 Hello everyone, welcome to lecture on OPAM feedback configurations. At the end of this session, students will be able to define and compare negative and positive feedback, also able to identify different feedback configurations. Now before starting with actual session, let's pause the video and let's think about what are the characteristics of voltage amplifiers. These are nothing but voltage gain, input impedance, output impedance and bandwidth. Now open loop OPAM. Now what is the mean by open loop OPAM? It is nothing but a simple operational amplifier which is having two inputs and one output. For this, it has formula Vo equals to A into bracket V1 minus P2, where Vo is nothing but the output voltage, A is nothing but the gain of the OPAM, V1 and V2 are two inputs. In this case, V1 is connected to plus terminal and V2 is connected to minus terminal. If V1 is greater than V2, then Vo is positive and if V2 is greater than V1, then Vo is negative. This OPAM has some characteristics. First, it has very high gain. So because of that, only a smaller signal which is having a low frequency amplified without any distortions. If you are using any other signals with higher frequency, it gets distortion in the amplification. Second one is a gain of is not constant. It varies with the temperature, power supply and in mass production techniques. Next large variation in voltage gain. So it is unsuitable for many linear applications. And the last one, it has a very small bandwidth which is almost nearly equals to zero. So these OPAMs is impractical in AC applications. Now to overcome the disadvantages, whatever you seen in the previous slide of OPENLOOP OPAM, for that we use feedback. So that it is called as a closed loop OPAM. So let's first see what is the mean by feedback. So feedback is nothing but the fraction of output is applied again back and recombined with the input. Why we need a feedback? As I said earlier, some parameters of amplifiers are needed to be changed as per the requirement in the applications. So these changes can be done by using a feedback in the OPAM circuit. So using this feedback, there are two types. One is a negative feedback, second one is a positive feedback. So let's see these feedbacks one by one. Now first negative feedback. Figure two shows OPAM with the negative feedback. Here V1 is applied as an input to the positive terminal of the OPAM and V0 is nothing but the output of the OPAM. So output of the OPAM is again taken back feedback and connected to the non-inverting terminal of the OPAM. So when feedback signal applied to input is 180 degree out of phase with respect to your input signal, then it is called as a negative feedback. So because of the 180 degree out of phase, it decreases the net input signal to the OPAM. So that's why it is called as a degenerative feedback amplifier. Because of this decreasing in the net input signal or negative feedback, it leads to stability. If output is less than VIN, in this case V0 is shoots to positive. And if output is greater than VIN, V0 shoots to negative. Now positive feedback. What is mean by positive feedback? If figure three shows OPAM with the positive feedback. In this case, VIN is nothing but the input voltage or input source which is connected to the negative terminal of the OPAM, V0 is the output which is again connected as a feedback to the OPAM to positive terminal. So it is also called as inverting operational amplifier. So positive feedback means when feedback signal is applied to input is in phase with your input signal, then it is called as a positive feedback. So because of it is in phase with your input signal, the net input signal to the OPAM is increases. So that's why it is also called as a regenerative feedback. Now in this case, suppose if you are having plus terminal is slightly higher than your VIN, so V0 goes to positive. So again that V0 is connected as a feedback to your OPAM means that this makes plus terminal more positive than the VIN. So that situation makes worse. So in practical, you can say that positive feedback is worse or not good for the SA application. Now this table shows some points in comparison with the positive feedback and negative feedback. We have already seen this in previous slides, but let's see one by one. Feedback signal in terms of positive feedback, it is in phase with your input signal. In negative feedback, it is 180 degree out of phase with your input signal. So because of this feedback signal, total input signal in positive feedback increases. In negative feedback, it is decreases. Again, it is increase in positive feedback. In negative feedback, it is decreases. Noise increases in positive feedback. In negative feedback, it decreases. So due to this stability is poor in the positive feedback, in negative feedback it is improved. So application point of view, positive feedbacks are used in oscillators and negative feedbacks are used in amplifiers. Now let us see what is mean by feedback amplifier. So the opamp that uses a feedback is called as a feedback amplifier. So it forms a loop, so it is called as a closed loop amplifier. It has two parts, first is a opamp, second one is a feedback circuit. Feedback circuit is composed of passive components, active components and or the combination of the both. Active components are nothing but the resistor capacitor inductors and active components are nothing but the transistor diodes. So by using this feedback it has four configurations. Voltage series feedback amplifier, voltage shunt feedback amplifier, current series feedback amplifier and last one is a current shunt feedback amplifier. Now let us see these four configurations one by one. First is a voltage series feedback amplifier. So figure 4 shows a voltage series feedback amplifier. As I said previous slide it has two parts, first is opamp, second one is a feedback circuit. So if you see the opamp having two inputs and output, output is nothing but VO is nothing but voltage across load resistor RLE. So that output of the opamp is connected as a input to the feedback circuit. So feedback circuit having a input is VO. Now feedback circuit have output which is nothing but the VF. So that VF is connected as a input again to the opamp. So opamp having two inputs, first one is nothing but your VIN that is voltage source and second one is nothing but the VF that is a feedback output voltage from the feedback circuit. So input to feedback circuit is a voltage across load resistor RLE and feedback quantity that is voltage here in this circuit is output of feedback circuit. Means from this you can say that VF is directly proportional to VO. Next one is a voltage shunt feedback amplifier. Number 5 shows voltage shunt feedback amplifier. It is similar to the voltage series feedback amplifier. Only the difference is what, whatever the output of your feedback circuit here it is not a voltage, it is a current. If you see the figure you can see that here opamp having two inputs, one is nothing but your VIN source connected to P number 1, second is nothing but the current which is having output from feedback circuit connected as a second input. So here the inputs are in terms of current. So opamp having two input currents, one is a IB, second one is a IF. Remaining part is same that is feedback circuit having input which is nothing but the VO which is nothing but the voltage across load resistor RLE of the opamp. So you can say that IF is directly proportional to VO. Now next is nothing but the current series feedback amplifier. Here you can see that it has a output of the opamp is nothing but the current that is IL which is nothing but the current flowing through load resistor RLE which is connected as a input to the feedback circuit and output of the feedback is connected as a input to the opamp. So opamp having two inputs first is nothing but the VIN which is nothing but the voltage source and second one is nothing but the VF which is nothing but the output of the feedback circuit connected as a input to the opamp means that input to the feedback circuit is nothing but the load current which is flowing through RLE and feedback quantity that is is nothing but the voltage to is output of the feedback circuit. So you can say that VF is directly proportional to IL. Last one is nothing but the current shunt feedback amplifier. Figure 7 shows the current shunt feedback amplifier. It is having IL which is nothing but the load current flowing through the load resistor RLE is output of your opamp which is again similar to input to the feedback circuit. Output of the feedback circuit over here is nothing but the IF that is current which is flowing through feedback circuit which is connected as a input to the opamp. So opamp having two input first is a VIN which is sourcing IN that is input current and second one is IF that is a feedback current which is connected as a second IN. So you can say that input to the feedback circuit is a load current and feedback quantity that is current over here in terms of current is output of your feedback circuit you can say that IF is directly proportional to the IL. These are the references. Thank you.