 Good morning, dear friends. I am Dr. Sachin R. Gengze, Professor and Head Department of Electronics Engineering at Vulture Institute of Technology, Sholapur. In today's session, we are going to have a look at some more DC characteristics of operational amplifier. The learning outcome of today's session are, after completing this session, student will be able to explain important DC characteristics of practical operational amplifier. The content are of course, some of the important DC characteristics of operational amplifier. As we know, operational amplifier is a directly coupled high gain amplifier. It is available as a single IC or integrated circuit. It is a versatile device and can be used for both AC and DC signals and some of the applications of op amp include mathematical operations like addition, subtraction, multiplication and integration on analog signal. We can design different applications like oscillator, active filter, comparator, voltage regulator, using op amp also. We already discussed about the characteristics of an ideal op amp which include it has infinite input resistance that is ri is equal to infinity, has a zero output resistance r zero is equal to zero. Infinite open loop voltage gain A is equal to infinity, infinite bandwidth that is bandwidth is infinity and there is no any phase shift between output and input signal and the op amp ideal op amp has a zero offset voltage. However, as we know there is no any ideal device exists and we have what is called as a practical op amp. Although we assume that an ideal op amp do not take any current from the source and its response is independent of temperature. As far as the practical op amp is concerned, there are three things that are happening. First, a practical op amp takes current from its source, then the two inputs which are nothing but non inverting and inverting input of op amp, they respond differently to voltage and current due to mismatch of the two transistors at the input stage. As we know at the very first stage of the op amp is a differential amplifier which consists of two transistors and these because of these two transistors are different, two input respond differently to voltage and current and most importantly the operation of the op amp is dependent on the temperature. Then we will understand few of the important characteristic of an op amp. The first one is called as an input resistance. Input resistance is nothing but a differential input resistance or input resistance r i. It is that equivalent resistance that can be measured at either inverting terminal or non inverting terminal with the other terminal connected to the ground and an ideal op amp has an input resistance of infinity. However, a practical op amp do not have an input resistance of infinity but it is of very high value. For example, the op amp 741C has an r i of 2 mega ohm. There is another op amp for example, mu af 771 which is a FIT based op amp and r i of that is a very huge of the order of 1000 giga ohm. The second important DC characteristics of op amp is called as an input capacitance. Input capacitance C i is the equivalent capacitance that can be measured at either inverting or non inverting terminal of the op amp and other terminal at that time is connected to the ground. The 741C has an input capacitance, a very small input capacitance of 1.4 picofarad. The next parameter is called as an offset voltage adjustment range. Now, if we look at the 741C op amp in between of the pin 1 and 5, we are connecting a variable part of value 10 kilo ohm and the variable of that is connected to pin number 4. Which is nothing but the minus VEE. You usually which is of minus 15 whole. Now, this part as we know we can adjust this part in order to make the output offset voltage equal to 0. If you remember output offset voltage is nothing but the small output voltage which is present at the output of the op amp when both its inverting and non inverting terminal are connected to 0. So, how to make this output offset voltage is simply we have to connect the inverting and non inverting terminal to input to 0 and then we have to adjust this part until the output offset voltage becomes 0. So, what is an output or what is an offset voltage adjustment range? The offset voltage adjustment range can be defined yes you can see over here op amp 741 has a pin 1 and 5 as a offset null by varying a part connected between 1 and 5 the output offset voltage can be reduced to 0. The output offset voltage adjustment range is the range through which the input offset voltage can be adjusted by varying this 10 kilo ohm part and 4741 C this range is of plus or minus 15 milliholes. So, by varying this voltage between plus or minus 15 whole it is guaranteed that the output offset voltage of 741 C becomes 0. Some of the op amp do not have this facility of connecting a part between pin number 1 and 5 they do not have this offset null pins like we have for 741 in that case we have to design an external circuit to reduce the output offset voltage equals to 0. Here you can pause the video for a while and answer this question how output offset voltage can affect the performance of the op amp is it desirable to have an output offset voltage or it is undesirable to have if it is undesirable how it is going to affect the performance is it an important parameter or can we neglect it for some of the application and if there is an output offset voltage how we can reduce it to 0 how we can get rid of it. So, that is a question you have to answer for pausing the video for some time I think you are now ready with the answer ok. Then we will move to the next parameter which is called as an input voltage range. Now input voltage range actually specify how much maximum input voltage we can apply to the operational amplifier as we know the input to the operational amplifier is given at inverting and non-inverting terminal. So, we also know that when the same voltage is applied to inverting and non-inverting terminal it is called as the common mode voltage. So, input voltage range is a common mode voltage that is the same voltage which we are applying to in inverting and non-inverting terminal of the op amp that can be applied without disturbing the performance of the op amp or without damaging the op amp how much of this voltage we can apply to inverting and non-inverting terminal as a common mode voltage. And for 741C this range is between of plus and minus 13 whole. So, that is another important parameter of op amp. The next parameter is called as the common mode rejection ratio or CMRR. It is defined as the ratio of differential voltage gain AD to the common mode voltage gain ACM. So, CMRR is nothing but a ratio of common differential voltage gain to the common mode voltage gain. As we know this AD is a larger signal gain or is also called as a differential gain and ACM is a common signal gain which can be calculated as VOCM divided by VCM where VCM is then input a common mode voltage input to the op amp and VOCM is the common mode output or rather it is an output when I am connecting the common mode input to the op amp. What are the values? Let us have a look at the typical values of the CMRR. Generally ACM is a very small and we know that ACM is a very small common op amp is designed to reject the common mode voltages and amplify the differential voltages. That is why ACM is very small and AD is very large. So, CMRR for 741C is 90 dB and that for precision 741C is of 120 dB. It means that the precision 714C op amp has a better ability to reject the common mode signal like electrical noise and perform better in a noisy environment because it has a better CMRR. So, it has a better ability to reject the common mode signal and to amplify the differential signal. The last parameter of an op amp that we would like to have a look at is called as the supply voltage rejection ratio which is also called as some of the literature is referring this also as power supply rejection ratio or power supply sensitivity. Now, we have two formulae for SVRR. If we are defining SVRR in case of absolute values then SVRR is equal to delta VIO divided by delta V where delta V is a change in the supply voltage and delta VIO is a change is the change in the op amp's input offset voltage. So, when we are changing the supply voltage the input offset voltage is also changing and that change is captured as an SVRR. If we are some of the books or some of the literature some of the data manuals they are representing SVRR in terms of dB and when we are representing in terms of dB the formula becomes 20 to the 20 log of delta V divided by delta VIO where delta V is the change in the supply voltage and delta VIO is the change because of when I am changing the delta whatever there is a change in an input offset voltage that is being represented by delta VIO. So, that is what is called as an supply voltage rejection ratio. For 741C SVRR is 150 micro hole per hole and for 741C it is 6714C precision op amp it is of 6.31 micro hole per hole. So, with that we come to the end of today's session. I am leaving you with two question first you have to explain what are the different DC characteristics of op amp and secondly how they govern the choice of op amp out of many available. We know that there are many op amps available and for a particular application how we are going to select op amp out of these available op amp. Reference for this are again the two books which we are using one is by Gayakwad, Ramakan Gayakwad and second one is by Roy Chaudhary and Shail Chek. Thank you very much dear student for patient listening.