 Good morning student friends. I am Dr. Sachin R. Gengze, professor and head department of electronics, Vulture Institute of Technology. So, in today's session, we are going to have a discussion on one of the popular application that can be designed using operational amplifier and that application is sample and hold. The learning outcome for this session are after completing this session, the student will be able to describe the working of sample and hold circuit and can describe one of the application of sample and hold. The content of this session includes basic introduction to a sample and hold circuit that is how sample and hold circuit can be designed using operational amplifier. Secondly, we will have a look at one of the popular application of sample and hold where this sample and hold circuit can be used for analog to digital conversion. And lastly, we will have a look at how the sample and hold circuit is available in IC that is integrated circuit form. So, to begin with what is the sample and hold? A sample and hold is the circuit which samples the input signal and holds on its last sample value until the input is sampled again. So, the input is a continuous signal and you take the samples of the input signal. When you take a sample, a particular value is available and then this particular value is hold at the output of the sample and hold circuit until the input is sampled again. So, during this sampling period, the input is taken and it is put on the hold. However, when the output is at hold the input may change, but the output is not going to change. Let us have a look at the waveform of the circuit. So, as you can see this is there are two waveforms you can see the one which is indicated by a grey line and the one which is indicated by the red line. So, the grey line is the input to the sample and hold circuit and the red line is nothing but the output of the sample and hold circuit. And these vertical line indicates the instances at which the input is sampled. So, the first sample is taken at this point and then that at this point whatever the instantaneous value of the input signal is appearing at the output of the sample and hold circuit. Now, during this period as you can see the input signal is changing, but the output of the sample and hold circuit is not changing it remain constant or it remains to the same value at which the it was previously sampled. Now, at this point the input is sampled again and now the instantaneous value of the input signal is this. So, the output of the now sample and hold signal is going to be this value and it will be hold constant. Now, during this period again the input is changing, but the output of the sample and hold circuit is not changing. So, as you can see input to the sample and hold circuit is a is a is a continuous signal and output of the sample and hold signal is a discrete signal in which the instantaneous value is the one which is a last sample value. Now, how this sample and hold circuit can be designed? Now, the sample and hold circuit can be designed using an operational amplifier and then we require a transistor. Now, in this example we have used N channel E MOSFET as a transistor then of course, for holding the charge we require a capacitor. Now, here we can see that the transistor which is an N channel E MOSFET transistor is basically used transistor as a switch. Then coming to the operational amplifier circuit one can very easily identify that as you can see the input signal is connected to the non inverting input of the op amp and then there is a feedback at the inverting inverting inverting terminal of the op amp and as over here there is no resistors connected in the feedback. Very easily we can say that this circuit is nothing but it is nothing but non inverting voltage follower. It means that whatever the input is is available at at the input of the op amp the same in same input is available at the output of the op amp. So, there is no gain offered the output simply follows the input as usual we can see that there is a plus 15 volt and minus 15 volt of the supply required for the op amp. So, basically as I said the operational amplifier is used as the non inverting voltage follower in this application. Now, how this circuit together works as a sample and hold. Now, for this we need to see that there are two signals applied first one is the input voltage signal as an example we have considered a sinusoidal signal over here. Now, this is an input signal which we need to sample and convert and then we have to hold this input signal. So, there is a another input as you can see there is another input waveform V s which is basically a square wave and this voltage is called as the sample and hold control voltage. Essentially this voltage controls the switching action of this particular transistor and that is why it is called as the sample and hold control voltage. So, there are two voltages the input voltage which we want to sample and then there is a control voltage which controls rather the sample and hold action of this circuit. Let us have a look at the waveforms. So, as you can see there is a input signal which is a V in which we say that this is a input signal over here then this is a V in and it is having a peak voltage of V p. Now, this is a voltage as you can see this is a square wave voltage control voltage which is applied at the gate of the transistor. Now, we need to analyze this circuit for two period the first period being the positive of the control voltage and the second one being the zero of the control voltage. Now, this input is continuously applied to the drain of the transistor. Now, let us analyze what happened during time this time period. Now, during this time period which is called as the T s which is nothing but the sample period the voltage applied to the drain of the transistor is drain is nothing but the input voltage and the voltage which is applied to the gate of the transistor is a positive voltage. Now, because of this because of this positive voltage the gate is high and the transistor becomes on and then the source voltage follows follows the drain voltage. So, essentially the input voltage is available over here. Now, this as you can see this capacitor quickly charges to the input voltage. So, the capacitor quickly charges to the input voltage. What is the voltage at the output of the op-amp? As we have already discussed the output is used as the voltage follower and the voltage at the output of the op-amp is nothing but the same as the voltage at the input of the op-amp which is nothing but the voltage across capacitor which is nothing but the input voltage. So, during this time period which is during this time period which is called as the sample period one can say that the output of the op-amp is a simply equal to input voltage. Now, during this time period remember the input appears at the output at the same time the input voltage has charged the capacitor to the its highest value. Now, during this time period the during this time period which is actually called as an hold period the voltage applied to the gate of the MOSFET is 0. Now, when the voltage applied to the gate of the MOSFET is 0 the MOSFET is turned off. Now, when the MOSFET is turned off this voltage no longer reaches to the source. However, remember that this capacitor has been charged to the maximum value over here and then the capacitor holds that value. Now, one may ask that why this capacitor is not getting discharged? The answer is very simple the input impedance or input resistance of the op-amp is very high and that is why the only discharge path available for the capacitor as this is already an open circuit the capacitor will not discharge through the transistor. The capacitor will not also discharge through the operational amplifier because of the very high input resistance of the operational amplifier. So, the voltage across the capacitor will keep on driving the at the input of the op-amp and the same voltage that was the voltage to which it was previously charged the same voltage will appear over here and as it is a voltage follower the same voltage will appear at the output of the circuit. So, one can see that during the whole period during the whole period whatever the previous voltage was there to which the capacitor was charged that was the last voltage of the sample input and to that value the output is available. So, one can see that if one can look at the waveform of the sample and hold during the sample period this is T s is the waveform during the sample period the output is equal to input and during the whole period the output is equal to previously sample value. So, again at this point again at this point we can repeat the same thing and we can say that during this time period the transistor becomes on and then again the V in will make this capacitor charged at the same time it will appear at the output and during whole period that voltage will be retained across the capacitor and at the input of the op-amp and hence it will be available at the output. So, as you can see that the input is a continuous waveform however the output is the output follows the input for the sample period and during the whole period the output remains to the last sample value. So, this waveform this kind of circuit is called as the sample and hold circuit which can be very easily designed using a few components apart from the operational fire operational amplifier like a transistor and a capacitor. Now, you can pause the video and answer the question you have to comment about the frequency of the V in and V s as you can see there are two waveform one is the V in which is an input waveform and then there is a V s which is nothing but the control voltage. So, we have to you have to discuss or you have to comment about what is the relationship or what is the rather desirable relationship between the input waveform as and the control waveform. So, I think dear student you are ready with the answer and very obviously one can say that the frequency of the control waveform must be much greater than that of the input waveform. Let us go ahead and quickly have a look at one of the application of the sample and hold you may be familiar with this kind of setup where there is a plant and then we are taking some analog input from the plant and we are converting that analog into a digital which is being processed by the digital computer or a process and then again this pass to through a d to n a converter and it is again some of the parameters in the plant are modified accordingly. Now, what I want to make you focus is on this block which is called as an analog to digital converter which essentially convert an unknown analog voltage into a proportional digital output. Now, before this analog to digital converter as one can see that we are using a circuit called as a sample and hold this is because the unknown voltage may be continuously changing. However, the analog to digital converter requires some time for the conversion of this unknown voltage and during that time period the input analog voltage should not change and should be held constant and that can be done using a sample and hold circuit. So, sample and hold circuit are finding number of application in the industry. So, one of the popular application is being with the analog and digital converter. Lastly, we come to sample and hold IC that is LF 398 as you can see the sample and hold can be realized using an op-amp and a transistor and a capacitor and few resistors. However, directly the sample and hold is available in an IC form. So, the part number is LF 398 as you can see that of course, the capacitor you need to the capacitor which is required for the sample and hold you need to you need to connect externally and it will operate with the 2 power supply plus Vcc and minus Ve or plus V and minus V and this IC will have 2 input one is the analog input which you need to sample and then of course, there is a control input which will rather control the sample period and hold period and then the output will be the sample and hold. So, one can design a sample and hold circuit using an operational amplifier or one can directly use a readily available IC like LF 398. So, with that we come to the almost end of this session in which we discussed about the sample and hold I am leaving with you for a discussion about can you can you can you sketch the input and output waveform in case the frequency of V s that is the frequency of the sampling is a less than that of the V in. So, how the output is going to what is going to be effect on this output. So, what is going to be the output of the sample and hold circuit when the when the sampling frequency is less than than the input signal frequency. So, I think I am leaving you with the discussion on this and then references we have used are the two books one is by Ramakan Gaikwad and second is by Roy Choudhury and Jain. Thank you student for a patient listening.