Understanding Operational Amplifiers: Structure, Function, and Applications

Operational amplifier is an electronic integrated circuit designed to amplify the voltage difference between two inputs. These components are widely used in audio equipment, measuring instruments, sensors, and automatic control systems. They enable amplification of weak signals, creation of filters, and execution of analog computations.

Let's explore the structure of an operational amplifier, how it works, typical circuit configurations, and what to consider when choosing an IC.

What Is an Operational Amplifier and How Is It Built?

An operational amplifier (op-amp) is a direct current amplifier with a very high gain and two signal inputs. Its key feature is that it responds to the voltage difference between the inputs, not to the absolute value of each one.

The name comes from the original use of these devices in analog computing machines, where they performed mathematical operations such as addition, subtraction, and integration of signals. Today, operational amplifiers are used much more broadly.

A typical operational amplifier has several main pins:

• Non-inverting input (+) - increasing the voltage here tends to raise the output voltage.
• Inverting input (−) - increasing the voltage here tends to lower the output voltage.
• Output - delivers the amplified or processed signal.
• Power pins - supply power to the internal electronic components.

Inside the chip, transistors are arranged in several stages. The input differential stage compares the voltages, subsequent stages provide amplification, and the output stage delivers the signal to the connected load.

Modern operational amplifiers often use field-effect transistors, especially in the input stages of models with high input impedance. Their structure and features are discussed in detail in the article How Field-Effect Transistors Work: Simple Explanation of MOSFETs, Structure, and Applications.

The most important specification of an op-amp is its open-loop gain. For some ICs, this value can reach hundreds of thousands or even millions. However, such high gain is usually not needed in practice; it is reduced and controlled using external components.

Other important parameters include input impedance, bandwidth, and maximum slew rate of the output voltage. These characteristics determine how accurately the amplifier can process signals of various amplitudes and frequencies.

How an Operational Amplifier Works

The operating principle of an operational amplifier is based on comparing the voltages at its two inputs. If the voltage at the non-inverting input is higher than at the inverting input, the output voltage increases. If the situation is reversed, the output decreases.

This relationship can be expressed with the formula:

U_out = A₀ × (U₊ − U₋)

where A₀ is the open-loop gain, and U₊ and U₋ are the voltages at the respective inputs.

For example, with a gain of 100,000, a difference of just 0.1 mV between the inputs can theoretically produce a 10 V output. However, the real output is limited by the amplifier's power supply and the capabilities of its output stage.

Why Negative Feedback Is Needed

Due to the extremely high open-loop gain, an operational amplifier without feedback can easily reach its maximum output voltage even with a small difference between the inputs.

To control the gain, part of the output signal is fed back to the inverting input. This mechanism is called negative feedback. It allows you to:

• Set the required gain using resistors.
• Reduce the impact of IC parameter variations.
• Minimize nonlinear signal distortion.
• Ensure more predictable circuit operation.

With sufficiently high gain and properly configured negative feedback, the voltages at the inputs become nearly equal. This property is often used in op-amp circuit calculations.

Note that input voltage equality is an approximation and holds only when the amplifier operates in linear mode. If the output hits voltage limits or the input signal frequency is too high, the circuit's behavior may change.

Main Operational Amplifier Circuit Configurations

Depending on how resistors are connected and where the input signal is applied, an operational amplifier can amplify voltage, change its polarity, or buffer the input signal. The three most common configurations are: inverting, non-inverting, and voltage follower.

Inverting Amplifier

In the inverting configuration, the input signal is applied to the inverting input through a resistor. The non-inverting input is connected to ground, and a feedback resistor links the output to the inverting input.

The main feature is that the output signal has the opposite polarity to the input. For instance, with a +0.5 V input and a gain of −10, the output will be −5 V, provided the power supply allows it.

The gain is calculated as:

K = −R₂ / R₁

where R₁ is the input resistor and R₂ is the feedback resistor.

This configuration is used in audio equipment, analog filters, and signal processing devices.

Non-Inverting Amplifier

In this configuration, the signal is applied directly to the non-inverting input. The inverting input is connected to the output via a feedback resistor and to ground through a second resistor.

Unlike the inverting option, the output signal maintains the same polarity as the input. For example, with a +0.2 V input and a gain of 5, the output will be approximately +1 V.

The gain formula is:

K = 1 + R₂ / R₁

The non-inverting configuration offers high input impedance, meaning the amplifier hardly loads the signal source. This is especially important when connecting sensors.

Voltage Follower (Buffer)

The voltage follower is a special case of the non-inverting amplifier with a gain of one. Its output is directly connected to the inverting input, and the signal is applied to the non-inverting input.

As a result, the output voltage nearly matches the input:

U_out ≈ U_in

At first glance, this configuration may seem pointless since the voltage does not increase. However, the follower serves a different purpose - circuit matching.

It has high input and low output impedance, allowing a weak signal source to drive a load without significant voltage drop. For example, a follower can be used between a sensor and the input of a measuring device.

All three configurations assume linear-mode operation with negative feedback. Actual output voltages are limited by the IC's specifications and power supply.

Where Are Operational Amplifiers Used?

Operational amplifiers are used in electronic devices where it is necessary to amplify, measure, or convert analog signals. Thanks to their versatility, a single IC can perform different functions depending on the circuit.

Key application areas include:

• Audio equipment: Amplifying weak microphone signals, preliminary sound processing, and tone control.
• Measuring instruments: Amplifying small voltages from temperature, pressure, and other physical sensors.
• Active filters: Suppressing interference and selecting certain frequencies without bulky inductors.
• Control systems: Handling feedback signals, comparing measured values, and generating control voltages.
• Analog computation: Addition, subtraction, and integration of electrical signals.

Another use is as a comparator. In this mode, the device compares two voltages and switches the output depending on which is higher.

For example, such a circuit can detect when a battery voltage drops below a set threshold. However, general-purpose operational amplifiers are not always suitable for fast switching. For these tasks, specialized comparators with appropriate specifications are preferred.

How to Select and Connect an Operational Amplifier

When choosing an operational amplifier, it's important to consider not only the gain but also the operating conditions. Using the wrong IC can distort the signal, limit the output voltage, or even fail to work in your circuit.

Key parameters to pay attention to:

• Supply voltage: Determines the IC's allowed operating conditions. Some op-amps support single-supply or dual-supply voltages.
• Input voltage range: Indicates which signals the amplifier can process correctly.
• Bandwidth: Defines the frequencies at which the amplifier maintains the required gain.
• Slew rate: Affects the ability to reproduce fast signal changes without distortion.
• Input offset voltage: Reflects amplifier error, especially important when working with weak signals.

With single-supply operation (e.g., 0 and +5 V), a standard amplifier cannot generate a negative voltage relative to ground. To process AC signals, a bias point is often used.

With dual-supply operation (e.g., −12 and +12 V), the amplifier can handle signals of both polarities relative to zero, but the output voltage may not reach the supply rails.

A popular example is the LM358 - a dual op-amp IC. It supports single-supply operation and is suitable for simple sensor signal processing circuits, but its input range does not reach the positive supply, and the output has swing and load limitations.

When connecting an op-amp, check pinout in the datasheet, observe the recommended supply voltage, and place a decoupling capacitor near the IC. For your specific circuit, also ensure the input and output voltages remain within the allowed ranges.

Conclusion

Operational amplifiers are versatile analog electronics components that allow amplification of weak signals, creation of filters, and processing of voltages with defined characteristics. Their operation is based on amplifying the voltage difference between two inputs, with negative feedback providing controlled gain.

Use a non-inverting configuration for amplifying signals without changing polarity, an inverting configuration to change polarity, and a voltage follower for circuit matching. When selecting an IC, consider the supply, input voltage range, and frequency characteristics.