he was riding the ferry home from work. This idea was a stroke of genius that seems obvious decades later.

Instead of trying to design an amplifier that would have the exact amplitude gain needed for the job, Black’s idea was to use an amplifier that had far more gain that was needed — thousands of times more gain, in fact — and then feed some of the output back into the input through a resistor. This feedback circuit would reduce the overall gain of the amplifier based on the amount of resistance in the circuit.

The circuit didn’t gain the name operational amplifier until the computer age began a decade or so later, and computer researchers figured out how to use the amplifier’s unique characteristics to perform basic mathematical operations like addition, subtraction, multiplication, and division on the input voltages.

Eventually, digital computers replaced the analog computers built from op amps. Op amps still play an important role in computers today, however, primarily to provide an interface with real-world input measuring devices such as voltage sensors and moisture detectors.

The original op-amp circuits were built with vacuum tubes. They were large, required several hundred volts to operate, and generated substantial heat. When transistors replaced vacuum tubes in the 1950s, op amps became smaller, and when integrated circuits were invented in the 1960s, op amps were among the first chips to be designed.