 Good morning students and Dr. Sachin Gengze, Professor and Head, Department of Electronics, Engineering, Vulture Institute of Technology, Sholapur. In today's session, we are going to have a look at negative clippers. The learning outcomes for this session are, after completing this session, students will be able to describe the general working of a negative clipper circuits. They can sketch the output waveform of a negative clipper for various input signals. The content of this session includes a basic introduction to wave shaping circuits. Then we are going to have a look at two circuits. The first one is a negative clipper with negative reference voltage. And the second one is going to be a negative clipper circuit with a positive reference voltage. As we know, what are the wave shaping circuits? Wave shaping circuits are the circuits which modify the shape of the input waveform to produce an output waveform which is different than that of the input waveform. There are many types of the wave shaping circuit. Few of them include the voltage limiting circuit, the clipping circuit and the clamping circuit. Wave shaping circuits are used in various electronic devices, especially like in digital computers, communication equipment like TV receivers and FM receivers. Wave shaping circuits can be easily designed using operational amplifier and then we need few more components like resistors and diodes. And of course, some fixed reference voltage source is also required. In today's lecture, let us have a look at a very popular circuit designed using a PAM called as the negative clipper circuit. As we are aware of, a clipper circuit clips up or chops up some of the portion of the input signal to obtain the desired output waveform. Now, a negative clipper circuit is a circuit which removes part of the waveform which is towards the negative of the input signal. This negative clipper circuit can be formed using operational amplifier and a diode in its feedback path. So, we are going to have a look at the two types of the negative clipper circuits. The first one is a negative clipper with a negative reference voltage and the second one is going to be a negative clipper circuit with a positive reference voltage. Let us have a look at the first circuit that is negative clipper circuit with a negative reference voltage. As you can see, this is a circuit diagram for a negative clipper with a negative reference voltage. This circuit is formed by using an operational amplifier in a closed loop. As we know, the most of the applications of the operational amplifier are in a closed loop and this circuit is also not an exception. So, operational amplifier is used in a closed loop. There is a diode D1 that is 1N914 is used in the feedback path of the operational amplifier. Basically, this circuit is a non-inverting amplifier because the input signal is connected to the non-inverting input and the feedback is applied to the inverting input. A fixed reference voltage. In this example, we are considering reference voltage equal to minus 1 whole. A fixed reference voltage is generated already as there is a supply of minus 15 volt available which is nothing but the power supply for an op-amp. Using the same supply and a potential meter, we can generate and adjusting the value of this 10 kilo ohm pot such that the voltage at this point becomes 1 whole. A fixed reference voltage of minus 1 whole is generated and then that can be applied to the inverting input of an op-amp. So, the circuit consists of an op-amp and a diode in a feedback path. The input signal, the signal to be modified, the signal to be clipped off is applied to the non-inverting input of the op-amp and there is a fixed reference voltage of minus 1 whole which is connected to the inverting input. Now with this circuit, let us see how this works as a negative clipper. For that, let us assume that we are applying a sinusoidal signal of sufficient amplitude as a V in or here. For this waveform, the voltage at inverting terminal is fixed voltage that is a minus 1 whole and the voltage at the non-inverting terminal is rising gradually from 0 whole. So, for all this voltage as you can see, the voltage at the non-inverting terminal is greater than that of the inverting terminal and hence the output of the op-amp V0 dash is sufficiently positive to make the diode D1 as a on. So, because if you look at the diode D1, its cathode is already connected to minus 1 whole and a positive voltage at an anode will make the diode on. Now, as soon as the diode makes on, the feedback path is completed and then we see that this circuit becomes a non-inverting voltage follower. So, the input voltage will directly appear at the output V0. So, as long as the voltage at the non-inverting terminal is greater than minus 1 whole, the diode is on and V0 is equal to V in. So, for all this voltage where the voltage at the non-inverting terminal is greater than minus 1 whole, the output follows input. Now, something interesting happens at this point. At this point, when the voltage the V in drops just below minus 1 whole then in that case, the voltage at the non-inverting terminal is less than that at the inverting terminal and the output V0 dash becomes negative. Now, when the output V0 dash becomes negative, the diode stops conducting diode become off diode and then this becomes an open circuit and the feedback path is not completed. Although this output V0 dash is a positive in that case, the output V0 dash is a negative voltage. It is no longer reaching to V0 and what is reaching to the V0 is a fixed voltage of minus 1 whole. So, for all this for all this part of the input waveform where the input V in falls below minus 1 whole the diode stops conducting and V0 is equal to fixed voltage of minus 1 whole. So, as you can see the waveform the output waveform has a two part the one when the D1 is on the output is equal to V in and when the D1 is off the input is no longer reaching to V0 and output is equal to a fixed voltage of minus 1 whole. So, why this is called as a negative clipper? This is called as a negative clipper because if you look at the output waveform carefully the part of the output waveforms towards the negative it chopped off. Let us have a look at the second circuit which is again a negative clipper but instead of applying a negative reference voltage let us see what happen if we apply a positive reference voltage. So, that is a negative clipper with a positive reference voltage. In this case again the circuit remains same however if you look now the reference voltage which is applied to the inverting terminal of the op-amp is generated from a positive supply and this potentiometer rp which is of 10 kilo ohm is adjusted such that the voltage at the inverting terminal equals to plus 1 whole. So, again the circuit consists of an op-amp there is a diode D1 in the feedback path of the op-amp the input signal to be clipped off the chopped off is applied to the non-inverting terminal and then there is a feedback at the inverting terminal and a reference of plus 1 whole is also applied to the inverting terminal. So, basically this circuit is again same this is nothing but a non-inverting amplifier because the input is applied to the non-inverting terminal. Now, in order to analyze the circuit once again we assume the same waveform is applied to the input so there is a sinusoidal signal of 4 whole peak to peak which is applied to the non-inverting terminal. Now, for this of for this much time period as you can see for this much time period if you remember the reference voltage is 1 whole and the whole take at the non-inverting terminal falls below 1 whole. So, if I consider this time period we can see that for all this time period the whole take at the non-inverting terminal is less than that of the inverting terminal and hence the output of the output of the op-amp goes to negative when the output of the op-amp goes to the negative diode stop conducting and hence this circuit becomes an open circuit and what is available at the output is the fixed voltage of plus 1 whole. Now, for this time period for this time period where you can see that the input is input is rising above 1 whole. So, when the input is rising above 1 whole you can see that the voltage at the non-inverting terminal is greater than that of the inverting terminal. So, the output of the op-amp becomes positive when the output is sufficiently positive the diode D1 becomes on when the diode D1 becomes on the loop is completed and this circuit get converted into a non-inverting voltage follower and that is why the output will be equal to input. So, again this waveform can be divided into two part for this much time period when the diode is on for this much time period the diode is off when the diode is on the output is equal to input and when the diode is off the fixed voltage of 1 whole is available. This is still called as a negative clipper because as you can see the waveform which is towards the negative of the input waveform is clipped off or chopped off. After this we will have a look at the special circuit which is nothing but a negative clipper reference voltage. So, this is a negative clipper because as you can see there is a diode in the feedback path and now instead of having a fixed DC voltage applied we are applying a zero reference voltage to this circuit and what is going to be output. So, you can pause this video and draw the output waveform for this circuit. So, this is output waveform so as simple for all this time period all this time period D1 is conducting and hence 0 is equal to V in. For this time period the diode D1 is not conducting it is a open circuit and output equal to 0 whole. So, for this circuit if I apply this input waveform this is what I am going to get as an output. So, what is this? This is nothing but a positive half wave rectifier. So, we can see that a negative clipper with a zero reference voltage can be used as the positive voltage or positive half wave rectifier. At last there is a question for discussion. We have seen that there is a negative clipper with a zero reference voltage design using op-amp and which can be used for the rectification. This is called the small signal rectifier and the question is why this is called the small signal rectifier because if you look at this signal the amplitude of this signal is very small it is near about 200 milliholt. So, what happened if I apply the same signal to your normal rectifier design using op-amp design without using op-amp. Whether I am going to get the required rectification or not and in this case when I am using an operational amplifier I can see that I get the desired rectifier. So, that is why this is called as the small signal rectifier. So, the question is why this is called as the small signal rectifier. So, friends in today's lecture we have discussed about a negative clipper with positive reference voltage and a negative reference voltage and we also seen that how negative clipper can be used as the half favorite rectifier for small signals. The books which we have used for this are thank you very much for the patient listening.