 This is my fifth lecture on related to analog electronics. Today's lecture is on feedback topologies and also on oscillators. Now, the last four lectures which I had, most of them were concerned about amplifiers. Now, what in all those cases, we did not much worry about feedback or most of the time, the amplifiers which we considered had no feedback. Now, feedback as a concept is an extremely important concept. Now, we are all familiar with it in our everyday life. The concept of feedback is extremely important in all fields of engineering. Now, there is not a single field of engineering which does not use the basic concept of feedback. Now, most of the time when we talk about feedback, we actually mean negative feedback. So, generally when just like when we say amplifier, we generally mean voltage amplifier. Similarly, when we talk about feedback, we kind of assume that we are talking about negative feedback. Now, why is feedback used? Now, feedback is used to make sure that the quantity of interest to us is stable and it stays stable irrespective of small disturbances. Now, in our context, we are talking about amplifiers. Therefore, most of the time, we are interested in having getting a stable voltage gain. When you did the experiment on the BJT amplifier, I am sure that there would have been considerable difference in the actual voltage gain you would have got. Maybe within the same remote center, maybe the 10 people, if you had compared the gain, there would be at least about 1 percent variation or sometimes even more than that. Now, this is because of the reason that the common emitter amplifier which we used did not use any feedback. In fact, the second part of the common emitter amplifier, you had negative feedback and in that case almost all of you would have got almost exactly the same gain irrespective of what gain you got in the same amplifier without feedback. So, you have already seen the use of feedback and you have some practical exposure to it in the context of amplifiers. Now, it is not just engineering where we use the concept of feedback. If you look at our human body, we use negative feedback very extensively. Now, think of the simple example of lifting an object. Let us say that you want to lift a let us say a notebook or let us say a textbook. Now, before you even touch that textbook, your eye would kind of make an estimate about the weight of that particular book and then your hand goes there and touches it and the brain has already instructed the hand. There are roughly the weight of the book, but when you touched the book, it was either lighter than what it was computed or it was more than that. So, accordingly the brain would immediately activate things. Similarly, when you climb a staircase, I am sure occasionally when the staircase size, the step height is not same, very often we all fall down, either way you are coming down. Most of the time, if you are coming down the stairs and if it so happens that one of the staircase steps is slightly having say less or more height, then you would almost will fall. Why is it so? Because at the first instant when you stepped on the staircase, your brain had an approximate idea. When you put the first step, it immediately calculated the amount of length by which your right leg or the left leg should move. Now, the second step that approximation whatever it did was even made perfect. After two, three steps, your system has adjusted very well and without any even without even you looking at it, you can go down. But if it by any chance, if the last step happens to be either having smaller step height or higher step height, we would almost all of us would fall or you would have a bad hit on our leg. So, we see that our body extensively uses feedback. Now, another very good example is the our sense of hearing. Now, maybe you could try this experiment, you could just talk to sometime maybe to your friend normally and then try to do this by putting an earphone into your ear. Now, if you do that, you would see that you will automatically use much louder sound than what you use without putting something in your ear. Why is it so? Because when we speak, our ear gets a feedback. Now, based on that the ear makes an estimation whether we are loud enough or not. If we are not loud enough, then we would speak louder automatically. So, we see that we have the concept of negative feedback is extensive and it is there almost in any field and it is not just limited to engineering. But we shall talk in the context of using negative feedback for amplifiers. So, we would limit it to that. Now, when we talk about feedback, there are two types of feedback. Now, the one which we would use in an amplifier is what is called negative feedback or it is also called degenerative feedback. And this is the one which is used extensively in amplifiers. The op-amp amplifiers which we are familiar with, they all use negative feedback. Now, you also have positive feedback and very seldom we come across positive feedback. But in today's lecture, we will talk about both negative feedback and positive feedback. We would spend more time on negative feedback and towards the end of the lecture, we will talk when the context of oscillators will talk about positive feedback. Now, positive feedback very often is thought that it is something to be avoided or very often we might think that may be positive feedback should be used only for say oscillators, sine wave oscillators. But interestingly, positive feedback is very useful in the in wave shaping circuits. Positive feedback is very, very useful. I am sure you are all familiar with op-amp based comparators and op-amp based Schmidt trigger. Now, what is the difference? What is the major difference? Both of them would compare the input and give you an output. Now, we know that the Schmidt trigger has a certain level of hysteresis. Now, in addition to that, another very, very important property of the Schmidt trigger is that it uses positive feedback to give you an output which is much sharper than what you would have otherwise got from a comparator. Now, what positive feedback does in the Schmidt trigger is that as soon as you cross either the upper triggering point or the lower triggering point, the output would either go high or low depending on your configuration. Now, the output would rise as fast as it can whereas in the case of a comparator, even if you have a certain level, even if you cross that level. Now, since a comparator is used in an open loop mode, there is a small region which is a linear region because of which the output will not rise fast as it does in a Schmidt trigger. So, we see that positive feedback is also very useful and it is used again extensively in electronic circuits. So, we shall discuss both of them. Now, what I have here is a kind of general structure of the negative feedback amplifier. A very similar structure is also very often used in control systems may be one of the first things to do in a control system course. Now, so the basic block diagram is shown here essentially gives you an idea about negative feedback and how it is applied in an amplifier. Now, we have a voltage source whose output let us say is X s. Now, X s may be either current or voltage. Now, this needs to be amplified in an amplifier. So, we have an amplifier which has a gain of A. Now, the output of the amplifier is X naught or X o and that is applied to the load. Now, in a negative feedback amplifier what you would do is you would take a sample of the output and a sample of the output is then fed back to the input and at the input it is summed rather it is subtracted from the source value and then applied to the input. Now, because of this kind of a subtraction at the input point we call it degenerative or negative feedback. So, we see in this case we have two directions. One direction the signal flows from the source through the amplifier to the load that is one direction. Now, side by side we have another direction the signal flows from the output through the feedback network backed to the input into a summer where it is subtracted the proportionately it is subtracted from the input and then fed to the amplifier. Now, if we write this in terms of equations we can we have just simple three equations from which from these equations we can get an expression for the voltage or the current or just gain with feedback. Now, the output let us say parameter X naught we know is related to the input parameter X i by the gain. So, we can write a simple equation as X o is equal to a times X i. So, that is the first equation we have. So, that is always valid. Now, the second equation relates the feedback the quantity of feedback. So, what is done here is we take a sample of the output and a certain proportion of it is fed to the input. So, this X f parameter is related it is like something like a let us say a gain parameter of the feedback network. So, X f is equal to beta times X naught and again a very simple equation. Now, the third equation we have relates to the way the signal is added at the input. Now, we see from the diagram that X i is nothing but X s minus X f. So, that is a third equation. Now, if we use these three equations and we for example, write an expression for X o by X s. So, X o by X s which we will define as the gain with feedback a f. We would see that you can write the expression for a X naught as a times X i and X s we can write as X i plus X f. And then we can express X f in terms of X i and then we would get the simple expression that a f is equal to a by 1 plus a beta. Now, this is an expression which is used extensively in many many areas. Let us try to understand what this particular expression means. So, what we are saying is the close loop gain that means the gain which we are going to get with feedback a f is equal to the gain which was there without feedback divide by 1 plus a beta where beta is the amount the say the proportion of the gain parameter of the feedback network. Now, the term a beta is called the loop gain and we see that this particular a beta is an extremely important parameter especially when we talk about oscillators we would see this is a very very important parameter. Now, the denominator 1 plus a beta is called the amount of feedback and again we would see this term 1 plus a beta coming repeatedly again and again we will see later. So, 1 plus a beta is called the amount of feedback and a beta is called the loop gain. Now, one very important thing which we need to assume in all our discussions is that the loop gain a beta must be positive for the feedback to be negative otherwise all that we did will not be valid. Now, the gain with feedback a f we see that is smaller than the open loop gain by the quantity 1 plus a beta. So, before applying negative feedback we had a as the gain now after applying negative feedback the overall gain reduced from a to a by 1 plus a beta. So, we see that this one of the most important say features or the let us say the peculiarity of negative feedback. Now, in negative feedback we always trade gain the open loop gain with the other parameters of interest. Now, we will again come a come discuss this particular topic this particular peculiarity or the feature of all negative feedback amplifiers. Now, the price we are paying here is the reduction in the gain, but we will see that we will see soon that we get many things in return as we all know in engineering in any design not in any design there is always a trade off we never get all that we want. We saw that even in the previous lecture where we talked about multi stage amplifiers we said that we never have a let us say the ideal single stage amplifier. So, to construct an amplifier for an application we try to put different non ideals amplifiers in such a way that we get something what we want. So, trade off is something which we are very very familiar in engineering. Now, another very very interesting thing to note here is that a f is equal to a by 1 plus a beta. Now, if a happens to be extremely high let us say the let us say much greater than let us say 1000 then we see that this close loop gain a f will be approximately equal to 1 by beta. Now, this is something we are very very familiar with we will talk about this later when we in the case of an op amp where we would see the same equation again coming. Now, in the case of an op amp this quantity a the op loop gain is the order of 2 into 10 power 5 extremely high this is precisely why we in a op amp amplifier say non-inverting amplifier we can get an expression for the close loop feedback independent. So, one very important observation regarding what we what we written here is that if the open loop gain is large a is large the close loop gain a f depends only on the feedback network. Now, this is an extremely important property an extremely important feature of negative feedback amplifiers. So, if we want to make our close loop gain independent of the amplifier all that we need to do is to increase the overall gain. So, in any design in a design of a negative feedback amplifier this is a very very very important consideration to keep the open loop gain very high. So, that when you apply negative feedback the parameter of interest gets stabilized very well. Another very interesting observation is that since our a f is now only dependent on beta which is the feedback parameter we can say that we said as right now that the gain depends only on the feedback parameter. Now, in the case of an amplifier this feedback network we have is made of only resistors. Now, therefore, we can make the gain dependent only on the resistors which we are going to put. Now, we know that assume that you have a very precise application where you are very particular about a particular gain value. In such an application just by choosing say precision resistors you could easily attain the close loop gain and that would remain stable irrespective of even if you change the op amp say tomorrow you would see that the close loop gain depends only on the resistors which you have used in the feedback network. So, these are extremely important observation and extremely important feature of negative feedback. Now, let us look at the kind of advantages of negative feedback. Why should we use negative feedback? Let us see them in a bit detail. Now, one of the first things we talk about when you talk about negative feedback is the gain desensitivity. What we mean by this is if you differentiate the expression for a f with respect to a you would see d a f is equal to d a by 1 plus a beta the whole square. Now, then if you write d a f by a f you would see that you can you will see that it is equal to 1 by 1 plus a beta square times d a by a. Now, what this particular expression is telling us is that the percentage change in a f is much smaller than the percentage change in a and it is related by the amount of feedback. So, for example, if you are a the open loop gain let us say changes by you know 50 percent or let us say it gets doubled or it gets halved. Now, the effect of that open loop gain on the close loop gain is determined by it will be only by that change divide by 1 plus a beta. Now, this is an extremely important feature to be kept in mind and this is one of the major advantages which we just now talked about we said that just now we said that in a negative feedback amplifier we said the close loop gain provided the open loop gain is above a certain particular value the close loop gain does not depend much on the open loop gain. So, gain desensitivity is an extremely important advantage of negative feedback. Now, another very very useful and advantage of negative feedback and very often negative feedback is used for this purpose is that by applying negative feedback the bandwidth would get expanded or we can say that the we get a extension in the bandwidth by applying feedback. Now, if like the amplifier let us say the common emitter amplifier if we assume the high frequency response to be characterized by a single pole which is a very good approximation we can write the response as a s is equal to a m by 1 plus s by omega h where a m is the mid-band gain we talked about mid-band gain we said mid-band gain is the region where the gain is steady and we said in a say in a common emitter amplifier we said the this is the region where the coupling capacitors can be considered as short circuits whereas the device capacitors can be considered open circuit. So, a m is the mid-band gain and omega h is the upper 3 dB frequency now by applying feedback we can show that the upper 3 dB frequency gets extended by 1 plus a m beta now this is extremely high now this can be easily proved and one of the best ways to prove this is from the example of non-inverting amplifier and I hope we are all familiar with this with it now we talk about the gain bandwidth product being a constant now we would see that in a in a non-inverting amplifier we see that by applying feedback we would see that the bandwidth of the upper cut-off frequency increases by the factor 1 plus a beta. So, this is an extremely important use or advantage of a negative feedback and very often we use negative feedback to get higher bandwidth there are two more advantages which may not be that important, but there are applications where this may be important now negative feedback and reduce the noise in an amplifier what we mean is by that is it can it can increase the signal to noise ratio of that amplifier. So, basically what we are saying is by using negative feedback we can reduce the effect of noise generated in an amplifier now we should be very careful when we talk about noise reduction do not confuse this with as though saying that if your input signal is corrupted let us say with noise by passing it through a negative feedback amplifier that noise whatever signal to noise ratio which you have at the input of an amplifier cannot be improved. So, that is not what we are talking about what we are talking about is if the amplifier which you which you have has let us say it generates noise internally now by applying negative feedback you can reduce the noise generated within that amplifier. So, that the overall signal to noise ratio would be much better than what it would have been without applying that feedback that is the meaning what we are trying to say about noise reduction. So, this must be very carefully the distinction must be very carefully understood now another occasionally another common application or advantage of negative feedback is that the non-linear distortion can be drastically reduced now we all know that a BJT is actually a non-linear device and we said when we talked about common emitter amplifier two lectures ago we said that we have what is called as small signal approximation we said that we need to keep the signal the AC signal much smaller we said within about 10 millivolt or so otherwise we said we would get distortion and that would make the output kind of non-linear you would get distorted output now by applying negative feedback this range can be drastically improved. So, an amplifier if it had some amount of tolerance by applying to let us say non-linearity by applying negative feedback you can increase that range which means it can now tolerate more I mean non-linearity. So, this is again an important application when we talk about say the context of non-linear distortion. So, we talked about four major advantages of negative feedback we talked about gain desensitivity we talked about talked about the bandwidth extension and then we talked about the noise reduction and finally, we talked about reduction in non-linear distortion. Now, let us talk about the basic feedback topologies now this is where we need to go slowly now we know that we have four types of amplifiers we talked about that again when we talked about common amplifier we talked about four types of amplifiers we said that we have voltage amplifiers we have current amplifiers we have trans contactance amplifiers and we also have trans resistance amplifiers. Now, there are four feedback topologies in fact catering to each of these four amplified types now these feedback these four feedback topologies essentially depend on two things one it depends on the parameter to be amplified now we know the parameter to be amplified it may be a voltage or it may be a current. Now, the topology also would depend on the output parameter now whether it is a voltage or a current now each topology is characterized by the connection at the input and at the output. So, let us look at them in some detail so let us name them one by one now this is where we need to go slow I will repeat it again this is where we need to understand this very well especially even if you do not remember all these names at least you should be able to say how to arrive at these names which we are given. So, you need not memorize these names, but you should very well know how did we arrive at these names then it is ok. So, let us look at them one by one now the first of the feedback topologies is what is called a series shunt feedback now it is also called voltage mixing voltage sampling now this kind of a feedback topology is suitable for voltage amplifiers. Now, it is very very evident from the second name we gave here that this particular feedback topology is actually meant for voltage amplifier we know that in a voltage amplifier both input and output are both voltages therefore, the whatever we are going to mix at the input of the amplifier is definitely a voltage and what you are going to sample at the output is also voltage. So, this we will see why it is called series shunt feedback, but at the moment we can say that this feedback is called voltage mixing voltage sampling and it is suitable for voltage amplifiers. Now, the second topology is called shunt series feedback or a name to understand from its application is current mixing current sampling. Now, again it is very very evident from the second name which we gave current mixing current sampling that both input and output parameter both of them are currents therefore, we know very well the such an amplifier is nothing but a current amplifier. So, this is the second topology which is useful for a current amplifier. Now, you could immediately notice something interesting here the voltage amplifier is called a series shunt whereas, a current amplifier with feedback is called shunt series we will see why let us go to the next the third one. Now, the third one is called series series feedback or the other name is voltage mixing current sampling. Now, again just like the way we argued in the previous case this particular feedback is suitable for transconductance amplifier. Now, we know that in a transconductance amplifier the input is a voltage whereas, the output is a current therefore, the second name we understand very well we have voltage at the input therefore, it has to be a voltage mixing because mixing we do at the input the output is a current therefore, what we sample must be current therefore, this current sampling. So, this second name which we have given is a very very easy way to understand what is being done and from that we will derive the other name. So, this is called series series feedback or voltage mixing current sampling. Now, the final feedback topology is what is called shunt shunt feedback again you can see is very interesting series series this is shunt shunt. Now, here this is also called current mixing voltage sampling. Now, again we see that why it is called current mixing voltage sampling because we know that this is suitable for the fourth type of amplifier which we are familiar with which is the trans resistance amplifier. We know that in a trans resistance amplifier the input to the amplifier is a current therefore, we must be mixing only current. Now, a trans resistance amplifier the output is a voltage therefore, we sample voltage. So, it is very easy to understand what the topology is from the second name we are given. So, once again to quickly recap we have four types the first type is called series shunt feedback or voltage mixing voltage sampling and this particular feedback is suitable for voltage amplifiers. The second one is called shunt series feedback also called current mixing current sampling and it is actually suitable for current amplifiers because we are actually both input and output parameters are current. In the case of a series series feedback we said that it is meant for a trans conductance amplifier and we know that in a trans conductance amplifier the input is a voltage the output is a current. So, therefore, we mix a voltage at the input we sample a current at the output. Now finally, we have the shunt shunt feedback which is a current mixing voltage sampling and this is suitable for trans resistance and we know that in a trans resistance amplifier the input is a current therefore, we need to mix current at the input the output is a voltage therefore, we should be sampling voltage. So, the second name is something which is extremely useful to keep it in mind. So, even if you forget the first name if you can remember the second name you can easily arrive at the first one. Now, let us see these each of this topologies in detail. So, let us see this in detail. What we have here is the block diagram of an ideal series shunt feedback and we said the other name for a series shunt feedback is voltage mixing voltage sampling and we also said it is meant for voltage amplifiers. Now, let us look at the scheme here the block schematic here you have an input which is a voltage source V s here now that is applied to the voltage amplifier. Now, we can see a mixing done here. So, a fraction of the output voltage again is sampled using the feedback network and that voltage is added in series. We have to always add voltage in series this is a very very very basic concept as far as voltage is concerned. We can never connect two voltages in parallel if we do that I mean there is nothing there. So, that is a very very basic thing. So, this is another very easy to think to remember that if you are talking about a voltage then if you want to alter that voltage you can only alter it by adding or subtracting you cannot alter it by connecting a voltage in parallel. So, this is why the the feedback voltage is subtracted and then fed into the amplifier. So, now, let us come and understand why it is called series shunt feedback. So, we see that at the input side we have a series connection we have a series connection and we said why this series because we need to add or rather subtract the the feedback voltage V f from the input voltage. So, our V i is nothing but V s minus V f the way the polarity also indicates that. So, therefore, this is called a series connection. So, the first term there indicates the way it is mixed. So, the mixing at the input being a voltage is series therefore, it is called the first word series there. Let us look at the output. Now, we know that if I want to measure a voltage using a multimeter I measure it by connecting two terminals across the terminals of the voltage source. So, if I want to sample then I have to sample it not by connecting something in series, but by measuring it across the source. Therefore, we have a shunt connection or a parallel connection. So, if I want to sample a voltage therefore, I always need to sample that voltage by a parallel connection or a shunt connection. So, now we understand why these two words have come series word has come because we are mixing the voltage in series at the input. The word shunt has come because we are sampling the output parameter in shunt and we said that if you want to sample a voltage then you have to do that by tapping it across. Mind you by doing this we do not want in the ideal case we do not want to disturb what is going on. So, this is the just like in a multimeter if you want to measure a voltage by measuring a voltage you are not upsetting the network. So, that is the way you sample. So, therefore, the word series shunt a series mixing at the input a shunt sampling at the output. So, now it is very clear voltage mixing therefore, is called series a voltage sampling therefore, is called shunt. So, even if you forget this word series shunt it is as long as you remember that whenever we mix a voltage we need to do that in series whenever we sample a voltage it has to be done in parallel. Now, let us look at the next topology now this topology the name we gave is shunt series feedback. Now, we said the name is also current mixing current sampling and we said it is meant for current amplifier. Let us look at the over our amplifier in detail. So, we have a current source here I s and it is given it is shown here in its not an equivalent circuit. So, you have an R s here which is the output resistance of the current source. Now, we know that essentially what we want to do the current flowing into the amplifier must be the source current and from that we need to subtract a certain amount of current just like in the case of voltage the previous case of voltage amplifier we subtracted a certain amount of feedback voltage from the source voltage to get the input to the voltage. Similarly, here the input is I s we need to subtract some amount of current which is I f here and the I s minus I f is I i flows into the amplifier. Now, again conceptually think about a current. Now, if I want to change a current I can never do that by connecting in series that will be absurd. Now, I can only change a current by tapping rather you know let us say that I have a load connected you have a something a current source and is connected to some load. If I want to change the current flowing into the load I can change it by connecting something in parallel to that load and tap away or just kind of shunt away some amount of current this precisely why the word shunt is coming here. Since, we are mixing a current here we can do conceptually you can never mix current in series if you want to do a mixing of current it has been done in shunt therefore, the word shunt here. Similarly, we need to sample a current. Now, how do we sample a current? Now, if I want to measure a current we all know it very well if I want to measure a current we have to connect the ammeter in series with the circuit not in parallel we measure voltage by connecting in parallel, but we want to measure current we connect the ammeter in series. So, same way here also if we want to sample the current then we must have a series connection you have some network which samples may be a proportion of that current. So, therefore, we have a series connection at the output because we are sampling a current extremely important. So, when I sample a current think about an ammeter similarly, when you sample a voltage think about voltmeter. So, a voltmeter we connect the terminals in parallel therefore, we have a shunt connection there a current if you want to measure or sample I have to do that by connecting a meter in series. So, that is a way you can think about therefore, I have a series connection. Therefore, the name here makes sense mixing is current therefore, I have the word shunt. Now, the sampling of current is done in series just like in an ammeter therefore, the word series. So, shunt series feedback is we it is primarily meant for a current amplifier. So, I hope these norms these names which we have given there makes some sense. Now, coming to the third type of feedback topology this is called as series series feedback or voltage mixing current sampling again let us now we can understand since we are discussed the other two these two names would be very fairly simple. Now, we know that in a trans conductance amplifier the input signal say voltage. So, what do we have here we have a voltage source connected to the amplifier trans conductance amplifier. Now, if you did not have a feedback we would have connected this particular terminal directly to the trans conductance amplifier. Now, because of the feedback we have introduced a certain voltage in series and we said if I want to subtract something from a voltage I have to do that in series not in parallel conceptually that is absurd. So, therefore, I have a series connection at the input because I have a voltage signal. So, therefore, the word series here because I have a input parameter which is a voltage. Now, the output in a trans conductance amplifier we know is current. Now, we know immediately that if I want to sample a current I have to come in connect the ammeter in series in this case the feedback network must be therefore, in series. So, therefore, I have a series connection at the output now again coming back. So, we have a series feedback because we are mixing a voltage and we said voltage has to be mixed only in series therefore, the word series. Now, we are sampling a current and we said current can be sampled only serially the way if we connect an ammeter. So, therefore, the word series, series feedback and now I think it makes lot of sense that for the trans conductance parameter the with feedback the feedback topology we would call it series, series feedback. Now, we are left with the last topology which I am sure you can mentally argue it yourself why this particular name shunt feedback mind you this is a trans resistance amplifier therefore, we are talking about a current at the input. So, essentially our input parameter is a current now if we did not have the feedback network we would have connected this directly and we would not have these two wires here. So, the output is a voltage now since the input is a current if I want to subtract something from that input current I have to do that in parallel and we said that very clearly because changing a current by a series connection is absurd it does not make any sense it has to be done in parallel conceptually that is the only way you can do it therefore, the word shunt. Now, here the output is a voltage and we said that if you want to sample a voltage we sample like we measure a voltage with a multimeter or a voltmeter. So, we need to measure or sample in parallel for shunt therefore, the word shunt feedback again just to just brush it just quickly once again these four topologies. So, I hope next time when you hear this words at least it will not be too complicated. Now, feedback unfortunately feedback topologies feedback amplifiers are dreaded by everybody definitely by students is something which is supposed to be very very tough and something which nobody can understand but actually it is not that difficult if you look at it carefully and tries to understand it from a basic point of view or a conceptual point of view. So, quickly again to recap what we discussed we have four feedback topologies we have a series shunt feedback meant for a voltage amplifier. So, if I have a voltage amplifier I have to use a series shunt feedback why because I need to mix a voltage at the input and I can mix voltage only by a series connection not by a parallel connection. Now, being a voltage amplifier my output parameter is a voltage and I sample a voltage like I measure voltage with a multimeter or a voltmeter and that has been done in parallel therefore, I have a series shunt feedback for a voltage amplifier. Now, coming to a current amplifier I need to mix current at the input and I can do that only by connecting a shunt connection therefore, I have a word shunt here. Now, being a current amplifier I need to sample the current at the output and we need to sample it like the way we measure a current if you see a multimeter or a ammeter we connect an ammeter to measure a current in by connecting it in series to the circuit therefore, you have a series connection therefore, the word shunt series here or current mixing current sampling. Now, the third category of the amplifier is a trans contactant amplifier now the feedback appropriate for this particular amplifier is the series series feedback why because input parameter is a voltage therefore, we need to add or subtract the feedback voltage in series therefore, we use the word series here. Now, the output is a current and my current sampling needs to be done in series therefore, the word series. So, series series feedback is the topology to be used for a trans contactant amplifier. According to the last topology we have a shunt shunt feedback which is meant for trans resistance amplifier. Now, in a trans resistance amplifier my input is current now therefore, mixing has to be done in shunt therefore, the word shunt here now the output is a voltage now a voltage I sample in parallel the way I do with a voltmeter therefore, the word shunt shunt feedback. So, I hope to some extent these names make sense now.