Operational Amplifier History

A Brief History of the Operational Amplifier (Op Amp)

The operational amplifier, or op amp, is a cornerstone of analog electronics, used in everything from signal conditioning to active filters and analog computation. Its development spans decades, with each milestone reflecting advances in technology and circuit design.

1941: The First Vacuum Tube Op Amp

The earliest op amp design appears in U.S. patent 2,401,779, filed by Karl D. Swartzel Jr. at Bell Labs. This circuit used three vacuum tubes to achieve a high gain of 90 dB, running on ±350 V rails. Unlike modern op amps, it featured only a single inverting input. Swartzel’s op amp was instrumental during World War II, notably in the M9 artillery director, which, when paired with the SCR-584 radar, dramatically improved targeting accuracy.

1947: Introduction of the Differential Input

John R. Ragazzini of Columbia University formally defined and named the operational amplifier in a seminal paper. A significant innovation came from Loebe Julie, who designed an op amp with both inverting and non-inverting inputs—a differential input stage using a long-tailed pair of triodes. This allowed for more versatile signal processing, though widespread use of differential inputs was delayed by the popularity of chopper-stabilized amplifiers.

1949: Chopper-Stabilized Op Amps

Edwin A. Goldberg developed a chopper-stabilized op amp, which used an AC amplifier and a mechanical or electronic chopper to convert DC signals to AC, amplify them, and then convert them back to DC. This technique greatly reduced output drift and DC offset, making these op amps the standard for precision applications, despite limiting the use of the non-inverting input.

1953: Commercial Availability

Op amps became commercially available with the K2-W model from George A. Philbrick Researches, Inc. This vacuum tube-based device, derived from Julie’s design, marked the beginning of widespread industrial use. The K2-W and its successors were pivotal in bringing op amps into mainstream electronics.  

1961: Discrete Transistor Op Amps

The invention of the transistor (1947) and the silicon transistor (1954) paved the way for solid-state op amps. By 1961, discrete transistor op amps like the P45 were available, offering high gain (94 dB) and operating on ±15 V rails. These were built on small circuit boards with hand-selected components to optimize performance.

1961: Varactor Bridge Op Amps

Varactor bridge op amps emerged, designed for extremely low input currents and exceptional common-mode rejection. These specialized designs could handle high input voltages and remain among the best for certain precision applications.

1962: Modular Potted Op Amps

Manufacturers began offering op amps in potted modules—sealed packages that could be plugged directly into PCBs. This innovation made op amps easy to use as standardized components, simplifying circuit design and assembly.

1963: The First Monolithic IC Op Amp

Bob Widlar at Fairchild Semiconductor introduced the μA702, the first monolithic Integrated Circuit (IC) op amp. Unlike discrete or hybrid designs, monolithic ICs are fabricated on a single silicon chip. Early monolithic op amps had limitations (uneven supply voltage, low gain, small dynamic range), but the release of the improved μA709 in 1965 set the stage for widespread adoption.

1968: The Iconic μA741

The LM101 (1967) and μA741 (1968) further refined monolithic op amp design. The μA741, with its internal compensation capacitor, became the industry standard. Its pinout and architecture are still used in countless modern op amps, and the 741 remains in production today.  

1970s: FET and MOSFET Op Amps

The 1970s saw the introduction of op amps using Field-Effect Transistors (FETs), which offered high input impedance and low input current. These were soon followed by MOSFET-based designs in the 1980s, further improving speed and input characteristics.

1972: Single-Supply and Quad Op Amps

Op amps like the LM324, released in 1972, could operate from a single supply voltage, simplifying power requirements. The LM324 also packaged four op amps in one IC, becoming an industry staple. Hybrid op amps—combining monolithic ICs with discrete components—were used for specialized, long-life applications but became less common as monolithic designs improved.

Recent Trends: Low-Voltage and Rail-to-Rail Op Amps

Modern op amps reflect the trend toward lower supply voltages (5V, 3.3V, or even 1.8V), matching advances in digital logic. Rail-to-rail input and output stages maximize signal swing, making op amps more versatile in low-voltage systems.

Summary

From bulky vacuum tube circuits to compact, high-performance ICs, the op amp has evolved to meet the needs of engineers and hobbyists alike. Today’s op amps are ubiquitous, reliable, and available in countless variants for every imaginable application.