Introduction to Operational Amplifier: Characteristics of Ideal Op-Amp

0:19 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

1:09 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

1:55 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

2:44 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

3:32 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

4:17 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.

5:05 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

5:55 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

6:42 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.

7:31 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

8:20 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

9:10 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.

9:57 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.

10:42 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

11:31 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.

12:19 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. If you like this video hit the like button and subscribe to the channel for more such

13:07 videos.