 Now we were looking for a word feedback and we said a feedback amplifier which is of interest to us. We have an amplifier which is say as a gain of AOL and it is called open loop gain and it gives you a output of X0 when an input of XS is given or XI is given and we also said that the part of X0 is returned to input and mixed with the signal to form the new XI and if XF is in opposite phase to XS that is the input signal is reduced by XF feedback signal then XI will reduce and if XI reduces X0 will reduce. And if X0 reduces the beta X0 which is the feedback signal also reduces so XS – XF will now increase and get a new XI which when amplified by AOL will get a new X0 is slightly larger and this feedback will finally settle to one XF XS and X0 values so negative feedback essentially stabilizes the gain X0 by XS this is the purpose of any feedback amplifier I want to stabilize the gain fixed gains so if there is any noise or anything which is putting in this and if I have a feedback ratio then I will see that the gain stabilizes that is the purpose of negative feedback that is what we started with we looked into few few things we talked about A beta we talked about this we also looked into few properties of negative feedback one of them we said about gain desensitivity and we said the closed look gain is A0 upon 1 plus A0 beta. Now please remember if you are not working with Cedra Smith many books this value will come X0 1 – A0 beta do not get worried too much what essentially they are saying that the expression which I derived in getting this expression I said XI is XS – XF so already thought thought it is a negative feedback what they are saying is they are actually giving it this plus okay and because of that they will get a 1 – A beta values there but essentially they are saying whenever XF is negative put a minus sign correspondingly okay so beta will become minus and 1 plus A beta will again become as is there so that is the trick of the trade so if some books you are following in which they are writing 1 – A beta they are essentially using this plus with no sign of XF right now but if it is a negative feedback it will subtract and if it subtracts then that 1 plus A beta term will appear okay so we said that if we have a feedback network the closed loop gain with feedback means the change in such gain with reference to his initial gain will be proportion to 1 upon A0 beta times A0 by D0 that means any change in this the closed loop gain will be much smaller by 1 upon A0 beta factor that means it is desensitized the gain variation that is what I was saying stabilizing the gain DA0 by A0 is the sensitivity of A0 if this sensitivity has to be reduced and if I divide by some fixed factor then the closed loop gain essentially shows less sensitivity compared to A0 that is the purpose of negative feedbacks this is what we did last time we also said by same logic we figured out that if there is a bandwidth and if there is a pole mid band upon something like this and we finally said in a closed loop gain the bandwidth will also increase by 1 plus A beta times omega 0 that means the increase of bandwidth was essentially occurring because of what because the closed loop gain was lower than the open loop gain AOL was higher than ACL we reduce that by 1 upon A beta factor and because of that by same factor the gain bandwidth bandwidth also increase is that point clear that the bandwidth increases by same ratios because what is what why it should be of same because gain bandwidth for an amplifier will still remain constant list transistors are changed is that clear so since we are not changing anything else so if the gain reduces by 1 upon 1 plus A beta the bandwidth will proportionally increase by that much amount this is the trick of increasing the bandwidth whenever you are looking for a larger bandwidth amplification where do you think you need larger bandwidth amplifiers for example of course these are not these are not the amplifiers which we use there but let us say we are using a mobile applications mobile systems in which there are amplifiers of course they are at very high frequency so is television signals both video and audio they are at 62 megahertz to 67 megahertz 5 megahertz bandwidth in case of GSM it may be almost around a gigahertz bandwidth so which means I want an amplifier which has a large bandwidth but I may not have larger gains at larger bandwidths this has to be and so in many cases I do not really need gain of 100 I need gain of 10 15 so but I need larger bandwidths so feedback is one way to improve the bandwidths correspondingly reducing the open loop gains okay this is universal and there is nothing to do with any of the kind of amplifier everything feedback always reduces the gain or negative feedback I must clarify negative feedback always reduces the gain why positive feedback what will do in fact it will gain will start enhancing or the output will start enhancing because every time you give a feedback input rises so output rises part of reason output is further feedback at the larger feedback so further output so it is called growth functions okay so now you can understand if I have a system in which I have two kinds of feedbacks one which is negative feedback one is which is positive feedback and I adjust their feedback such that the net feedback is 0 such systems will be called oscillators we will see what exactly are oscillators okay so it must grow but it should not grow randomly so it should reduce something and that kind of system will follow a law which we will see use using this theorem itself later called Barkhausen criteria okay and we will see if the Barkhausen criteria has not violated or is what is violated oscillations dance down okay or infinite growths can start in either case we also I think last time showed you there are four kinds of possible feedback amplifiers the mixing is the word use at the input that is the feedback signal mixes with input signal and sampling is the word use part of the output is feedback what is the part coming from there okay that part is essentially called sampling I have a output part of it is feedback okay so that is called sampling so I can either have sampling done on currents or I can have sampling done on voltages same as I can have mixing of voltages at the input source or I can have current at the same node which may also mix with the current at the input essentially there are four possible amplifiers in which the input signals and feedback signals are of this kind output is of this kind and mixing can be series or shunt depends on the way we define will show you that little later and these amplifiers are also therefore classified as voltage amplifier in which there is a series mixing voltage is series mixed okay and the output is sampled also voltage wise okay and therefore it is called voltage output divided by voltage inputs such an amplifiers are voltage amplifiers please remember all amplifiers can be called voltage amplifiers in the end why even a output is the current if I put it across a load resistance it will create a voltage at the output so do not think that I am meant by current amplifier means only I am looking current I am saying current will enhance and correspondingly IR drop may also enhance but the gain essentially I will define only with the increase of water relationship I am taking so one will be current amplifier one will be if the input is voltage and output is current then we say it is a trans conductance if the voltage input is current and output is voltage this is a trans resistance amplifier so we four amplifiers I repeated the saying that all amplifiers can be current amplifiers or voltage amplifiers per se that is output to input ratio you can choose anything you like but the basic amplification is voltage to current or voltage to voltage or current to voltage is what we are saying by these four terms we will see this now when we start having an example we said there can be four possible mixing and sampling here is two kinds of mixing we showed I think instead of this I can directly show you the better ones I mean they are separated now I mixed both of them so you can see I have in a voltage amplifier which has a voltage gain so if I it receives an input Vi please remember it receives a input Vi here okay and the output of an amplifier is V0 output of an amplifier is V0 this is an input this is an output so if there is no feedback what will be the gain of this transistor this amplifier V0 by Vi is always called open loop gains is that correct V0 by Vi is the open loop gain is that what clear to you because feedback has been taken away then we say whatever is received at the in this case what may happen VS may become ratio of RS by RI plus whatever into Vi but Vi is the real input to a transistor and therefore the gain is VS by a VO by Vi okay and that is called without feedback whatever gain occurs we say it is open loop in the feedback one can see from here I have taken taking a voltage across RL okay these two terminals are voltage I am picking voltage across the output load RL and I am go passing that voltage across a feedback network which has a gain factor of beta since it is a voltage it will give beta times V0 will be output somewhere here beta times V0 will be output here which I defined as VF beta times V0 is the sampled output so the what kind of sampling I did here this is my V0 and I am putting a shunt across across the load I am picking the voltage so this is a shunt connection what is series connection series to the this will be series connection across is shunt connection so I am picking across outputs so it is a shunt connection here so it is called shunt sampled is that correct this has shunt sampling this receives a voltage VF which is beta times V0 and gives you a series voltage to the signal VF is equivalently saying there is a voltage source here as if here in series to the signal is that correct so how I am mixing series series mixing I am doing and since I am doing a series mixing and I am putting voltage plus voltage minus or plus sign correspondingly then I say it is mixed series way at the input is that clear so input is also voltage output is also voltage feedback is also voltage such amplifiers will be voltage amplifiers is that also called series shunt amplifiers what are these called why it is called series shunt the mixing is in series and sampling as at the output is shunt so it is called series shunt amplifiers okay if I am looking for shunt series amplifier then I do little trick I say okay the series connection at the output can only be taken if there is a current which can be input in series voltages there is no voltage source except coming from the amplifier so what we say if there is a current amplifier use output is I0 passing through RL in normal case this will be connected okay so what will be actual voltage drop to you I0 minus I0 RL minus is because of the phase shape phase shifts so V0 is still output if you need is that clear but what is real output for this amplifier I0 I0 is the real output of a this amplifier so I0 by II if I use it then I say it is current amplifier okay but the feedbacks we start looking if this is my current and series to this part of this current is fed back to a network which gives me beta times I0 as my feedback current now this current I connected to a node this node for example then I say if I have to connect it at a node then I say it is a shunt any node what is shunting across at any node if you put a current you say you are actually shunting at that point is that correct shunt means what between two nodes so I am adding a current and ground is other side so I am actually putting a current at a node and I say it is getting into that okay so this you can see the way I have fed I have connected I which is the path for feedback current it goes like this and comes like this is that correct it goes like this and comes like this okay so this is essentially we say shunting the current at the input remember what is being entered here current this is a current source I am entering a current here that is getting modified by this current source which is feedback current okay that what is the II current will be here can you tell me now if this is opposite phase IS-IF will be the actual current II flowing inside the amplifier is that correct if this current is leaving this current is entering so this current is IS-IF is whatever is going to the amplifier so if IF increases what does that mean that is feedback is larger what that means II is smaller if II is smaller I 0 is smaller if I 0 is smaller IF will become smaller next time if IF reduces II increases so I 0 increases and this will in a loop kind it will start reducing slowly slowly and finally it will settle to a value in which I 0 by II is the gain of the amplifier and fixed IF will start is that correct this is the purpose of negative is that point clear what is negative feedback if it about it reduces if it is below it output shifts okay is that point clear why it stabilizes if that current increases then the feedback will reduce it negative feedback if it reduces the feedback is lower and therefore input rises so output rises so it settles itself to a value which is essentially called shunt inputs shunt currents here and currents are series wave picked up okay from the output such amplifiers are called shunt series amplifiers by same logic we can do the other two this is very interesting both two you can see from here this is scratch part I am taking a same sampling current sampling that is I 0 amplifier output is what what is output of this amplifier current I 0 please remember I keep on keep on telling I 0 into RL is going to give you V 0 anyway is that point clear to you so voltage can always be picked up by any current outputs okay but this amplifier is essentially has I 0 and II is the current flowing here sorry voltage is the voltage across this amplifier is VI so what we are now saying that whatever is I 0 is proportional to VI by what factor what is the gain here trans conductance current by voltage so such an amplifier is a trans conductor output is current input is voltage so current by voltage is trans conductance how input is voltage output is current so we have a trans conductance amplifier and it is given by term GM okay then our case we keep calling a everywhere but as a a can be GM a can be RM a can be current AI it can be AV but in our definition of feedback we keep a constant okay is that point clear how many is can be a can be AV a can be AI a can be GM a can be RM but essentially there are four separate amplifiers is that correct now in this case beta this whatever is voltage across I am creating so network is such that I put a series voltage in series to the source then whatever this voltage is negative to this correspondingly VI will change and correspondingly I 0 will change part of it feedback gain is a negative feedback circuit input is that series output at that series is that correct current is also in series and voltage is also in series this is called series series amplifier or also called trans conductance amplifier by same logic both side are shunts this is voltage this is current current voltage divided by current is resistance so these amplifiers are shunt shunt which are essentially called trans resistance amplifiers is that correct so feedback allows you to create four possible amplifiers okay now why are we want to do because many of the circuits which we have done earlier without knowingly that there was a feedback you will see suddenly for example is source follower or an emitter follower which will take an example later which kind of amplifier do you see this it is a voltage is being sampled and voltage is in series to the input itself RE is common between the whole output and input part of the voltage is picked up across RE may be one here because the same is fed back beta can be unit in that case so whatever is received at the output is also a part of input RE is common to both sides then series of that so we say it is a voltage I mean you can see it is a series at the input and what is that the output series so we can similarly if I have a capacitance or a resistance between gate and the drain is that correct that will be a shunt because output is voltage but input is at that a node so we say it is a trans conductance amplifier sorry trans resistance amplifier so each name we may not have given there but the amplifiers we already seen is that correct whenever there is a connection between output and input by any impedance whether it is resistance or capacitance or RC networks these will form a feedback amplifier is that correct the nomenclatures we are now defining with the word feedback Bada Nam Deke as an idea we are already done those amplifiers like for example the common source amplifier common source amplifier with source degeneration the feedback is from where from the node side is that correct so the and but the feedback is in from this side you are feeding it back so one is current and one is voltage okay so now that we are already done those amplifiers okay but we are never named at which kind of amplifiers we are using because there we always say we are looking into voltage by voltage all amplifier here also we can look as a voltage by voltage after all a current source is equivalent to a voltage source and a series resistance in feminine a Norton source is equivalent to a feminine source so essentially we are not actually trying to say we are done we are doing something great all that we are saying we are doing more generalized way of doing it and all other cases are more specific cases of that okay that is what you have to say but the advantage what we get probably there we did not do you remember in common source amplifier with source degeneration or common emitter with emitter what is the gain the load divided by the degenerated resistance typically all other factors are small so R R RL by RE or RL by RS that is what we said what is the advantage we said there the gain has reduced but it is independent of transistor stabilized the word we did not say very specifically that time stabilized is that correct yes feedbacks stabilizes the gain and gain reduced is that correct so please why I am bringing this fact again again just to tell you that we are already done most of the amplifiers in some way or the other many of them where feedback amplifier and that part we did not care because we saw the Kirchhoff equations is that correct if it is a feedback so do no car though equation like the so we did we actually took care of feedback without actually telling it is a feedback whenever between drain and gate we connected it was a feedback output input was connected or between collector and base we connected we were connecting it is that correct so without telling you that this was a feedback amplifier we saw as a Kirchhoff law without thinking because then I do not have to think anything so what is this feedback theory is trying to tell I need not do all that analysis for all of them I can do something interesting I can calculate the open loop gain by and show how to get it I can calculate a beta for that okay and then I directly cannot a beta upon 1 plus a upon 1 plus a beta is feedback gain and I can find input impedance and output impedance okay that is something what I want to do quickly once I know that but otherwise even if you do not know any feedback we were any way solving the circuit is that correct so do not think that this is trying to do something over and above that it is only trying to generalize the same thing which we did earlier okay their individual case we solve here is say you do not have to solve you particular case is a specific case of this case okay so this is what I keep telling you that the books do not want to tell this they have a separate chapter telling as if this is an individual extraordinary work we are doing no it is not another thing which feedback provides us is something called a word which we call non-linear distortions we have to tell something in non-linear distortion meaning you have first let us forget distortion linear and non-linear cat or movie if I have a function fx and if it is linear it may have a coefficient a 0 okay linear system but if I have a term it has become non-linear is that correct higher terms the orbit that is non-linear it will keep on increasing okay as we could keep term for term but if x is your signal sinusoid signal let us say vi is a sin omega t so what is vi square a square sin square omega t sin square omega t goes over expand that sin 2 omega t is how much 1-2 sin square omega t 2 omega t is that correct sorry it is either way so what does that mean square terms will be give what what turns to us the omega t terms is coming from the first one but 2 omega t terms will coming from the square terms okay and higher order terms will give higher order free these are called harmonics okay omega is called fundamentals any number proportion to this omega 2 omega 3 omega these are called harmonic but whenever harmonics are present output will also show you corresponding those frequency outputs V0 will also have a components of 2 omega t 3 omega t apart from omega t omega what does that mean in real life I will see to it that I receive power only at omega my fundamental and I do some what we call tune circuit there which will pick up only signal at omega but what is it means then that the power which has or energy which has gone to 2 omega 3 omega or n omega I have lost so the efficiency of an amplifier will keep reducing as nonlinearity starts increasing is that correct because you will actually not get equivalent voltage if you have got a linearity some part of it is as if is lost by me is that correct so if you see this is what theory I was telling and you can now see very easy there is a V0 Vn characteristics from some value say here to here 40 to 40 volt millivolt it is linear output is minus 5 to plus 5 yeah it is not necessarily this is from open so one typical value that shows from an open so we say let us say this linearity has a gain of 100 any any gain can be assumed just for the heck of it so if Vn is less than 40 millivolt or greater my actually minus 40 from here to here one can see that output will be proportional to Vn 100 times Vn is that correct but if you look for little term which is ahead of this where nonlinearity has started it is not maybe that does not cube terms are smaller there that is a3 a2 coefficients are smaller but at least square terms have been up here because not linear so second term so if we say V0 is 100 within this range plus minus and the second term will now V0 at least I will be a quarter of guys across square with a quite for a kind of Vn minus 0.04 square and as I just now said when I square the inputs I will see in outputs also 2 omega equivalent terms now so we say from 40 volt to 60 millivolt we may say only second harmonic appears that is the dominant other frequency than omega is 2 omega but if we go beyond 60 millivolt the gain is really becoming flatter now which means there will be more cube and maybe fourth order fifth order terms will also start appearing that means gain will start reducing drastically so beyond this we may say voltage become fixed to 5 volt maximum does that occur in opium if opium as a power supply of minus 5 plus 5 it saturates at plus 5 or it saturates at minus 5 so I just what I said that typically opium characteristics are chose to explain my term so I want to see that if there is a non-linearity how much non-linearity is tolerable to be so we want to see since our swing is now 40 millivolt only minus 40 plus 40 but I have a signal which is 100 millivolt then what will happen where the output will go fix 5 volt nothing else can be done now okay but I want actually linearity there so one method of improving the linearity of a non-linear system is to give a negative is that correct so what is the advantage of negative feedback will be seen if I give a negative feedback but at what cost I will get it lower gain if I do if this is my normal amplifier 100 okay I have reduced the gain 10 times okay but what did I achieve the input signal range has now increased by 10 times okay 450 or 400 millivolt to 4 so large signal amplification any karna hai to kya karna padega feedback amplifiers use karna padega so do you know now I have reduced non-linear distortion in this why what do what do I why do I say I have reduced now because now up to even 450 millivolt or minus 450 millivolt there is no non-linearity in that range then we say there is no distortion in that range is that clear if the gain a linearity is up to higher values of signal there is no non-linearity so by actually providing you a feedback what did I achieve linearity was improved by me by order because the gain I reduced by an order is that correct so this is another advantage I see whenever I have a large signal amplification desired what does that mean where does this large signal amplification will be required first stage of a normal amplifier may not be a feedback amplifier up ne paile 5 millivolt or 100 ka gain hai to 500 millivolt output as a input or be come karna padega but next samyoh aapka 400 millivolt output aaya hai first stage se to next stage ko to fir wo karne hi the go to saturation me paile but I want to amplify still this signal which is higher voltage so what should I do then I put a feedback for the second stage reduce the gain but still boost that is that correct by increasing my linearity range to higher input range I have now been able to actually have a two stage amplifier with a larger outputs available for inputs which are larger now okay this is the advantage of having non-linear distortion reductions is that correct yes saturation is the ultimate now it may be 100 polynomials non-linearity or 1000 polynomials is that correct but even at the when the linearity starts bending down and signal output based on bending there is a non-linearity term has appeared and that means the first order term case I second order terms are there or third order so at the age also if you have a normal amplifier TK your chalice ke bhajaya 45 tat nahi dega usse kya farak bala mujhe tha 500 chaiye tha oh this is just to give an example young man wherever it will start bending and why so you say why feedback 10 I am only trying to say you that reduce by increasing the if I want increase input range I can actually put a feedback to reduce the gain and then increase the linearity this is the purpose that means at least in this range linearity is maintained by me therefore non-linear distortion does not occur in this case in this case even here itself I have a non-linearity starts okay so my input swing is very low so if my input swings are higher than what do I do and I want linearity normally in what we call tuned amplifiers which are different kinds we are just now looking for amplifiers called class A but if you are looking for amplifiers like class C or class D or class C class F we are not interested in linearity any way there is that clear since we are not top please remember there are class A class B class C class D class C class F 6 kinds of class of amplifier also exist all class A amplifiers are essentially linear amplifiers is that correct output is directly proportional to inputs so all the amplifier so far we are talking is class A all that we said if I want in a class A amplification in which I do not want distortion I can put a feedback is that correct but not every amplifier need to be worked there is also another in between called class AB okay so I think maybe at the end of the day I may show you quickly what are these classes and where do they use or radars for example will be using either the class C if it is sinusoid if it is a pulse transforms then you will have class D class C or class F amplifiers like all mobile systems mobile transmissions which is running at 890 gigahertz to above or Bluetooth running at 2 to 4 gigahertz there we are using class F class C amplifiers is that clear but there we are not worried about we actually using nonlinearity is that correct so it is not my contention that nonlinearity is always bad is that point clear to you what I am saying in a class A amplifier if I have to improve the input range for this distortion free input then I must put feedback is that point here so having told you this let us do something more okay so here is something which I will like you to so this is a graph which is called signal program okay this is only shown for this circuit but can be shown by any circuit what is say this is my amplifier okay this is my signal in VS input to a 0 the output of this is some V in and from V in to input of signal we will say why it is V in dash a hat then there is the amplifier is 0 and then this another term we say let us say all distortions are taken care as a noise at the output okay all distorted outputs are taken on the equivalent of a VD noise at that point so we say whatever is coming from amplifier will be added by this VD is that correct but if you have a feedback here the directly the it will come as a VD but if you have a feedback like this one can say V0 is not only AF VS but plus VD times divided by 1 plus a 0 beta you can do this analysis as we did earlier okay so what does this essentially means now the noise factor is actually reduced by 1 by 1 plus T factor is that correct gain of course has also reduced doing so earlier it would have been a now it is AF but by doing that I have actually seen that the distortion outputs or noise outputs also get reduced by 1 plus a beta times so one can say this term being very very small output is proportional to input that is what the graph I was trying to show looking at the PS to win there is no any network why this one has been put here because there is a direct transmission here okay but if there is a RS or some combination this itself will have some gain there less than one but RS upon something by this so this yeah say yeah I need to have it again one those go gain lick the one okay we also right now don't tell you but why I want hats I will come back when I draw feedback full amplifiers from V in to V in hat there is no change it's same connection so we say it has also a gain of one V in hat goes through an amplification of a to get to a V0 at this point we have a distortion through a sigma and we say all of it is connected at the output if there is a network here also it may further reduce like in feedbacks so here is V distortion and all of this through a gain of beta is fed back to be is that correct yes I say we say V in I am V in say V in hat I am V in hat say V0 ka pahla wala I am here pari VDV mila or yeah yeah say output through a gain of beta is returned to this win whatever I am showing here this as a gain of one this as a gain of beta times whatever it is okay so we say whatever amplifier we say we can represent that circuit by a signal flow graph is that correct is ka aro kesa hoga another thing there is a arrow you have to show everywhere what does that arrow means signal path going from where to where from this node to this node signal is going in this direction so this node to this node signal has gone from this direction okay from here it has gone to V in hat with a gain of one so it has gone to this node which is V in hat node by gain of one so here also I must have an arrow from V in hat there is an amplifier to go to V0 so I come to V0 which has a gain of A at the same point I have a distortion input with a gain assuming here say yeah one is same input where I so it is this point and from here I am feeding back beta gain kesa so any amplifier circuit can be represented by signal flow graph is that correct so what about you exam the problem not exam but they may not show you all that they say okay this is the signal flow graph for the system design and this and this for you okay but essentially it is this system please remember there is nothing great in signal flow graph okay all that it does it is larger such a blocks of circuits can be represented by only lines and they can also represent the function going through these are called signal flow graphs is that correct now why do we are why are we interested in signal if not trivial we need a shift the kind of a naked one so what the guy on the graph but you will know it since I have a graph I can manipulate it better than anything okay and then I can my solutions will be faster and then only I had to solve smaller parts okay so signal flow graph do help us at the end to give a smaller circuit solutions okay therefore signal program theory is relevant okay the most important thing in a feedback amplifier is feedback gain but that is smaller value we are already said a by a b 1 plus a beta gain reduced but doing this do we achieve something else or do we lose something I call it achieve or call lose so we say okay the other two parameters of interest in an amplifier one is gain of course also a bandwidth right now please remember I am not talking of bandwidth part this will do separately what band bandwidth we did say that bandwidth increases okay but we will actually see Bode's plots of feedbacks and how do they change okay but as said right now we are not talking of bandwidth so what two other parameters of an amplifier of interest to us is the input resistance and the output resistance so we say start looking into it because of feedback does it happen differently than so we figured out yes feedback has also influences on the RI and RO both sides around both sides so we see why what are in four cases which we had two series one series mixing shunt mixing series sampling shunt sampling so all Charo case may RI RO we like to see okay so we said okay let us look at this is a normal amplifier okay for the simplicity right now I I may neglect RS we can always add this vs can be modified by something into this and use the new source feminine source so right now we say vs is the source and this is beta times V0 this is series mixing what what more thing I am talking about series mixing voltage is in series to the signal and the input input resistance is seen before the feedback that is after the source after this is RI will be seen please remember RS is not part of RI is that correct RS is not part of RI so whatever voltage actually I am looking at here is I am calling vs is that correct though it is it may be ratio of something but essentially I am saying because I am only interested here okay so this is my source this is my feedback which is reducing vs because a negative feedback and I want to know what is the input resistance here without this feedback how much is our input resistance RI there is nothing else if I put a voltage source there we buy there is nothing to see about there is only one resistor sitting here if I see it only I see that nothing else I see okay is that correct but something else I may see this RI may include the bias resistor network or any other thing which you say may include in this RI but for me it is only RI is that correct so I am seeing here RN is only RI but if I have feedback then I want to know I say AF which is the gain which is V0 by vs AL by A beta which is this is our feedback gain for an amplifier we say RI is vs hat that is the voltage at this point okay divided by II which is my in the first case this is vs hat I put it so it is RI with the new one I find RIF is vs divided by current is that correct what is the resistance seen here voltage divided by current they go fits it is that correct so vs divided by II is the resistance seen by the source into the circuit is that correct but now I see this II current it is not same as it is now getting as if modified by feedback if that is not there II will be same as the old current okay but if there is a feedback I now see II is nothing but VI divided by RI is that clear VI divided by RI but this I know II have vs divided by VI vs divided by VI now we say vs you can see here VI plus VF is vs this this voltage drop plus this voltage drop is this vs is this voltage plus this is that correct so VI plus VF is vs which is essentially VI and what is VF beta times V0 but what is V0 a beta times VI so we say vs is nothing but 1 plus a beta times VI substitute here so I get RIF which is the feedback resist input resistance as RI times 1 plus a beta or 1 plus T so it is series mixing head to input resistance budget negative conceptually cab also take your brother what is the current in the circuit decided by the voltage divided by a resistance is the current in the feedback the voltage reduces that is why negative feedback will voltage reduces across this if it reduces the current also reduces resistance we have to fix the kind of so what essentially feedback is doing it is reducing the current in the input mesh is that correct and because of that the input resistance is is that physical work here because that reduces current reduces that means the net resistance increases and the feedback essentially is telling the input resistance of a series connected at the input mixing will always be higher than conscious circuit may have me a arrive jada dekata kisi be in emitter circuit may be resistance hey okay though input make it not be a guy with a bipolar transistor a though beta times are I will be reflected inside is that correct beta times are I will be reflected so r pi plus beta times are I are II will be the actual input seen by the transistor is that clear what does that mean input resistance increases whenever R e is present unbiased common source or common emitter amplifier with source degeneration or emitter and so will increase R I is that correct will also increase R I so whenever there is a series mixing where the input voltage is modified by another feedback voltage the resistance at the input will always increase so you can do it but where do you need your input input resistance be higher generally amplifiers have smaller R0 is that correct now if I have a smaller R0 and I am feeding it to a next stage which also has equivalent resistance there so what does that mean in terms of maths if I am having a circuit whose R out of one is lower and I have a next stage whose R02 in if these are comparable part of the current is going here part will go here okay what is it called essentially the resistance will actually reduce and therefore the voltage at the input will not remain voltage which you are feeding is that correct so what should be ideally R02 should be much greater than R01 then what will happen all the current is only here so this voltage remains constant so the next stage sees same output of a first stage otherwise it will get modified and what does that word in other circuit we call loading the next stage starts loading the first stage is that correct so the first stage amplifier kebar bhoot baar kya lagana patta next stage buffer buffer and a RE unbypassed or RS unbypassed so it is input impedance beta times again times input maya so it will be larger there so any source now coming from there will not get modified for the next stage is that point clear that is why buffers are required is that correct what is the advantage of buffer other than the input resistance match its output is still low okay so if you have a circuit which requires load next which is lower this you have a RO lower at the same time you are matching with the first stage is that correct that is why it is impedance transformer as we can say higher impedance connected to a lower impedance is that and what is the gain of a such stage normally buffers unity gains because a common emits source follower or emitter followers okay is that clear to you always a emitter follower or source follower is kept whenever impedance need to be matched between two individual stages which may buffer longer okay is that clear so this is the technique we use everywhere so this is why we say we want to reduce this effect of loading we need to know some kind of impedance modifications is that correct in some circuit you may require RO reduction or RO increase we can do all four we can increase RI decrease RI increase RO decrease RO we are all pole possible oh there is no hand the hat was done here because you know you would have asked me is it vs here this vs okay call it here no actually we are not what we are trying to say whenever I calculate input impedance I do not consider real source and it is series resistance is that correct to why I am vs hat is that clear I am not saying RS is 0 because that is something tomorrow we will say sir I have never said RS 0 there RS 0 any signal source you put there is a series I cannot avoid it any voltage source will have a series resistance is that clear logo 50 almost okay okay if I do shunt mixing at the so at the input please remember shunt what kind of mixing can be done by shunt not by voltage voltage cannot be shunt across a voltage source across the voltage source do not run out of the cash on current sources so this is my current source is and this is the feedback IF and I am putting an RIF here okay and I want to know what is the RIF now I know IF is beta times why I said X 0 generally because I am not seeing either the at the output sampling can be voltage or sampling can be current but that the input it is always shunt is that what clear to you input is always shunt but output is always a can be voltage or series or shunt either case so many don't know what you mean earlier so is that point clear it can be V 0 or I 0 either of them can be X 0 but I have made a generalized this because it does not matter I X 0 I say be nickel jagam is like it does not matter whether sampling was shunt or what we are looking essentially input side up to input may mix out a way important for gain Kelly up to which number pattern so X 0 is 8 times I I now as I say it can be either of kinds okay what is II current IS minus IF is II which is entering RI IS minus beta X 0 IS minus a beta II 1 plus a beta II IS so RIF is RI upon 1 plus 8 whenever I want to find an impedance input impedance what do I do in real life I short all outputs is that correct I remove all independent sources other than the for which I am finding that is the case I was so may as if VX VX by so that is what all that I am doing is that point clear so what is that disadvantage from the system or advantage is that clear because current is a feedback the net current II essentially reducing sorry II is sign wise if you see that means V by II upon 1 plus beta that means yeah the factor have a resistance go with me ratio say come cut the palace make a key I brought the other make a key I come cut as I am output because at that time what you are going to input call again a short okay or output see they can show you coming in that may load set up again is that clear loads are never considered and then calculating output so here is one more case I will solve and will stop on this I want to find shunt sampling let us say AXI is the gain again what is XI XI which is appearing AXI is anything may plus minus of a voltage equivalent source D Khadya us ka series resistance joys voltage source kahaibohai R0 and this I now apply a VX IX VX is the source I applied the output and measure the current IX with feedback XI will be 0 input will be shorted is that correct for output resistance input kithna hogar 0 XI to 0 karthya XI 0 kya and a voltage 0 kya kya current 0 kya no no because XS wo wo heparan a shot here so the usko short karthya hain to whatever resistance seen here is same as RO okay so now without feedback the output resistance is the RO of the source RO of the source okay but in the feedback hain yes a kiya hain abhi dekhye ROF is VX by IX XS is XI plus XF and X can be current or voltage whatever it is with XS equal to 0 ma you said it no XS equal to 0 XI minus is XF which means beta way 0 so XI is minus beta VX because V0 is same as VX so XI is minus beta VX substitute what did I do this is a shunt sampling going on is that correct so current voltage co sample kya gaya is that correct I applied a VX source and I want to see what is the current coming out to get my resistance VX by IX so I said at the input side I shunted the source shorted the source short short so XS is XI plus FXF XS kumine 0 kiya so XF is minus XI is not it but what is XF beta times V0 it na feedback mila beta times V0 a voltage sample in a so beta times V0 is the voltage sampled at the input as a feedback so XI is minus V and V0 is how much VX so XI is minus beta VX is that point clear this is voltage which is same as VX sampling is beta times V0 because session sample feedback is voltage now beta V0 equations may input side may they can get those source plus is equal to XI in this at the input of the amplifier plus the feedback is that correct are we happening expression they cannot if XS is 0 the feedback is opposite of XI but XF is beta V0 so XI is minus beta V0 or beta VX XI is the input to this so this amplifier may ye jo yeha par jo bhi input ata hai usko XI is that correct XS is here so XS me kya yeh voltage plus feedback yeh milke XS aina yeh voltage plus a voltage is this voltage is that clear so that is the word which I use there so what is XI minus beta VX so if I now say IX which is the current flowing here is VX plus a beta VX by R0 which is my current now is Sade circuits a current Nikonov IX is equal to VX plus drop a beta VX coming from here XI aina V what is XI beta VX I mean I call Anna usko subs add Karo sources go VX plus a beta VX divided by R0 is the current is that point clear this voltage plus this voltage VX is plus yeh divided by this current is the current by this resistance is the current is that correct so VX plus a beta VX upon R0 is the IX so VX by IX is RO upon 1 plus a beta so it is ROF is 1 RO by 1 plus T RO bada ke come over shunt sample may come over so the RO co reduce karna chaate hai toh consa sampling hona chaate shunt sampling hona chaate we did not do it do it for the opposite case a second last case I have a series sampling series me kya sample hothaya current sample hothaya by similar logic if you solve that you will get ROF is equal to how much 1 plus a beta times R0 so the feedback output resistance is higher than the resistance with the output is a without feedback if you have a current sampling going on is that correct so now you are four cases series mixing shunt mixing series sampling shunt sampling so now you could you get I can get both lower one higher one lower first lower next higher are both higher by a proper choice of mixing is that point clear I repeat since I have four amplifiers I can increase RI and increase RO simultaneously I can increase RI but decrease RO I can decrease RI increase RO I can increase both RO and RI all four possibilities four amplifiers will give me so whatever requirement I have in my actual requirement I can choose one of the feedback blocks of that kind and I will use them in between to create the RI and ROF my choice is that clear this is the advantage of feedback that at what cost all that I am doing every time the gain gain kuchhora hai main to ek baas circuit ka hai ya nature ka bhi hai ya apna human ka bhi hai linear system mera to linear system the output grade is essentially proportional to your input work do whatever you say thank you very much.