Introduction to Operational Amplifier: Characteristics of Ideal Op-Amp

Summary

This video from the 'All About Electronics' YouTube channel introduces the operational amplifier (op-amp), a high-gain differential amplifier used for amplifying input signals and performing mathematical operations like addition, subtraction, integration, and differentiation. The script explains the op-amp's circuit symbol, input terminals, and the concept of open-loop gain. It also covers the saturation behavior of the op-amp and its applications in various electronic circuits, highlighting the importance of choosing the right op-amp based on specific characteristics like slew rate and common mode rejection ratio for different applications.

Takeaways

• 😀 The video introduces the concept of an operational amplifier (op-amp), which is a high-gain amplifier used to amplify input signals.
• 🔍 The term 'operational amplifier' originates from its early use in performing mathematical operations such as addition, subtraction, integration, and differentiation.
• 🔌 The op-amp typically has two inputs (non-inverting and inverting) and one output, and can operate with either dual power supplies or a single power supply.
• 📶 The operational amplifier functions as a differential amplifier, amplifying the difference between the two input signals, with the output being proportional to the gain (A) times the difference of the inputs (V1 - V2).
• 🔄 The open-loop gain of an op-amp refers to its gain without any feedback, which can be extremely high, in the range of 10^5 to 10^6.
• 🔁 The op-amp's output is restricted by the biasing voltages and will saturate at certain levels, which is useful for comparator applications.
• 🛠️ The op-amp is a versatile component found in various applications, including active filters, oscillators, waveform converters, and analog-to-digital converters.
• 📊 The voltage transfer curve of an op-amp shows the relationship between the differential input and the output voltage, highlighting the gain and saturation points.
• 🌐 The ideal op-amp is characterized by infinite input impedance, zero output impedance, infinite bandwidth, infinite gain, and infinite slew rate, with zero output when input is zero.
• 🔄 Slew rate is the measure of how quickly an op-amp can respond to changes in input voltage, particularly important for square wave inputs.
• 🔄 Common mode rejection ratio (CMRR) is a parameter that indicates how well an op-amp can reject common input voltages and amplify the difference between the two inputs, with an ideal op-amp having an infinite CMRR.

Q & A

What is an operational amplifier (op-amp)?

An operational amplifier, or op-amp, is a type of amplifier that amplifies the difference between two input signals. It is a versatile integrated circuit used in various applications like signal amplification, mathematical operations, filters, oscillators, and more.

Why is it called an 'operational' amplifier?

The op-amp is called an 'operational' amplifier because, before digital computers, it was used to perform various mathematical operations such as addition, subtraction, integration, and differentiation by connecting a few resistors and capacitors.

What are the two input terminals of an op-amp called, and what do they represent?

The two input terminals of an op-amp are called the non-inverting input (marked with a positive sign) and the inverting input (marked with a negative sign). The non-inverting input produces an output in phase with the input, while the inverting input produces an output that is 180 degrees out of phase with the input.

What is meant by the 'open-loop gain' of an op-amp?

The open-loop gain of an op-amp refers to the gain of the amplifier when there is no feedback loop from the output to the input. It is typically very high, ranging from 100,000 (10^5) to 1,000,000 (10^6).

How does the op-amp output behave when a small input signal is applied in an open-loop configuration?

In an open-loop configuration, even a small input signal can cause the op-amp's output to saturate, reaching the positive or negative biasing voltage limits, depending on the input signal's polarity.

What are some common applications of op-amps?

Op-amps are used in a variety of applications, including comparators, active filters, oscillators, waveform converters, analog-to-digital converters, and digital-to-analog converters.

What are the ideal characteristics of an op-amp?

The ideal op-amp has infinite input impedance, zero output impedance, infinite open-loop gain, infinite bandwidth, and infinite slew rate. Additionally, when the input voltage is zero, the output should also be zero, and it should have an infinite common-mode rejection ratio.

What is slew rate in the context of an op-amp, and why is it important?

The slew rate of an op-amp is the rate at which the output voltage can change in response to a step input voltage. It is usually expressed in volts per microsecond (V/µs). A high slew rate is important for accurately reproducing fast-changing signals, such as square waves.

What is the common-mode rejection ratio (CMRR) in an op-amp?

The common-mode rejection ratio (CMRR) is a measure of an op-amp's ability to reject common-mode signals, which are the same at both input terminals, while amplifying the differential signal, which is the difference between the two input voltages. Ideally, the CMRR should be infinite.

How do practical op-amps differ from ideal op-amps?

Practical op-amps have finite input and output impedances, a finite open-loop gain, and non-infinite bandwidth and slew rate. They may also have a small output even when the input is zero, known as offset voltage, and their CMRR is finite.

Outlines

00:00

🔌 Introduction to Operational Amplifiers

This paragraph introduces the concept of operational amplifiers (op-amps), emphasizing their role as amplifiers that can enhance input signals. It explains the historical use of op-amps in performing mathematical operations before the advent of digital computers, such as addition, subtraction, integration, and differentiation, by simply connecting resistors and capacitors. The paragraph also describes the basic circuit symbol of an op-amp, which includes two inputs (non-inverting and inverting) and one output, and mentions that most op-amps require two power supplies, although some can operate on a single supply. The operational principle of op-amps as differential amplifiers is outlined, with the output being the amplified difference between the two input signals, governed by the open-loop gain (A). The paragraph also touches on the behavior of the op-amp when it receives input at the non-inverting or inverting terminals, including the phase inversion that occurs with inverting input.

05:06

📊 Characteristics and Applications of Op-Amps

This paragraph delves into the characteristics of op-amps, starting with the voltage transfer curve that illustrates the relationship between the input signal and the output voltage, highlighting the high gain values typical of op-amps. It discusses the saturation behavior of the output voltage, which is limited by the biasing voltages, and how this makes op-amps useful as comparators. The paragraph also outlines various applications of op-amps, including in active filters, oscillators, waveform converters, and analog-to-digital and digital-to-analog converters. The versatility of op-amps is attributed to their unique characteristics, which are further explored by examining the equivalent circuit of an op-amp, including input and output impedances and the open-loop gain formula. The paragraph introduces the concept of ideal op-amp characteristics, such as infinite input impedance, zero output impedance, infinite bandwidth, and slew rate, and contrasts these with the practical limitations of real op-amps.

10:11

🔍 Ideal vs. Practical Op-Amp Parameters

The final paragraph contrasts the ideal characteristics of an op-amp with those of practical op-amps, providing a list of parameters and their typical values for general-purpose 741 op-amp ICs. It explains that while ideal op-amps have infinite input and output impedances, infinite open-loop gain, bandwidth, and slew rate, and perfect common mode rejection, real op-amps have finite values for these parameters. The paragraph mentions that practical op-amps may have an offset voltage even when the input is zero and that different op-amp ICs are optimized for different parameters, such as slew rate or open-loop gain, depending on the application requirements. It concludes by looking forward to the next video, which will discuss the effects of feedback in op-amp configurations and encourages viewers to engage with the content by asking questions or providing feedback in the comments section.

Keywords

💡Operational Amplifier (Op-Amp)

An operational amplifier, commonly known as an op-amp, is a type of amplifier used to amplify input signals. It is central to the video's theme as the discussion revolves around its functions, characteristics, and applications. The op-amp can perform mathematical operations like addition, subtraction, integration, and differentiation, which is why it is called an 'operational' amplifier.

💡Open-loop Gain

Open-loop gain refers to the amplification provided by an op-amp when there is no feedback loop from the output back to the input. It is a significant concept in the video as it describes how an op-amp amplifies the difference between its input signals, with values typically ranging from 10^5 to 10^6. Understanding open-loop gain is crucial for grasping how op-amps function in different circuit configurations.

💡Inverting and Non-inverting Terminals

These are the two input terminals of an operational amplifier. The inverting terminal, marked with a negative sign, inverts the phase of the input signal, while the non-inverting terminal, marked with a positive sign, does not invert the signal. The distinction between these terminals is essential for understanding how op-amps manipulate signals and why they produce different outputs depending on which terminal is used.

💡Differential Amplifier

A differential amplifier is a type of amplifier that amplifies the difference between two input signals. The operational amplifier is described as a differential amplifier in the video, which means it outputs a signal proportional to the difference between its two input voltages. This characteristic is fundamental to the op-amp's operation and its ability to perform various signal processing tasks.

💡Saturation Voltage

Saturation voltage refers to the maximum output voltage an op-amp can produce, which is limited by its power supply. When an input signal causes the output to exceed this limit, the op-amp output 'saturates' at this maximum value. The video discusses how an op-amp behaves when its output reaches saturation, which is important for understanding the limitations of op-amp circuits.

💡Feedback

Feedback in the context of op-amps refers to the process of routing a portion of the output signal back to the input. This concept is introduced in the video as a way to control the behavior of the op-amp, such as stabilizing the gain or creating specific circuit responses. Understanding feedback is key to designing and analyzing op-amp circuits.

💡Comparator

A comparator is a circuit configuration of an op-amp that compares two input voltages and outputs a signal indicating which input is higher. The video mentions using an op-amp as a comparator, highlighting its application in detecting and responding to voltage differences. This functionality is a common use of op-amps in various electronic circuits.

💡Slew Rate

Slew rate is the speed at which an op-amp's output can change in response to a rapid change in the input signal. It is measured in volts per microsecond. The video explains this as a key characteristic of op-amps, especially relevant in applications where the speed of signal processing is critical. A high slew rate is desirable for fast, accurate signal amplification.

💡Common Mode Rejection Ratio (CMRR)

CMRR is a measure of an op-amp's ability to reject input signals that are common to both input terminals, amplifying only the difference between them. A high CMRR is desirable for accurate differential signal processing. The video touches on this parameter to explain the quality of an op-amp in distinguishing between desired signals and noise.

💡Ideal vs. Practical Op-Amp

The video contrasts the characteristics of an ideal op-amp, which has infinite gain, input impedance, and bandwidth, with those of a practical op-amp, which has finite values for these parameters. This comparison helps viewers understand the theoretical ideal performance versus the realistic limitations encountered in actual circuits.

Highlights

Introduction to the basics of the operational amplifier (op-amp).

Explanation of the term 'operational amplifier', highlighting its use in performing mathematical functions like addition, subtraction, integration, and differentiation.

Description of the op-amp circuit symbol and its components: two inputs and one output.

Differentiation between the non-inverting and inverting input terminals of an op-amp.

Operational amplifier as a differential amplifier with a single output that amplifies the difference between two input signals.

Concept of open-loop gain and its significance in the operation of an op-amp.

Behavior of the op-amp output when a sinusoidal signal is applied to the non-inverting or inverting input terminals.

Limitations of the op-amp output due to biasing voltages and the concept of saturation.

Visualization of the voltage transfer curve of an op-amp and its implications on gain and saturation.

Utility of the op-amp in open-loop configuration for applications like comparators.

Versatility of the op-amp in various applications including active filters, oscillators, and waveform converters.

Characteristics that make the op-amp a versatile integrated circuit (IC).

Overview of the equivalent circuit of an op-amp including input and output impedances and open-loop gain.

Definition and explanation of ideal op-amp characteristics such as infinite input impedance and zero output impedance.

Introduction to slew rate and common mode rejection ratio as key parameters of op-amp performance.

Comparison between ideal and practical op-amp characteristics, including finite input and output impedances and open-loop gain.

Discussion on the selection of appropriate op-amp ICs based on specific application requirements.

Upcoming discussion on the effects of feedback in op-amp configurations in future videos.

Transcripts

Hey friends, welcome to the Youtube Channel ALL ABOUT ELECTRONICS.

So, in this video, we are going to talk about the basics of the operational amplifier.

And in the upcoming videos, we will talk more about this operational amplifier.

And we will see, how we can design the different circuits using this operational amplifier.

So, as its name suggests, this op-amp is basically amplifier.

And the basic job of an amplifier is to amplify the input signal.

Now, let's understand why it is known as the operational amplifier.

So, in early days when digital computers were not evolved, at that time the different mathematical

functions like addition, subtraction, integration, and differentiation were performed using this

operational amplifier.

So, just by connecting few resistors and capacitors, it is possible to perform the different mathematical

operations.

And that is why this amplifier is known as the p[erational amplifier.

So, now if you see this circuit symbol of the operational amplifier, it can be represented

by this symbol.

So, it consists of two inputs and one output.

And most of the operational amplifiers consist of two power supplies.

The positive and the negative power supply.

But there are many op-amp IC's which runs on the single power supply.

So, now in this operational amplifier, the input terminal which is marked by this positive

sign is known as the non-inverting input terminal and another input terminal which is marked

by this negative sign is the known as the inverting input terminal.

And it will get cleared to you very shortly why it is known as the non-inverting as well

as the inverting input terminals.

So, now if you see this operational amplifier, it is one kind of differential amplifier with

the signle output.

It means that this amplifier amplifies the difference between the two input signals.

So, let's say V1 and V2 are the input signals which is being applied to this operational

amplifier and let's say the gain of this operational amplifier is A, then the output will be equal

to A times the V1 minus V2.

So, let's say if we have applied the single input to this operational amplifier and we

have grounded another input terminal then at the output you will get A times V1.

Where A is the open loop gain of this operational amplifier.

The reason it is being known as the open loop gain is that it is the gain of the operational

amplifier when there is no feedback from the output to the input side.

So, suppose if you are applying the sinusoidal signal over here, then at the output that

sinusoidal signal should be get multiplied by the factor of this gain and at the output,

you should get the amplified sinusoidal signal.

Now, here the phase of this output voltage will be the same as the input voltage.

Likewise, whenever we are applying input to this negative terminal, and we are grounding

another terminal then the output of this amplifier will be equal to minus A times the V2 because

the difference between these two input terminals will be equal to 0 minus V2, that is equal

to minus V2.

So, suppose let's say if we are applying the sinusoidal signal at the input then at the

output we will get the amplified sinusoidal signal which is having a 180-degree phase

with respect to the input signal.

That means the output will be get inverted by 180 degrees.

And that is why this input terminal is known as the inverting terminal.

Because the output will be get inverted with respect to the input.

So, now here suppose if we apply the input signal between these two positive and the

negative terminals then at the output we will get A times this differential input signal.

Where here this A represents the open-loop gain of this operational amplifier.

Now, this operational amplifier is a very high gain amplifier.

And the value of gain used to be in the range of 10 to the power 5, to the 10 to the power

6.

So, let's say, even if we apply the 1 mV of a signal between these two terminals, and

let's say if the gain of this op-amp is 10 to the power 5, then at the output theoretically

we should get 1 mV signal that is multiplied by the 10 to the power 5.

That is equal to 100V.

Or let's say if we apply 1V of a signal, then theoretically, we should get the output as

10 to the power 5 volts.

But that is not possible.

And the output of this op-amp is restricted by the biasing voltages that are being applied

to this op-amp.

So, the output voltage will be between these biasing voltages.

So, if you see the voltage transfer curve of op-amp then it will look like this.

So, here this X-axis represents the differential input that is applied to this operational

amplifier. and the Y-axis represents the output voltage

of the amplifier.

And here the slope basically represents the gain of the amplifier, which used to be in

the range of 10 to the power 5 to the 10 to the power 6.

Now, here let's say if the gain of the op-amp is 10 to the power 6.

And let's say we are applying 1 microvolt of a signal.

Then at the output, we should get 1V of a signal.

Likewise, let's say if we apply 10 microvolts of a signal, then at the output, we will get

10 V of output.

But as we increase this input signal, then we will find that after some value of input

signal, the output will get saturated to the value +Vsat, which used to be less than the

positive biasing supply.

So, in this way, as soon as the input voltage goes beyond some certain value, the output

will be get saturated to the plus saturation voltage.

And same is true for the negative input voltages.

So, as soon as the input voltage goes beyond some threshold value at the output you will

get minus saturation voltage.

So, whenever this operational amplifier is used in open loop configuration that means

there is no feedback from the output to the input side, at that time even if we apply

small input signal between these two input terminals, then also you will find that the

output will be get saturated towards the positive or the negative biasing voltages.

So, this particular characteristic of the op-amp is particularly useful when we use

this op-amp as a comparator.

So, this is one of the applications in which this op-amp can be used.

But if you see this op-amp, this op-amp can also be used in som any other applications.

Like, in designing the active filters, oscillators, waveform converters and analog to digital

and digital to analog converters.

And if we count the list, then the list will go on.

So, basically this op-amp is very versatile IC and you will find this op-amp in so many

applications.

Now, the reason this op-amp is used in so many application is because of its different

characteristics.

So, let's see the different characteristics of the op-amp because of which it is so versatile

and it is being used in different applications.

So, before we see that let's see the equivalent circuit of the op-amp.

So, as you can see here, this Ri is the input impedance of this op-amp.

Likewise, this Ro represents the output impedance of this op-amp.

And the output voltage of the op-amp will be the open-loop gain multiplied by the difference

between the input signals V1 and V2.

So, now before we see the different characteristics of the op-amp, let's see the different characteristics

of the ideal op-amp.

So the ideal op-amp should have this input impedance Ri that is equal to infinity.

So, that whatever input that is being applied between the input terminals will directly

get applied to the op-amp.

Similarly, the output impedance of this op-amp should be equal to zero.

That means whenever we are applying the output load to this op-amp then the output voltage

should directly come across this output load.

Then if you see the bandwidth of the ideal op-amp, the bandwidth of the ideal op-amp

should also be equal to infinity.

It means it should support all the frequencies starting from the zero Hertz to the infinite.

Similarly, the gain of the ideal op-amp should also be equal to infinite.

Apart from that whenever these two input terminals are zero, that means the input to this op-amp

is zero, at that time the output of this ideal op-amp should be equal to zero.

Now, apart from these characteristics, there are few more characteristics of the ideal

op-amp that is slew rate and the common mode rejection ratio.

So, will see more about these different characteristics in detail in separate videos.

But let's see the basics of this different characteristics.

So, in a simple way, if I say, the slew rate is basically how fast the op-amp is able to

reach its final value.

In that is particularly useful when we are applying a square wave to the op-amp.

So, let's say we have applied the square wave to the input of this op-amp and at the output,

we are getting this output waveform.

That is varying from zero volts to the V saturation voltage.

So, the ideal op-amp should be able to reach from the zero volts to the Vsat volt in zero

time.

So for the ideal op-amp, the slew rate should be equal to infinity.

Generally, this slew rate is defined in the unit of Volt per microseconds.

That means the how fast the op-amp is able to respond to the output voltage.

Then there is another parameter, which is known as the common mode rejection ratio.

So, let's understand very briefly what do we mean by this common mode rejection ratio.

We will talk more about it in a separate video.

So, let's say if we are applying the same input voltage to this V1 and V2 then the difference

between these two voltages will be equal to zero and at the output, we should get zero

volts.

Likewise, when we are applying different input voltages V1 and V2 to this op-amp then at

the output the difference between these two voltages will be get amplified by certain

amplifier gain.

So, this common mode rejection ratio basically defines how well the op-amp is able to reject

the common input voltages that are being applied to both its input terminals and how well it

is able to amplify the difference between the two voltages.

And it is generally defined as the ratio of differential gain divide by the common mode

gain.

So, for the ideal op-amp, the value of this common mode rejection ratio should be equal

to infinity.

So, here is the list of different ideal op-amp characteristics.

So the ideal op-amp has infinite input impedance, zero output impedance, infinite open loop

gain and infinite bandwidth and slew rate.

And in this ideal op-amp, whenever the input is equal to zero then at that time the output

is also zero.

And this ideal op-amp has infinite common mode rejection ratio.

But if you see any practical op-amps, they used to have finite input as well as the output

impedance.

Generally, this input impedance is in the range of megaohms, while the output impedance

is in the range of few ohms.

Similarly, the open loop gain of the op-amp is not infinity but it used to be in the range

of 10 to the power 5 to the 10 to the power 6.

Likewise, for the practical op-amps, when the input is equal to zero, at that time also

you will get some output at the op-amp.

Generally, it used to be in the range of few mV.

That is known as the offset voltage.

And we will talk more about it in the separate video.

So, here is the list of different parameters and the values of different parameters for

the general purpose 741 op-amp IC.

So, now if you see the different op-amp ICs, they are optimised for the different parameters.

So, let's say if one op-amp is optimised for the very high slew rate, while another op-amp

is optimised for the very high open loop gain.

And if you see some other IC, you might find that it is optimised for the very low offset

voltages.

So, depending upon your application you need to decide which parameter is critical for

your application and based on that you can decide which op-amp is suitable for your particular

application.

Now, so far we have seen this op-amp in open loop configuration.

That means, there was no feedback from output to the input.

Now, in the next video, we will see what happens when we provide the feedback from the output

the input side.

So, I hope in this video you understand the different characteristics of this op-amp.

So, if you have any questions or suggestion do let me know in the comment section below.

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