The Tried-and-True Operational Amplifier (Op Amp): An Overview

Are People Still Using Operational Amplifiers?

Working the floor of a trade show years ago, I was asked, “Are people still using op amps?” I couldn’t help but smile, as the operational amplifier market is estimated to be over three billion dollars and growing. But to the questioner’s point, the tried-and-true op amp has been around for a long time, and although it has experienced improvements due to process technology, enhanced architectures and design techniques, it is fundamentally the same. Given the ubiquitous nature of the op amp, let us take a step back and review the elements of an operational amplifier and why it is still so useful in today’s electronics.

An operational amplifier is an electronic device that utilizes two inputs—an inverting input and a non-inverting input—an output and a dual power supply, as shown in the schematic symbol below.

The op amp amplifies the difference between the two input signals to produce an output signal. An ideal operational amplifier is a theoretical device that has the following characteristics:

• Infinite open-loop gain: The output voltage will be equal to the difference between the input voltages multiplied by infinity. This characteristic allows the op amp to amplify even the smallest input signals.

• Infinite input impedance: It draws no current from the input source. This allows the op amp to be used with high-impedance sources, such as sensors and transducers, without affecting the input signal.

• Zero output impedance: It can drive any load without affecting the output voltage. This characteristic makes the op amp very useful for driving low-impedance loads, such as speakers.

• Infinite bandwidth: It can amplify signals of any frequency. This characteristic allows the op amp to be used in applications that require high-frequency amplification, such as RF amplifiers.

• Zero input offset voltage: The output voltage will be zero when the input voltages are equal. This characteristic allows the op amp to be used in applications that require precise signal processing.

• Infinite slew rate: It can respond to changes in the input signal instantly. This characteristic allows the op amp to be used in applications that require fast signal processing, such as in high-speed data transmission.

While no op amp can meet all these ideal characteristics in the real world, modern op amps come close to meeting many of them. Designers choose the op amp that best fits the requirements of their specific application, considering the real-world limitations of the device.

As noted earlier, op amps continue to be used in a wide range of electronics. Some of the most common applications for op amps are as follows:

• Amplifiers: Op amps can be used as voltage amplifiers to increase the amplitude of an input signal. The gain of the amplifier is determined by the ratio of the feedback and input resistors.

• Filters: Op amps can be used to build a variety of filters, including high-pass, low-pass, band-pass and band-stop filters. These filters are used to remove unwanted frequencies from a signal.

• Comparators: Op amps can be used as comparators to compare two voltages and produce an output signal that indicates which voltage is higher. This is useful in applications such as level detection, threshold detection and pulse-width modulation.

• Oscillators: Op amps can be used to build oscillators that produce a sinusoidal waveform at a specific frequency. There are various types of op amp oscillators, such as Wien bridge, phase-shift and crystal oscillators.

• Voltage regulators: Op amps can be used to build voltage regulators that maintain a constant output voltage regardless of changes in input voltage or load. Voltage regulators are used in a variety of electronic devices, such as power supplies, audio amplifiers and motor controllers.

• Instrumentation amplifiers: Op amps can be used to build instrumentation amplifiers that amplify very small signals, such as those from sensors or transducers. Instrumentation amplifiers have high input impedance, low output impedance and a high common-mode rejection ratio.

• Integrators and differentiators: Op amps can be used as integrators and differentiators to perform mathematical operations on signals. Integrators produce an output signal that is proportional to the integral of the input signal, while differentiators produce an output signal that is proportional to the derivative of the input signal.

Overall, op amps are an essential component in many electronic circuits, and their versatility and ease of use continue to make them a popular choice for designers and engineers.