Op-amp (Operational Amplifier)

Op-amp is an abbreviation of the English term "Operational Amplifier" and is referred to in Turkish as "İşlemsel Kuvvetlendirici". Op-amps are widely used in electronic circuits and amplify small input signals to produce larger output signals through their high input impedance and low output impedance characteristics. Although originally developed for analog computer applications, they now play a significant role in signal processing, filtering, oscillator design, and various control systems. Op-amps are analyzed considering both ideal and real-world properties.

Structure and Basic Characteristics of Op-amp

An op-amp is typically an electronic component with differential inputs and a single-ended output. It has two input terminals: the inverting input (–) and the non-inverting input (+). The output is determined by the voltage difference between these two inputs. An ideal op-amp is assumed to have infinite gain, infinite input impedance, and zero output impedance.

Basic Structure

An op-amp circuit contains a multilayer structure composed of transistors, resistors, and capacitors. Generally, the following stages are present:

• Input Stage: Functions as a differential amplifier.
• Gain Stage: Amplifies the input voltage difference.
• Output Stage: Provides the current necessary to drive the load.

In the ideal op-amp model, the following assumptions are made:

• The input current is zero.
• The input voltage difference is zero (under negative feedback).
• The gain is infinite.

In real op-amps, these values are limited, but ideal assumptions provide sufficient accuracy for many practical applications.

Applications of Op-amp Circuits

Operational amplifiers are used in a variety of applications with different circuit topologies. These applications are typically based on fundamental circuit configurations.

Inverting Amplifier

The inverting amplifier circuit produces an output signal with a phase shift of 180 degrees relative to the input signal. The gain value depends on the ratio of the feedback resistor to the input resistor.

Gain formula:

Av = – (Rf / Rin)

Here, Rf is the feedback resistor and Rin is the input resistor.

Non-Inverting Amplifier

In the non-inverting amplifier circuit, the input signal is applied directly to the non-inverting input. In this configuration, the signal phase is preserved.

Gain formula:

Av = 1 + (Rf / Rin)

This configuration is suitable for applications requiring high input impedance.

Voltage Follower (Buffer)

The voltage follower circuit is used to transmit a signal without amplification. Its gain is 1. Due to its very high input impedance and very low output impedance, it provides circuit isolation.

Summing Amplifier

A summing amplifier circuit is used to combine multiple input signals. It is commonly used in audio engineering and data acquisition systems.

Output formula:

Vout = – ( (V1 * Rf / R1) + (V2 * Rf / R2) + ... + (Vn * Rf / Rn) )

Differential Amplifier

A differential amplifier is used to amplify the difference between two input signals. It is effective in noise rejection and signal processing applications.

Op-amp Characteristics

Several important characteristics must be considered when selecting and applying op-amps:

Input Bias Current

This is a small current component flowing through the op-amp input terminals. It is zero in an ideal op-amp but has a finite value in real circuits.

Offset Voltage

This is the output voltage that exists when the input voltage difference is zero. It is ideally zero.

Gain Bandwidth Product

The product of gain and bandwidth remains constant. It becomes critical in high-frequency applications.

Slew Rate

This is the rate at which the op-amp’s output voltage changes per unit time. Its unit is V/μs. It is a key parameter for high-speed signal processing.

Important Considerations in Applications

When designing op-amp circuits, the following points must be observed:

• Negative feedback ensures stable and linear operation.
• Positive feedback is used in oscillator circuits.
• The supply voltage must be chosen within an adequate range to keep the op-amp operating within its full range.
• An appropriate op-amp model must be selected based on signal levels and frequency requirements.

Types of Op-amps

Different types of op-amps exist based on their intended use and performance criteria.

General Purpose Op-amps

Designed for standard applications. Models such as the LM741 belong to this category.

High-Speed Op-amps

Used in applications with very high signal frequencies. Examples include the AD8000.

Low Power Consumption Op-amps

Preferred in battery-powered devices. Models such as the MCP602 provide low power consumption.