 Good morning and Dr. Sachin R. Gengze, Vulture Institute of Technology and today we are going to have a look at positive clipper circuits. The learning outcomes for this session include after completing this session student will able to explain concept of what is a clipper. They can describe working of a positive clipper and can sketch the output waveform for a positive clipper for various input signals. The content for today's session include a basic introduction to wave shaping circuits. Then we are going to have a look at positive clipper with a positive reference voltage and a positive clipper with a negative reference voltage. What are the wave shaping circuits? Wave shaping circuits are the circuits which modify the shape of the input waveform. There are different wave shaping circuits and the popular includes limiting circuits, clipping circuits and clamping circuits. Wave shaping circuits are commonly used in digital computers and they are also used in communication equipments like TV receivers and FM receivers. These wave shaping circuits can be easily designed using op-amp, resistors and diode. Very few components are required like op-amp, resistors and diode and of course we need a certain reference voltage source for that. So let us first understand the first type of wave shaping circuit which is a clipper. We have two types of the clipper, a positive clipper and a negative clipper. What is a clipper circuit? A clipper circuit clips off or chops off some of the portion of the input signal or input waveform to form the desired output signal or waveform. As I say there are two types of the clipper circuit. A positive clipper removes a positive part or the part towards the positive axis of the input signal. A positive clipper can be formed using op-amp and a diode in its feedback path. So let us have a look at the circuit diagram of a positive clipper. As I say this positive clipper again the reference voltage that we are applying to the positive clipper can be a positive reference voltage or a negative reference voltage. Let us have a look at first circuit which is a positive clipper with a positive reference voltage. As we can see over here the positive clipper circuit can be formed by using an operational amplifier then there is a diode put in the feedback path of the operational amplifier. Then the reference voltage as you can see there is a reference voltage of one whole. The reference voltage as it this reference voltage is positive this can be very easily derived by putting a potential meter for the same power supply which is used a positive power supply which is used for the op-amp. So this value of this RP is adjusted such that the reference voltage at this point which we are calling as a reference voltage is equal to one whole. The signal to be shaped that is the input signal is applied to the non-inverting terminal of the op-amp and as with most of the circuit there is a feedback path or the closed loop path at the inverting terminal of the op-amp. So how this circuit works? This is a waveform for the circuit as you can see let us assume that we are applying a sinusoidal signal of plus 2 and minus 2 voltage that is 4 whole peak to peak to the input of the op-amp that is at the non-inverting terminal. Now let us analyze this circuit let us this analysis can be made by considering 2 half cycle the positive half cycle and a negative half cycle. Let us understand what happened during the positive half cycle now during positive half cycle as you can see the voltage which is present at the inverting terminal is equal to reference voltage in this case as an example we are taken that as a one whole. Now as long as the V in start rising from zero hold and it is continuously increasing during positive half cycle as long as the voltage at the non-inverting terminal is less than one hold then the output of the op-amp as long as the voltage at the non-inverting terminal is less than one hold that is a reference voltage the output of the op-amp is negative. Now when the output of the output voltage is negative as you can see the voltage at this point is one hold and this voltage output voltage is sufficiently negative which makes the diode on when the diode is on the feedback path is completed and when the feedback path is completed we can see that this circuit behave as a simple voltage non-inverting simple non-inverting voltage follower. So the output voltage will be same as the input voltage which is nothing the V in so as you can see that as long as the non-inverting input is less than one hold or input signal is less than one hold output is equal to output is equal to input. Now as soon as the voltage at the non-inverting terminal goes just above one hold then the output becomes negative the output of the op-amp becomes negative when the output of the op-amp becomes negative the diode D1 is off or then it act as an open circuit. Now when the diode act as an open circuit there is no feedback path so you can see that there is no feedback path and the output of the op-amp is no longer reaching to V 0 but the output V 0 will be simply equal to reference voltage. So for this much for this portion as you can see the diode is off and the output is equal to reference voltage when the input voltage again falls below one hold then again the output of the op-amp becomes negative the diode start conducting and then the output will follow the input. So as you can see if you look at the output waveform as long as the input voltage is less than one hold the output is equal to input and when the input voltage goes above one hold then the output is remaining to be one hold. So if you look at this waveform and this waveform you can see that all the portion towards the positive above reference voltage that is all the portion towards positive above one hold is chopped off and then that is why this circuit is called as the positive clipper. Let us understand what happened when we have a negative reference voltage so as you can see now the same circuit is redrawn but now the reference voltage is taken from the negative supply let us assume that this reference voltage is of minus one hold then what happened the circuit behaves very similarly let us have a look at the waveform. The rule remains same as far as the input waveform is above one hold the output is clipped off when the input voltage is less than one hold then the output is following the input. So as you can see over here the input voltage start rising from the zero hold for all this portion for all this portion the voltage this voltage is one this voltage is minus one hold and this voltage is above minus one hold and that is why the output is clipped off. However when the input voltage falls below minus one hold and at that time the diode start conducting and then the output is following the input voltage. So as you can see this is still called as a positive clipper because all the portion which is above reference voltage towards positive is clipped off and all portion which is below reference voltage is remaining as it is so that is why this is a positive clipper with a negative reference voltage. So we have seen two circuit both are of the positive clipper but one with a positive reference voltage and the other one with a negative reference voltage. Let us have an interesting circuit as you can see in this case the reference voltage is now connected to zero. So we can say that this is a positive clipper with a zero reference voltage. So you can see that here instead of having a voltage divider we have directly connected this reference voltage to zero. So this is nothing but a positive clipper with a zero reference voltage. So now you can pause the video and draw the output waveform for this circuit which is nothing but a positive clipper a special case reference voltage is equal to zero hold. So this is how the output will look like. So as you can see the rule is when the input waveform is above reference voltage then that part is clipped off. So now reference voltage is equal to zero hold and that is why all the waveform all the part of the waveform which is above reference voltage is clipped off and all the portion of the waveform which is below reference voltage is remain as it is. So as you can see these two positive half cycles are clipped off and this is this remains as it is when the diode is conducting. And what is this if you compare the input waveform and output waveform we can say that this is nothing but negative half cycle rectifier. So this is a negative half wave rectifier for small signal. So a clipper a special application of clipper is also a small signal rectifier. Now let us have discussion. So we have just seen a circuit in which we have a positive clipper and the reference voltage is equal to zero hold. Then we have a op amp and a diode in a feedback path of the op amp. Now why this why this circuit the point of discussion is why this circuit is called as the rectifier for small signal. If you look very carefully the levels of the voltages here are zero hold hundred milliholt and minus hundred milliholt mean that the signal is of 200 milliholt peak to peak amplitude. So this is this is a very interesting and a special application of op amp which can be used for rectifying small signals or the signals which are of small amplitude. Now why a normal you may think about why a normal rectifier fails to fails to rectify the small signal and why for the small signal we are using this special circuit which is designed using an op amp. So I leave this point to you for the discussion and the references used for today's discussion are operational amplifier and linear integrated circuit by Ramakan Gaihar and linear integrated circuit by Roy Choudhury and Shail B. J. Thank you.