What is an operational amplifier?

The benefit of operational amplifiers is that they can amplify weak electrical signals. First, let's take a look at the basic configuration and main functions of operational amplifiers.

Basic configuration of an operational amplifier

Main functions of an operational amplifier

An operational amplifier has several functions, but its fundamental one is to amplify the voltage difference of weak electrical signals input to its two input terminals. This property is called the amplification function, and the amplification level is expressed as “gain” or “amplification factor.”

Physical phenomena such as light, heat, and sound are analog signals. We use sensors and other devices to capture these physical phenomena and convert them into electrical or digital signals that we use in our daily lives and businesses.
For example, converting the temperature measured by a sensor into a numerical value for easier handling on a computer or other device is also a form of converting a physical phenomenon into a digital signal.

However, the electrical signals converted by the sensor are very weak, so they need to be amplified before a computer can use. This is where an operational amplifier comes in, amplifying the signal and making it easier to handle. This is the amplification function of an operational amplifier.
In addition, operational amplifiers also have filtering functions that enable noise processing and extraction processing. The filtering function of operational amplifiers removes electrical signals other than the electrical signals you want to process, or so-called noise, and extracts only signals in specific frequency bands.

Furthermore, operational amplifiers can also be used to create various arithmetic circuits (addition, subtraction, differentiation, integration). Specifically, these include non-inverting amplification circuits, inverting amplification circuits, buffer circuits, comparator circuits, and oscillator circuits.

Classification by semiconductor device (transistor)

The semiconductor devices (transistors) that constitute operational amplifiers can be broadly divided into two types: bipolar transistors and unipolar transistors. They are called and classified as “bipolar operational amplifiers” and “unipolar operational amplifiers,” respectively.

The main characteristic of bipolar operational amplifiers using bipolar transistors is that they have high voltage resistance. On the other hand, they are also characterized by the fact that they tend to generate input bias current, which is the cause of input errors, and offset voltage, which causes malfunctions such as outputting voltage even when no input signal is input.

In contrast to bipolar operational amplifiers, unipolar operational amplifiers, which use unipolar transistors, have the features of low input bias current and low power consumption. They also have a low breakdown voltage characteristic compared to bipolar operational amplifiers.
Unipolar operational amplifiers include types such as CMOS operational amplifiers and JFET operational amplifiers. CMOS operational amplifiers use CMOS, which is a combination of P-type and N-type transistors, and are characterized by their low power consumption. JFET operational amplifiers use JFET, short for junction field effect transistor, and are capable of high-speed switching.

Classification by power supply method

Operational amplifiers can be classified by the type of power supply they use: single power or dual power. In the case of dual power, the ground (GND), a reference voltage, is the center of the positive and negative voltages, but in the case of single power, it is either the positive or negative voltage.

Classification by input circuit

The categories in classifying operational amplifiers by input circuit include “P-channel MOSFET input differential pair,” “N-channel MOSFET input differential pair,” and “full-range input differential pair.”
An input differential pair refers to amplifying the difference between two input signals. P-channel MOSFET input differential pairs are suitable for circuits that handle high voltages, while N-channel MOSFET input differential pairs are suitable for low voltages. Full-range input differential pairs are made up of a combination of P-channel MOSFETs and N-channel MOSFETs, and can operate over a wide range of voltages. A typical example of full-range input differential pairs is the rail-to-rail input differential pair, which can cover almost the entire input range from the power supply to ground (GND).

Classification by electrical characteristics

Operational amplifiers are also sometimes classified by their electrical characteristics. Among these, low offset voltage, low power consumption, and low noise are important metrics when selecting an operational amplifier, as we will discuss later.

3. Main applications of operational amplifiers

Operational amplifiers, whose functions include signal amplification and filtering, are essential to many electronic devices used in our daily lives and businesses. This section introduces some typical applications.

Mobile devices and household goods

Operational amplifiers are used in the light sensors and pressure sensors of smartphones and tablets. In addition, operational amplifiers are essential to many of the electronic devices around us, such as those that detect the flames of gas stoves and hot-water units.

Industrial equipment

Operational amplifiers are also used in industrial equipment. High-precision measuring instruments are an example.

Automobiles

In the engine compartment of automobiles, operational amplifiers are essential for sensors that measure current, acceleration, temperature, and others. Operational amplifiers are also used in peripheral devices such as airbags, car navigation systems, and immobilizers.

Medical equipment

Operational amplifiers are used in electrocardiogram monitors, blood pressure monitors, pulse oximeters, and other medical devices. For example, in electrocardiogram monitors, the role of the operational amplifier is to amplify and remove noise from the weak electrical signals produced by the beating heart.

Audio system

Operational amplifiers are used in the analog circuits of audio equipment, such as preamplifiers that amplify signals and equalizers that make use of filtering functions. In audio systems, the performance of operational amplifiers often affects sound quality, so it is not an exaggeration to say that operational amplifiers determine the performance of the audio system.

Requirements for the electronic devices and applications to be used

The first step is to choose an operational amplifier that meets the requirements of the electronic device or application you are using.
Specifically, these requirements include a low offset voltage, low change in characteristics due to temperature changes, and whether it can run at low voltages.

Operating power supply voltage range

The operating power supply voltage range is listed in the data sheet for the operational amplifier. If the power supply voltage you intend to use is not within this range, it may not run properly and you must check it.

Input voltage range

The input voltage range is the range of voltages that can be applied to the operational amplifier. As long as the voltage you intend to apply to the operational amplifier's input terminal is within the range of values shown in the input voltage range, there will be no problem.

Input offset voltage

An input offset voltage refers to the small voltage difference between two input terminals. If you want to use a high-precision application, employing an operational amplifier with a low input offset voltage is the better option.

Gain Bandwidth Product (GBW)

The Gain Bandwidth Product (GBW) is a metric of signal amplification performance that indicates the upper limit of the frequency at which an operational amplifier or the like can work. To ensure signal amplification performance in the required frequency range, it is a good idea to select an operational amplifier with sufficient GBW.

Temperature range, temperature drift

You must also check the temperature of the environment in which the operational amplifier will be used. Of course, it is important that the temperature of the operating environment is within the temperature range specified in the data sheet, but another important point is that the operational amplifier should have low temperature drift, or a low rate of accuracy degradation due to temperature changes.
There are also products on the market called zero-drift amplifiers, which have almost zero temperature drift.

For more information on operational amplifiers, talk to MinebeaMitsumi to access our wealth of products and knowledge

Operational amplifiers are a technology that supports the normal operation of various electronic devices, and recent years have seen high expectations for their use in areas such as smart factories, smart homes, and wearable devices.

MinebeaMitsumi and our group company ABLIC offer a wide range of operational amplifiers our lineups. If you have any questions or concerns about operational amplifiers, feel free to contact us at any time.