Characteristics of an Ideal Operational Amplifier

Posted By: Jahid | Posted On: 28/Sep/2022 | Posted In: EEE | No Comment

An operational amplifier as an amplifier is a typical linear electronic device. Actually, it is an electronic unit that behaves like a voltage-controlled voltage source. It is an electronic component widely used for amplifying electronic signals as well as for filtering, conditioning, and mathematical processes such as addition, subtraction, integration, differentiation, etc. In simple words, it is an active circuit element that is designed to perform mathematical operations of addition, subtraction, multiplication, division, differentiation, and integration. In this post, we have discussed the characteristics of an ideal operational amplifier, its type, and application, and also discuss some formulas of Op-Amp.

An operational amplifier can amplify the input signal hundreds of thousands of times. Its application-wise use is easy as operational amplifier characteristics such as gain, input impedance, output impedance, bandwidth, etc. can be determined through a feedback network.

Table of Contents

What are the Applications of Operational Amplifiers?

In modern electronics instrumentation, Op-amp is a fundamental building block. It is widely used in many devices including resistors and other passive components. It has a lot of practical applications including instrumentation amplifiers, analog computers, digital-to-analog converters, calibration circuits, level shifters, filters, inverters, summers, integrators, differentiators, subtractors, logarithmic amplifiers, comparators, gyrators, rectifiers, oscillators, regulators, current-to-voltage converters, voltage-to-current converters, and clippers.

Additionally, in the practical world, Op-Amp has applications in various electronic devices ranging from general consumer devices to industrial and research-oriented ones. Its use in analog circuit blocks is popular because of its many practical applications. As some of the widely used operational amplifier ICs are available in the market at a cheap cost, there are expensive ICs in the market for performing special functions.

What is an Ideal Op Amp?

The ideal operational amplifier is an extension of the concept of an ideal amplifier. It has infinite open-loop gain, infinite input resistance, zero output resistance, and a fixed gain at all frequencies. In simple words, An ideal operational amplifier(op-amp) has infinite input resistance and zero output resistance but has infinite gain.

To better understand, look at the below points:

Infinite open-loop gain, (A ≃ ∞).
Infinite input resistance, (Ri ≃ ∞)
Zero output resistance, (Ro = 0).

The General Structure of Operational Amplifier:

Operational amplifiers are available in the market as integrated circuits (IC). The operational amplifier (Op-Amp) has two input pins for differential input. One of the two pins is called the inverting input(PIN 2) and the other is the non-inverting input (PIN 3). These are denoted by V+ and V- respectively (abbreviated using just + and – symbols).

It has two pins for the power supply, as PIN 7 is used to supply positive power, and PIN 4 is used to supply negative power.

But in some cases, there may be two separate outputs. Besides, there may be two more pins called offset to eliminate manufacturing errors. Below is the circuit symbol of the operational amplifier.
Figure- Operational amplifier

What are the Important Terminals of Op-Amp?

There are five important terminals, such as:

The inverting input, pin 2.
The non-inverting input, pin 3.
The output, pin 6.
The positive power supply Vcc(+), pin 7.
The negative power supply Vcc(-), pin 4.

Characteristics of an Ideal Operational Amplifier:

Open loop gain is infinite.
Input impedance is infinite.
No offset voltage is required.
The output impedance range is infinite.
Bandwidth is infinite.
The common mode rejection ratio is infinite.
The power supply rejection ratio is infinite.

Functioning of Operational Amplifier:

The operational amplifier depends on its inputs. If there is any difference between the two inputs, it will amplify it and the amplified signal is available at the output.

Types of Operational Amplifiers

Op-amp can be divided into the following categories:

The Discrete Type.
IC Type
Hybrid.

The Discrete Type is usually made up of discrete transistors or vacuum tubes. At present, IC type operational amplifier is the most widely used. Based on usage, IC-type operational amplifiers can again be divided into various segments, such as dual inline package, cage package, flat pack, etc.

Based on Input/Output Voltage Range:

Based on input/output voltage range Op-amp can be divided into three main types, such as:

Dual Supply,
Single Supply, and
Rail-to-Rail.

Usage-based Circuit.

Depending on the use of Op-Amp is divided into two types, such as:

Open Loop Amplifier.
Closed Loop Amplifier.

1. Open Loop Amplifier.

In this case, between the input and output of the operational amplifier, no feedback network is connected. Hence it is called an open loop amplifier. The open loop gain of an operational amplifier is very high (typically a hundred thousand or more). So a small voltage difference between the two differential inputs causes the output to become saturated (excited resulting in a maximum voltage at the output).

As a result linear amplification becomes difficult in open loop amplifiers. However, open-loop operational amplifiers can be used in various comparator circuits. Such as-

Voltage Comparator:

A circuit diagram using the operational amplifier as a voltage comparator is shown in the above figure. In this case, if the input voltage is slightly higher than the reference voltage, the output voltage is equal to the supply voltage. Here the reference voltage is provided from the voltage divider created through resistors R.

2. Closed Loop Amplifier.

A closed loop amplifier is a portion of the operational amplifier’s output that is applied to the input with the help of a specific feedback network.

In the image above, we are seeing an example of a closed-loop amplifier. Here R1 is the reference resistor and R2 is the feedback resistor. Essentially, the feedback R2 directly connects the input to the output and makes this circuit a closed-loop network. In closed-loop gain, the ratio of input voltage to output voltage is called closed-loop gain.

.And in this kind of management, the operation of the operational amplifier depends entirely on the feedback network. So the use of op-amp is maximum in this management.

a. Positive Feedback:

In a positive feedback amplifier, a fraction of the output is fed to the non-inverting input of the operational amplifier with the help of a feedback network. Generally, positive feedback is implemented in various types of oscillator circuits.

An important use of positive feedback is in Schmitt’s trigger circuits. The following figure shows the Schmitt trigger circuit diagram. There are also some circuits where positive and negative feedback is applied simultaneously, such as Triangular Wave Oscillators, Active filters, etc.

b. Negative Feedback:

In the case of Negative Feedback, a part of the output is fed to the inverting input of the operational amplifier with the help of a feedback network. Negative feedback operational amplifiers are again of two types. namely-

Non-Inverting Amplifier.
Inverting Amplifier.

Mathematical Operations:

Based on Mathematical operations Op-amp has the following types of uses, such as-

Summing Amplifier,
Difference Amplifier (Subtraction),
Multiplication Amplifier,
Division Amplifier,
Differentiation, and
Integration.

Golden Rule of Operational Amplifiers:

Two assumptions are considered to be the golden rule of an operational amplifier to easily explain its operation. And two Golden rules are mentioned below.

1. Since the input impedance of the operational amplifier is infinite, its input does not draw any current.
2. The voltage difference between the inverting and non-inverting inputs is zero.

Common Mode Rejection Ratio.

This is a special feature of operational amplifiers. Due to this characteristic, when the same voltage/signal is applied to both inputs of the operational amplifier, it shows no signal at the output. That is if both inputs have the same voltage. The operational amplifier is bypassed and no signal amplification is obtained at the output. This common mode rejection ratio is much higher in practical operational amplifiers.

Bottom line:

In 1947, John Ragazzini introduced the word operational amplifier. The first operational amplifiers used vacuum tubes rather than transistors. Since that time it is an electronic component widely used for amplifying electronic signals. Mostly it is used for filtering, conditioning, and mathematical processes such as addition, subtraction, integration, differentiation, etc. And research on op-amp is still contuinig. So, research more to know more. We are happy if you like any of our posts. Then, Hope you stay with us.