 assistant professor, department of electronics, WD Solaapur. Now in this video we are going to look after the negative feedback in amplifiers. Now these are the learning outcomes. At the end of this video, students should be able to describe benefits of negative feedback. Then he should be able to specify different negative feedback topologies and he is able to analyze the series feedback network. Friends we know here feedback is one of the main concept here. Now in the feedback we are going to provide or we are going to add the part of output signal back to the input here. Now why feedback is required here? Now we know here in the amplifier gain is one of the main component here. When we design this amplifier, we expect that the gain should remain constant. But it is found that the gain is not remaining constant because of certain parameters here. For example, there is a change in temperature, the Q point changes here, the output tends to change here. It means that the circuit is not stable. So stability may change here due to some different changes in the environmental conditions here and some circuit parameters also there. Now friends in this video we are going to focus on negative feedback more. So the figure on the left hand side, you are seeing that there is a sign on the input mixer. The input is fade here to the mixer. It is having positive sign and whereas in network there are two components. There is amplifier having gain as A and there is one more component called as a feedback network having the value beta here. The output of beta network that is output of feedback network is added in the negative fashion with input here. That is it is out of the phase here. That is why it is normally called as the negative feedback. So as a summary here in the negative feedback we are going to add the part of output signal out of phase with the input here. Now these are some of the benefits of negative feedback. So with negative feedback we always found that the input resistance get increased here is quite required quantity. It is beneficial quantity here because for a good circuit it should possess very high input impedance. Similarly the negative feedback is also going to reduce the output impedance here or resistance we can say. There is a increase in the bandwidth then distortions are get decreased here. Then it is found that with negative feedback sensitivity is also decreased here. It is quite a good factor to be decreased here because we want to make circuit non-sensitive with certain parameters here. It is found that gain is more stable with the feedback in the circuit there. Now the different types of feedbacks are possible here. So normally it is called as feedback topologies. Topology means it is a mechanism by which we are going to connect different components here. So there are four main types here. These types are depending upon the input signal and the output signal here. That is we are going to feedback the output signal back in the input here. And the way we are going to feedback this signal will give us different type of topologies here. So we are listing here the four main types one is called the voltage phase feedback. It is also called as the voltage amplifier because the gain with the feedback is taken to be VO upon VS. Where VO is the output voltage and VS is the input supply voltage that is the signal voltage we can say. Second is the voltage shunt feedback. In this the feedback with the in this case the feedback is the current quantity. So the gain with the feedback is defined as VO upon input current here that is the signal current here. This also is referred as the trans resistance amplifier. We get one more is called current phase feedback also termed as the trans conductance amplifier. And finally we get current shunt feedback also termed as the current amplifier. So all these terms are showing you how we are going to feedback these voltages back to the input here. Now let us friends we are going to focus in this video more about the voltage phase feedback. Now in this case the RMS shows very clearly we are going to feed the output voltage which is across the load here. As input voltage to the feedback network called beta there. The output of this beta network is taken to be the voltage VF is normally called as the feedback voltage here. Now this voltage VF is defined as the voltage VO that is across RL multiplied by the beta factor that is of feedback network. Now this voltage is fed back out of phase with the input signal here that is VS. So that the effective voltage across the amplifier input is coming as the VI. It means that the voltage VI should be equal to VS minus beta times V0. Now friends just try to analyze the voltage feedback network here or the circuit. In this case we have first tried to find here the voltage gain with the feedback in the given amplifier. Now from the given block diagram we can define the voltage VO is equal to A into VI where A is normally called as the gain without feedback. Now we know here the VI is defined as the voltage VS minus VF and we know here the voltage VF is equal to beta times V0. Now we replace these values in above equations here so the voltage V0 is defined as A into in the bracket VS minus beta V0. Now when we arrange all the terms here and we define this voltage gain with the feedback is ratio of voltage VO upon VS. It gives me the equation as A upon 1 plus A beta where A is the open loop gain that is again without feedback. And beta is normally called as a feedback factor. Now friends we know here so the gain with the feedback is get decreased here because we get A is an open loop gain. And we divide this gain by a factor called as 1 plus A beta. So certainly the voltage gain with the feedback get decreased here. Now let us try to find out here two more parameters which are important. One is the input impedance other is the output impedance here. Friends just see on the left hand side of the figure. Here the ZIF is called as the input impedance with the feedback is defined as the voltage VS that is the input voltage upon the input current that is II. Here the feedback voltage is given as beta into V0. Let us try to find this derivation now for the input impedance with the feedback. The ratio of VS upon II. Now we know here the voltage VS can be defined as the sum of the voltage VI plus voltage VF. And we know here the VF is defined as beta times V0. So when we arrange all these terms we are going to get a modified expression as ZIF is equal to ZIF into 1 plus A beta here. Or otherwise we can say that the input impedance with the feedback is equal to 1 plus A beta times the input impedance. So we know here friends the value of this A1 plus A beta is always more than 1. So we can say that the input impedance with the feedback gets increased here in this case. Now try to focus on one more parameter called as the output impedance with the feedback. Which is the ratio of the voltage V output across the load upon the output current that is I0. Now friends in this case we take one condition to find out this output impedance with the feedback. We are going to make this input signal voltage VS equal to 0 volts here. Now we know here from the given diagram we can define this I equal to the voltage V0. The voltage across load minus the voltage source here is A into VI. Now friends in the diagram we are replacing transistor by its equivalent circuit here. So that this I0 is coming as this one that is V0 minus A into VI upon Z0 here. And we know here the VF is always is a feedback factor here. So we can define this VS equal to VI plus beta times V0. Or we can say here we are going to make this VS as 0 here. So the value of VI is coming as minus beta into V0. So we put this value in above equation here and we rearrange all the terms here. So the value of this output impedance with the feedback is coming as Z0 upon 1 plus A beta here. So from this we observe that the output impedance with the feedback is equal to output impedance without feedback upon 1 plus A beta. So friends we are going to summarize all the things together here. They are the properties or features of this voltage series feedback here. We found that the voltage gate decreased here. Then we increase the input impedance with the feedback. These are output impedance gate decreased with the feedback here. One more parameter called as a GBW is called gain bandwidth product here. It is always constant for amplifier. And we know here in the case of voltage series feedback the voltage gain with the feedback gate decreased here. So automatically the bandwidth gate increased here which is the very welcoming term here. Friend this is a practical example of this voltage series feedback. You see that it is an amplifier in which the output is taken across the resistance across emitter here. That is RE. Now we know here the output is across RE. And if you see carefully here the input to the amplifier is between base and the ground. And the voltage from base to the ground is nothing but equal to it is sum of the voltage VBE plus the voltage across RE. And we know here the voltage across RE is also the output voltage here. It means that RE is a component is common for both input and output here. So this gives me the feedback component in the circuit. So it is the example of this voltage series feedback. In the next video we are going to analyze this circuit in detail. Friends these are my references.