 Good morning, dear friends. I am Dr. Sachin Gengze, head of the department electronics engineering at Valkan Institute of Technology, Seoul. In today's lecture, we are going to have a look at some more characteristics of op-amp. Since last few sessions, we are discussing about the different characteristics of op-amp. Today will be the last session about the characteristics of op-amp. The learning outcome of this session includes, after this session, the student is able to explain few important DC and AC characteristics of practical op-amp. The content include important DC characteristics of op-amp and important AC characteristics of op-amp. As we know, operational amplifier is directly coupled high-gain amplifier and it is available as a single integrated circuit or IC. It is a very versatile device and we can design different applications that can perform mathematical operations like addition, subtraction, multiplication, integration of analog signal and op-amp can also be used to amplify DC and AC signal. We can design different applications like oscillator, active filter, comparator, regulator using op-amp. In the last few sessions, we have discussed about important DC characteristics of op-amp, which include input bias current, which is nothing but the current entering into the average of current entering into the inverting and non-inverting terminal of op-amp. Input offset current, which is an absolute difference between the current entering into the inverting and non-inverting input of an op-amp, differential input resistance or input resistance, which is nothing but the resistance between the two input terminal of the op-amp, that is inverting and non-inverting terminal. Similarly, the capacitance between these two terminal, the equivalent capacitance between these two terminal is called as an input capacitance. As we know, we have two pins dedicated to 741 like op-amp, pin number 1 and 5, where we can connect the pot and can adjust the output voltage, offset voltage equals to 0 and that voltage is the input voltage or rather the voltage between the pin number 1 and 5, which is required to adjust the output offset voltage equal to 0. The range of this voltage is called as an offset voltage adjustment range. The common mode rejection ratio is an ability of the op-amp to reject the common mode signal and amplify the differential signal. Input voltage range is the maximum voltage that you can apply, maximum common mode voltage that you can apply at the input of the operational amplifier and supply voltage rejection ratio indicate the ability of the operational amplifier in order to reject the changes in the supply voltage that is plus VCC and minus VEE. So, these are few of the characteristics that we have already discussed. Let us go ahead and understand few more characteristics of operational amplifier. The next characteristics is called as the last signal voltage gain, which is defined as the ratio of output voltage of the op-amp divided by the input differential voltage. So, a is equal to V0 divided by VID, where which can also be written as V0 is equal to a into bracket V1 minus V2, where V1 is the non-inverting input voltage, V2 is the inverting input voltage of op-amp and V0 is the output voltage. A is called as an open loop gain or which is also called as large signal voltage gain and usual value of A is very high. For example, for the op-amp like 7 4 1 C, the value of A is 200000. We can understand this large signal voltage gain using the equivalent circuit of a practical op-amp also. As you can see over here, the input at the non-inverting terminal is V1, that at the inverting terminal is V2, the differential voltage in between of this is called as a VID and the input resistance is R i. Now, then this block indicate or rather this indicate that the op-amp is able to amplify the differential input voltage with a large signal gain of A. So, if I look at the output of the op-amp V0, V0 is going to be A into bracket V1 minus V2 or V0 is equal to A into VID, where VID is the differential input voltage. The same thing can also be understood using this voltage transfer curve. So, on x axis, we have a differential voltage on. So, this indicates a positive differential voltage. It means that non-inverting input is greater than that of the inverting input. On this side, you can see that the differential voltage is negative. It means that the inverting input is greater than non-inverting input. So, x axis represent the differential voltage, while the y axis represent the output voltage of an op-amp. So, as you can see, when the differential voltage increases, the output is also increasing and after some time, whenever the differential voltage is greater than this particular voltage, the output of the op-amp goes into the positive saturation. Similarly, when the differential voltage is negative, up to certain point, you get an output which is linearly proportional. And after this point, the output of the op-amp goes into the negative saturation. Now, the slope of this line, the slope is nothing but the ratio of the change in the output divided by the change in the input. So, the slope of this line is nothing but the open loop gain of the op-amp, which is going to be very high. So, you can imagine, if I have the slope of this line, which is equal to 200,000, this line is going to be very steep. And even a small differential voltage will drive the op-amp into a positive saturation or into a negative saturation. So, this particular diagram is called as the voltage transfer curve of a practical op-amp. The next parameter or the next characteristic of an op-amp is called as an output voltage swing. Output voltage swing indicates the value of positive and negative saturation voltage output of the op-amp. And we know that it is never going to exceed the limit of the supply voltage of the op-amp, which is nothing but the plus VCC and minus VEE. So, theoretically from this, even theoretically from this equation, for example, if I assume A is equal to 200,000, and suppose V1 is equal to 2-hole and V2 is equal to 1-hole, then one may say that the output of the op-amp is 200,000 into 1, which is equal to 200,000-hole. That is not possible because the output of the op-amp is limited by output voltage swing, which is always lesser than the supply voltage. So, that voltage, the saturation voltage of the output of the op-amp is called as the output voltage swing. For 741C, output voltage swing VOMax is guaranteed between plus 13-hole and minus 13-hole. The next parameter is called as an output resistance. Output resistance R0 is the equivalent resistance that can be measured between the output terminal of the op-amp and the ground. And as we know, the output resistance of any amplifier is supposed to be as low as possible. And for op-amp also, it is very low. For example, for op-amp 741C, the output resistance is only of 75 ohms. So, these were some of the DC characteristics of an op-amp. Let us have a look at a few AC characteristics of the op-amp. The very first characteristic AC characteristic of an op-amp is called as the transient response. Now, we know that for any of the amplifier or any of the circuit, if the input is changing, the output is also changing and the measurement of that is called as the transient response. The response of the op-amp when the input has reached to a steady state value and remain at that level is independent of time and that response is called as the steady state response. Similarly, the transient response of an op-amp is characterized by two factors. One is called as the rise time and the second one is called as an overshoot. What is a rise time and overshoot? The time required by the output to go from 10 percent to 90 percent of its final value is called as the rise time and overshoot is nothing but the maximum amount by which the output deviates from its steady state value that is called as an overshoot. Usually, overshoot is measured in terms of percentage and for 741C, the rise time is 0.3 microsecond and overshoot is about 5 percent. The next important parameter of op-amp is called as the sleeve rate. Sleeve rate is defined as the maximum rate of change of output voltage per unit time and it is usually expressed in holes per microsecond. So, sleeve rate is defined as a dv0 by dt where v0 is the output voltage. So, dv0 by dt represent the change of the output voltage with respect to time and it is always indicated as the maximum value. The sleeve rate is actually indicating how fast the output of the op-amp can change in response to the input and for a given operational amplifier the sleeve rate is fixed and hence that puts a limit on the high frequency signal that we can use with that particular operational amplifier. For example, for the popular op-amp 741C has a very low sleeve rate of 0.5 holes per microsecond. It means that the output of this op-amp 741C cannot change by more than 0.5 holes per microsecond. Dear student, let us stop the video and answer this question. Now, the question is as we know the 741C has a low sleeve rate of 0.5 holes per microsecond. Do you think that it put any limit on the frequency of the input signal for 741C and if yes what that limit is going to be and can you please calculate by assuming certain value of the saturation voltage of the output of op-amp positive and negative saturation voltage can you calculate the limit that can be put on the frequency of the input signal used with the 741C. Lastly, to end this session we are going to compare the characteristic of the ideal and practical op-amp. For example, the voltage gain in case of ideal op-amp it should be infinite, but in case of practical op-amp it is very high. A typical values include 10 raise to 5 to 10 raise to 8. Input resistance of ideal op-amp is also infinite, but practically it is going to be not infinity, but it is going to be very high between 10 raise to 6 to 10 raise to 12 ohm. Output resistance of an ideal op-amp is 0, but practically it is not 0, it is very small say 75 ohm for 741C. Bandwidth is supposed to be infinite for an ideal op-amp, but we know that for any amplifier the gain bandwidth product has to be constant and that is why if the gain is very high practically the bandwidth of an open loop op-amp is going to be very low. And lastly the output offset voltage is expected to be 0 for ideal op-amp, but for practical op-amp it is not 0, it is infinitely hold. So, with that we come to the end of the today's session and I am leaving you with two questions. The first one is that what are the different AC characteristics of an op-amp and how the different AC characteristics govern the choice of the op-amp. We know that many op-amps are available on the shelf and which of that we are going to use for a particular application. So, how these AC characteristics decide which op-amp to be used for a particular application. So, references for today's session are the two books one is by Ramakan Gayakbar and the second one is by Roy Chaudhary. Thank you dear student for Patient Listed.