 Good morning everybody. I am Dr. Sachin R. Gengze, Professor and Head Department of Electronics Vulture Institute of Technology, Swalapur. Today, we are going to discuss and learn about open loop comparators designed using operational amplifiers. The learning outcomes of this session are after completing this session, the student will be able to explain working of non-inverting comparator and inverting comparator and they are going to explain a special type of comparator that is called as the zero crossing detector. Let us look at the content. In this session, we are going to discuss about three types of the circuit. First is non-inverting comparator. Second one is inverting comparator and the third one is zero crossing detector. What are the comparators? The comparator is a circuit with two input voltages and produce the output depending on which of the two input voltages are greater. So, if input A is greater, it will have some different output and if input B is greater, it will have something different output. These type of comparator for analog voltages can be designed very easily using operational amplifiers. They can be designed in non-inverting configuration as well as into inverting configurations. They can also be designed in for open loop application as well as for closed loop application. So, let us in today's session discuss how these comparators can be designed using operational amplifier in open loop applications. This is the first circuit that we are going to discuss about is called as the non-inverting comparator in open loop because there is no feedback provided for the op-amp. Let us have a look at what it contains. First, it contains a popular op-amp 741. As we know, there are two type of the power supply required for 741 is plus VCC and minus VEE. Then, there are two inputs. One is an inverting input and the other one is a non-inverting input. What is connected to inverting input is a reference voltage, a positive reference voltage and for this particular example, we have selected an arbitrary value of one hold. What is connected to non-inverting comparator is a sinusoidal signal VIN. This is called as a non-inverting comparator simply because the input signal, the sinusoidal signal is connected to non-inverting terminal and reference voltage is connected to the inverting terminal. How this work? This work with a very simple principle of when as we know when the operational amplifier is in open loop configuration, when the input voltage at the non-inverting terminal is greater than that at the inverting terminal, output of the op-amp is positive saturation voltage and when the input voltage at the inverting terminal is greater than that at the non-inverting terminal, the output is a negative saturation. So, let us apply the same rule for this circuit. Once again, I remind you the reference voltage of plus one hold is connected to inverting terminal and the varying sinusoidal signal is connected to the non-inverting terminal. So, let us have a look at the comparison of the waveform between input and output voltages. In first case, we assume that the V reference is positive. So, V reference is plus one hold. This is what happened. See, as you can see, there are as far as the inputs are concerned, there are two lines, one there are two signals rather the one is corresponding to a reference voltage of plus one hold, which is a fixed voltage, which is connected to inverting input and the varying sinusoidal signal, which is connected to the non-inverting terminal. As far as this portion is concerned, that is between 0 to 1 hold. In this case, the input connected to non-inverting terminal is a less than that at the inverting terminal, because if you remember the input connected to inverting terminal is one hold. So, for this small period of time, the output goes into the negative saturation. As the input connected to the non-inverting terminal crosses one hold for this portion, as you can see for this portion. Now, the non-inverting input is greater than the inverting input and that is why the output is a positive saturation. Similarly, from this point up to this point, the input at the non-inverting terminal is less than the inverting terminal and that is why the output is negative saturation. So, as you can see, when the unknown input voltage, which is connected to the non-inverting terminal is compared with the plus fixed voltage of plus one hold, which is a reference voltage and when the input voltage is greater than one hold, the output is a positive saturation, where it is less than one hold, it is a negative saturation. What happen when the V reference is negative? So, instead of connecting plus one hold to inverting terminal, if I connect a minus one hold to inverting terminal, still I can apply the same rule as far as the input at the non-inverting terminal is greater than that for the inverting terminal and as you can see for all this period of time, the input at the non-inverting terminal is greater than that of the inverting terminal. The output is positive saturation. At this point of time, the input at the non-inverting falls below minus one hold, which is nothing but the voltage connected to the inverting terminal and for this time period, the output goes into the negative saturation. So, as you can see, this is a circuit which is called as a non-inverting comparator, in which the reference voltage, which may be a positive or a negative, is a fixed voltage applied to the inverting terminal and the varying signal or sinusoidal signal is connected to the non-inverting terminal. Let us go for the next circuit, which is called as an inverting comparator, which is very similar to the non-inverting comparator with the difference now that the fixed voltage is now applied to the non-inverting terminal and the varying sinusoidal signal is now applied to the inverting terminal. As you can see, here I have here in this circuit, just to have a variation, what we have did is that we have put a we have put a variable resistor over here, so that the signal that can be applied to non-inverting terminal can be of any polarity, depend on the position of the varying arm, the signal can be either a positive or a negative. So, this is an inverting comparator, I repeat in this again, the fixed voltage is connected to the non-inverting terminal and the variable input is connected to the inverting terminal. Let us see how it works, let us assume that the reference voltage, which is connected to the non-inverting terminal is plus V reference, which is for example, which is one hole. So, as far as this portion is concerned, as we can see till the voltage start from zero hole up to it goes up to V reference, the input connected to non-inverting terminal is greater than that of the inverting terminal and that is why the output is a positive saturation. Now, at this instant of time, the input at the non-inverting terminal is greater than that of the inverting terminal and that is why the output goes into the negative saturation. At this point of view, as you can see again the input at the inverting terminal falls below that of the non-inverting terminal and output goes to the positive saturation. So, here again once again the rule is same, when the input at the inverting terminal is greater than that of the non-inverting terminal, the output goes into negative saturation and when the input at the non-inverting terminal is greater than that of the inverting terminal output goes into a negative saturation. This you can see that what happen when the V reference is negative in this case again the same thing applies as far as this part is concerned as far as this part is concerned the input at the input at the inverting terminal is less than that of the non-inverting terminal and that is why the output is negative saturation for this it falls below the 1 1 hold V reference minus 1 hold V reference and output becomes a positive saturation. So, as we have seen there are two types of the circuit which is an inverting comparator and the non-inverting comparator. This a general circuit of a general comparator can be converted into a special comparator and that is called as the zero crossing detector. So, this is a circuit of a zero crossing detector this is a non you can say that this is a non inverting zero crossing detector because sorry it is an inverting zero crossing detector because the variable V in is connected to the inverting terminal and fixed reference voltage is connected to the non-inverting terminal. But to wonder the fixed voltage here is of the zero hold and that is why it is called as the zero crossing detector. So, now dear student you can pause the video for some time and then you can draw the waveform you can draw the waveform relationship between the input voltage and the output voltage. So, this is how it will work if you remember if you remember the input is connected to the inverting terminal and zero hold is connected to the non-inverting terminal. So, as far as this portion is concerned input at the inverting terminal is greater than zero hold and that is why the output is a minus V sat and for this much period of time the input at the inverting terminal is less than that at the non-inverting terminal and that is why the output is a positive saturation. So, as you can see the zero crossing detector is a special circuit because the reference voltage is a zero hold and this can be used to convert a sinusoidal waveform into a square wave waveform of the same frequency if you see the frequency of the sinusoidal waveform and the square wave waveform they are same. So, this is about the open loop comparator in which we have studied the general comparator as well as the special circuit called as a zero crossing detector. However, there are problems with the open loop comparator that first problem is that they can produce an NNOS output or oscillation output with a noisy input or a slow varying input. If the input the sinusoidal signal is slowly varying or it is a noisy then the output can be errorless. One way to overcome this problem and to avoid the problem from going into the oscillation is to provide a positive feedback around the comparator and a comparator which is having a feedback a positive feedback is a special type of the comparator and this is called as the Schmitt trigger. So, in some other lecture we will discuss about the Schmitt trigger also. At the end of the session I would like you to have some discussion about the first question I am putting or first discussion I would like to have with you is that what is the maximum peak to peak voltage of the input waveform that will be allowed for these comparator. As we know we have seen that there is one fixed V reference and the other one is a sinusoidal varying voltage. So, what can be limit to these voltages which can be applied to the input of the open and the second discussion I would like to have is what will be the output of the non-inverting comparator if I apply plus whole DC to non-inverting terminal and plus 6 whole DC applied to the inverting terminal. So, instead of applying one signal as a DC and other signal as a sinusoidal what happened if I apply both signal which are DC one is of plus 5 whole and the other one is of 6 whole DC. So, I keep this question open for discussion amongst you. References for what we have done is the first reference is of course the operational amplifier and linear integrated circuit by Tranakan Gaikwad and the second book you can refer is linear integrated circuit by Roy Choudhary. Thank you very much.