 Consistent Professor, Electrons Department, The Blood is Soul of Food. Now in this video lecture we are going to focus on the feedback in amplifiers. Now at the end of this video lecture, these are the outcomes that after the end of this video, the student is able to describe types of feedbacks, then he will analyze effect of negative feedback and is able to specify various negative feedback topologies. Friends, we know in the amplifier the number of parameters which are of utmost importance. For example, gain, bandwidth, stability. Now we expect that we design amplifier for certain gain, for certain bandwidth and many parameters. And we expect that these parameters should remain constant. Now we know here we design this amplifier for certain gain. And we hope that this gain parameter should not change at all. Even if there is a change in supply voltage or there is change in the temperature and various parameters. Notice observed that even if you design this circuit, first required parameters here. And even if I use a very good quality components, it is seen that some of the parameters are keep changing. Because in the amplifiers use transistors, now these devices are non-linear. That is they show different changes when some parameters are changing. For example, if I change temperature, it is seen that the current in the collector or current in the base is get affected. And so that the output of this amplifier does not remain constant. And hence there is a change in the gain, there is change in the bandwidth and number of parameters. So that we have to make confirm that for a good amplifier, there should not be any changes in all this parameter. Even though there is change in some parameter around the circuit. That is why we are going to employ a concept of. Naturally in the feedback, there are three main components. That is one is amplifier, second is the feedback. And there is one more concept when I employ this feedback, we are having amplifier with the feedback. Now we know here, as I said here earlier, the feedback is given back to the input here. So feedback may be in the phase of input signal or feedback may be out of phase with input signal. Now whenever we feed the outputs signal back to input in the same phase. So that the feedback signal and the input signal get added together. Now this kind of feedback is called as a positive feedback. Whereas in the other case, whenever we are going to feedback the input out of phase with the input here. That is the part of output is feedback to the input here is normally called the feedback signal is out of phase with the input signal. This kind of feedback is called. Whenever we get feedback here, it is seen that the system gain and the response will be get stabilized here. Because we know here, the circuit characteristics such as system gain and response can be precisely controlled here with the help of feedback. Secondly, the feedback gives me one very important achievement here. That is the output is independent of variations in the supply voltage and variations in the environmental conditions such as temperature and the pressure. It is also observed that with the feedback in the circuit, the signal distortions are get minimized because we are trying to make the output independent of the components in the circuit. Because components are not linear or nonlinear in nature of working there. And moreover, as I said earlier, the frequency response gain and bandwidth can be controlled precisely as we require. So these are the benefits of feedback here. And that's why we are going to employ the feedback in the circuit. Now what is feedback? Now this diagram shows you how the feedback is achieved here in the circuit. On the right hand side, you are going to see two blocks here. One is the amplifier. It is having again a voltage gain is called AV. This one feedback network called the feedback circuit that is beta. Now you see that the input is the voltage VI and the output is voltage VO. Now the voltage VO is fed back to the feedback circuit having beta as a feedback factor. Now he is going to modify the output by the factor beta. Normally beta is having value less than 1. So that the modified V input as I said here as I shown here in this case will be given as the collection of this VO and the feedback voltage. Here VIF is called the feedback voltage that is beta into VO. On left side, we are going to show the same thing here. There is a summation box here that is adding the voltage that is the feedback voltage to the input voltage. It is either in phase or out of phase. It is suppose out of phase it is called negative feedback and so on. Now these are the basic elements of this feedback network. First is the input signal. As I said this is an input given by the input source here that is going as a AC input. Output voltage is a voltage of amplifier and it has got two states. Suppose feedback is not there. It is normally called the output without feedback. I mean feedback is employed. It is called output without with the feedback here. Secondly, the feedback network. Now in this network, we are going to use some electronic components. Normally there are passive components such as resistors. And they are going to modify the voltage according to the output here. Now in this area, we are going to show two simple forms of the feedback here. On left hand side, we are feeding back the voltage as input of feedback network here. It is normally called as a voltage feedback. Whereas on the right hand side, we are going to feed the output current and that creates the effect as a feedback through the feedback network here. So it is normally called the current feedback. So that we can summarize here. The feedback can be either voltage or the current. Suppose feedback is the voltage. It is normally called the feedback voltage feedback. Suppose feedback is the current quantity. It is normally called as a current feedback. Suppose voltage feedback is out of phase with the input here. That is called negative voltage feedback. And suppose the current which creates the signal across the input here out of phase with the input voltage or the input current here. It is normally called the negative current feedback. Let us try to find out here some simple calculations. We are going to first find out here the open loop gain. Now in this condition, the amplifier is having no feedback. So input is the voltage Vi and the output is voltage Vo. So the open loop gain is normally a ratio of voltage Vo upon Vi is called as a open loop voltage gain. That is AB equal to Vo upon Vi. Let us try to find the close loop voltage gain. Now in this case, we are going to assume here the feedback is negative. That is the feedback voltage is out of phase with the input voltage. Now we know here. Now here in this circuit, Vo dash is a voltage with the feedback. So it is normally a product of the voltage gain of the amplifier into the voltage given as input to the amplifier. So here the feedback voltage is nothing but the Vo dash. That is, that is now given as the Vo dash into 1 plus AV beta is equal to A into Vi. Just see the diagram here. We are going to find out this expression easily. Now by arranging all these terms, we can say that the voltage gain with the feedback is a ratio of Vo dash upon Vi. So it is coming as AV upon 1 plus AV beta. As we know here, beta is always compressing passive components such as resistors. So when I use a very good called resistors in the circuit here, we find that the beta normally stand constant here. So we can say that the voltage gain with the feedback normally remains very constant here. And this not bending one, the supply voltage and other parameters such as temperature or pressure in the environment. So that we can confirm that with the help of feedback, we are going to stabilize the voltage gain to some constant level. Now these are some important parameters you must remember. Again we are going to just summarize here the benefits of negative feedback. The first is this increase in input resistance, decrease in the output resistance, bandwidth get increased, distortion get minimized. There is less distortion as output here. Then it is also found that sensitivity of the circuit get decreased here is a good parameter and stability in the gain is get increased. Friends, before I end here, just let's put some focus on different types of connections also called topologies that we can use in the negative feedback system. The first type is called the voltage-size feedback. Second is the current series, current shunt and the voltage shunt. I feel friends, the video is very helpful to you to understand the concept of feedback. And we hope that we are just going to carry over some more topics in the same field here in the coming videos. These are my references. Thank you friends. Thank you very much for watching my video.