 So, in the previous session, we talked about diodes. Now, we are going to the next level of complexity. Now, when we talk about BJTs, all my 17 years of teaching, I had lot of difficulty teaching BJTs. I am sure you would have had the same difficulty. So, we have to innovate ways of teaching. This is by definition a complex device. Again, coming back to the importance of, let us say, analog electronics in particular. Now, in modern times, for example, with the IT, everybody has gone into computers. Similarly, we ask students which part of the syllabus in basic electronics they like? What is will be the answer? Digital electronics, very good. That is always the answer. Digital electronics is what they like. And you ask them what you hate most, they will say analog electronics. Now, at the same time, that is why I say, it is always a very big challenge to teach analog electronics in a way they understand. Now, today, anybody with a hardware knowledge is a rare commodity. Now, why is it so? What do you think? What is the reason? I was very fortunate when I was studying in Kerala in 1974 to 78. There was a lot of emphasis on experiment, lot of emphasis. In fact, I was so enthused, I did not have much money at that time. But still whatever money I had, I used to buy components, solder them in my hostel and bring it to the lab. I never wired in my lab. So, thrilled to see what will I get. You know, that helped me a lot. What I am trying to say is analog electronics actually by definition is tough. Digital that way is by far much, much, much simpler. But why is it tough? Because, you know, the actual understanding of a device. Once you understand a device, then things simpler. And the understanding of a circuit. So, our job is to make that, you know, that learning curve, you know, simpler. But another very, very important thing, I am sure. I am sorry if I sometimes, you know, talk as I am talking to students. But I am sure your experience also is the same thing that when you teach, one of the basic things they talk in the beard or somewhere, they always say, you know, we all learn by repetition. So, in what I, what I always do in my all classes, irrespective of PG, UGA, always first thing I ask in my class always is what I taught in the previous class. And you can always see they looking back and there is a very good evidence that they did not look at it. I mean, I am sure you do not have the same situation there also. But one of the things is repetition. I remember last semester when I taught basic electronics course transistors, I had to repeat my basic lecture at least three times. But I was very happy. I was very happy. Each time I could see a smile on their face. And you could see that is a delta exchange. So, what I am trying to say is, a learning curve which you have picked up may be 20 years, may be I have picked up in 30 years and whatever it is, do not ever expect a student to pick up in one hour. So, we have to give that time. Only our job is to make that job simpler and hopefully, you know, enjoyable. That is all we can do. We cannot make it simpler. But that is where I think a lot of applications. So, what I had done in BJT, what I had done is I have taken a route which I found very useful for with my student. I believe students are same all over. And what matters is motivation than anything else. So, I have, I have, I have the way I teach transistors. That is the way I put it, put up the lecture. Now, as we all know, BJT, invention of BJT in 1948, let us say, it was the starting point of all ICT we talked about today. In fact, whenever I teach a course called introduction to electronics, and I tell about two major inventions which led to what is today. Can you name the two basic inventions? Transistor is one. And maybe if you go one level up, you can say, let us say, ICs. Then it went to ICs, but it started all here. Then what is second one? And I happened to work in both areas. That is why I always take pride in that. Second one is fiber optics. I mean, I happen to teach both. So, that is why I always tell this. Now, so here you see transistor as a device. Any idea why this name transistor was coined? Transistor of resistance. Transistor of resistance. That was supposed to be something which is not possible. Today, we do not ever talk about transfer of resistance. Today, we do not talk about that. But the time when it was invented, that was the main issue, that here is a device which can transform something at an input level, at a lower impedance to at a higher impedance, which was supposed to be something great. Now, here again, let us bring back the same concept of a three terminal device. And this is something I always found useful to tell students that being a three terminal device, you cannot have just one characteristic. You need at least two. And because you have three terminals, you have many combinations. All these I think helps them in understanding. As opposed to a diode where is a two terminal device, here you have a three terminal device. Therefore, you have many combinations possible. So, in that way, then things become much simpler. And basic principle of a diode, sorry, a BJT is to use the voltage between two terminals to control the current flowing in the third terminal. Now, again PNP and PN, I will skip it. Now, one very important thing to know about is again very commonly, students have a question. Why do you say this is emitter and this is collector? Even though I with a multimeter measure, I will look like. So, this is something which actually we need to clarify. I mean, I do not know how many will ask that question. They will care to ask that question. But at least once in a while, you will have at least once such student would ask the question. Now, one thing we must remember is the word emitter itself means that this emitter region emits carriers. And collector, it collects. Because of the basic requirement itself, the way they are doped are very different. Emitter is very heavily doped, because and another reason why and base is very lightly doped. Because of that, you in collector has another, what is the other requirement of collector? One is collecting, what is the other requirement? Another very major requirement, how dissipation. This is again a very good point to illustrate, very, very important. Because of that, in an actual thing written that here, collector base regions. Now, the collector will always have the largest area. If you look at the actual physical area, you will see that the collector has a large area. Why? Even though the current flowing through emitter and the collector are the same, the why you should have, why does it have higher power dissipation requirement? Why area? Let me repeat the question. The current flowing through, let us assume that the current flowing through emitter and collector is same. Actually, emitter is more. Even though the current flowing through emitter is more than collector, we say that the collector has a larger area. Why? P is equal to v into y. Then, why? Across the? The important thing is, power dissipation is v into i. i is same, but v is vcb, collector to base voltage, which is much higher than vbe. Vbe is only 0.7, whereas your vcb may be 5 volts, 10 volts. So, at least 10 or 20 times. This is why the collector is generally connected to. If you have a metal transistor, collector is always connected to the body, so that you can dissipate heat. Some of these things, I think, are very, if you can show, again, a device will be very useful. Most of the time, when I teach in a class, there will be a power, sorry, a publicator system. What do you have in a publicator system? You will always have some power transistor sitting Pj. So, it is very nice to show them. Look here. That is collector. That helps them to understand what you mean by power dissipation. If you touch, most of the time, it will be hot also. So, it helps. I mean, some of these touch and feel, I would say, is very, very important. Very important. You learn it today, especially today. Then, it sinks into them and that keeps them motivated. As I said, today's youth, they do not have more than 15 minutes of attention span. I look at my son. I can say that. Keep him occupied. I mean, you have to make. So, you have to make sure that, in fact, most of the time, you have to be actors in the class. Sometimes, you have to tell stories nice related to the subject. You have to keep them interested and motivated so that they study. It is a tough job to be a teacher. Yeah. Now, the very important thing, which is not mentioned here also. Can you think of a problem because of the very high doping of emitter? There is a very major problem. In fact, very often, it is not realized and most of the times, people damage transistors like this. Fine. Collector has very high dissipation requirement, but because your emitter has very high doping, you have a problem. If you apply, I am talking in terms of any reverse bias, which you would connect to the base to emitter. What is the problem? You know, but see, for example, one very important thing to remember is, you should never apply a negative voltage between base and emitter. The breakdown voltage, base emitter junction is typically 5 volts as opposed to breakdown between the collector base or collector emitter junction is typically 50 volt, 30 volt, 100 volt. Trastic difference. That is because of the very high doping of emitter. So, something very often people damage transistors like this. So, if anybody is connecting some input, especially you are trying to measure a transfer characteristics and you would connect some input, make sure that your base does not get in a negative voltage by putting a diode or something like that. Very, very important. The significance of, you know, the high doping. Now, for the same coin, you cannot reverse. If you reverse, what will happen? Anyway, we will come to that in a minute. Let us look at the modes of operation. This question will be answered there also. When you talk about modes of operation, now, most of the time we are talking about active mode, where your emitter base junction is forward biased, collector base junction is reverse biased. That is what it is designed for. But you have situations where you also need cutoff and saturation, where? Which application do you know cutoff and saturation? Switching application. Switching application. Whereas, in a linear application, which is an amplifier and oscillator application, you are always talking about active mode. Now, again, if you are talking at a first-year level, this is something you do not want to ever talk about. But at least if somebody asks, reverse active is what? What is reverse active mode? It is the other combination, which he never used. That is, emitter to base junction u, reverse bias and the collector base junction u, forward bias. Now, if you do that, what will happen? Obviously, it is not designed for that. Now, essentially, your emitter has changed. Now, your emitter is something else, collector and collector is emitter. It will work. But what is the problem? It will work, but with a very major difference. In an active mode, the beta is very high, normal active mode. That active mode is also generally called normal active mode. The beta is very high. But here, beta is less than 1. So, basically, it is not designed for it. But there are applications where this is used. Can you tell where it is used? It is used in a very famous thing, which you are all using every day. You might not have realized it. It is used. This reverse active mode is used, actually. In switching circuit, one, something you are all familiar with. In a TTL gate, the first transistor, which is used there, is used in reverse active mode. It is used in both forward active mode and reverse active mode. In fact, this is just out of… If you are interested, you can see where reverse active mode is used. The reverse active mode, if beta is less than 1, what is the inference? The base current will be larger than the collector current, which is unusual. Something we do not ever think about. The reverse active mode, the base current will be larger than the collector current. And that is used, that property is used in a TTL gate. The first transistor is both working in normal mode, rather saturation, as well as in the reverse active mode. But this is something I never ever mentioned in my first-day class. Never. I said something like that, but let us not… Let us forget about it. And you say forget, they are very happy. Now, let us come to cut-off and saturation. My always, the biggest problem in teaching transistors is teaching saturation. Cut-off is easy. Cut-off, very easy. That is where I want to talk about the strategy I use, and I found it very, very useful. Before talking about common emitter amplifier and I take another circuit and explain, we will do the same thing. And I found that works always. If necessary, I repeat it twice, twice, but they understand the end of it. People who understand, and I like that root, I will use the same root. Now, one of the important things we must stress when we talk about, before we teach about other things is to, is the basic application of KCL. That in a transistor, whether it is PNP or NPN, you would see that the KCL is valid always, where the saturation, where it is cut-off, where it is active, it has to be the basic law. Kirchoff's current law has to be correct. And the equation which you are familiar with is nothing but KCL. So, that is something which they appreciate, and they must always remember it. Why? This is very important for the saturation case. We will see that in a minute. Whereas, when we talk about in a BJT in a normal active mode, your IB is much less than IC. And we talk about current gains, both alpha and beta, where alpha is defined as IC by IE. IC will be always less than IE. So, therefore, alpha will always be less than unity. Since, beta is defined as IC by IB, beta is also called the common operator gain, and beta will always be greater than, much greater than 1. Now, at this point, we must be careful about the word beta. In fact, but I think in the first year course, we make this mistake is not a problem. Now, we must be very careful about beta. When you say beta, beta is definition, what? Current gain, but it is by definition, beta is IC by IB. Now, in a saturation case, beta, what will happen to beta? Beta will, beta is not fixed. In active mode, there is no problem, because beta is a constant number. What about in a circuit which is as saturation? They keep changing. You will see a circuit right now, where beta keeps on changing, but do not. So, always careful, the word beta is defined as IC by IB. So, different values of IC and IB can have different values of beta. So, such times, when in saturated circuits, you would use, do not use the word beta. Generally, we do not say in saturation beta will be less than, generally the word uses beta in a saturation case is less than beta DC. That is the way it is used, but you must be careful with the word, especially teaching a electric engineering student. Now, again, this is something you are familiar with, which can be directly derived. Alpha is equal to beta by beta plus 1 and beta is equal to alpha by 1 minus alpha. Now, typical values of alpha and beta, if beta is typically in the range of 50 to 400 and alpha typically 0.99 and so on. In fact, in the lab, you would have some problem. Madam and all, we all have a lot of problem in the lab. Do you know why? Today, the transistor you get are too good. We had problems because of the gains. Beta was for the order of 400. We wanted a transistor with less beta. Finally, we were able to get a transistor with less beta. I will show you one another. So, today, be careful about beta. It is always good to play with a BJT, which has less beta. Most of this BC148 and all of them are very high betas, 300, 400. You will get into trouble. If you do a calculation and you see that nothing works. So, this is important to realize. That is why it is important to do the actual experiments and even measuring beta. I prefer this particular circuit. I found always this is a much better way of explaining all these three modes. This mystery, we must somehow make sure that our students do not get scared about all this. I found this is the best circuit to explain all the three modes. All three modes are present here and they understand it well. I sometimes go over it two, three times and then they get it very well. If I think, then I tell them you remember this circuit. If you remember this, I said you know everything about transistors. Then it is only a question of changing and I like this. I always use this and I found it very, very, very useful. So, let us take a BJT inverter even though this is an inverter in a logic circuit. This is the best one to teach all these three modes present in the circuit. One shot and they understand it well. So, let us take a BJT inverter where you have input, you are varying from 0 to 5 volts and you are taking output from here. Let us see what happens. Now, we can divide the operation of this particular one in the very clearly into three regions. Let us take the first region where we will assume something. We will assume that the VB is 0.7 volts, beta is 50, VC sat is 0.2 volts and again some of these are very, very complex things for students to understand. We will talk about it later. So, let us start with this and the good thing about this circuit is as you change V in from 0 to VCC, the BJT goes from cutoff to active to saturation. All the three modes. So, that is the beauty of this circuit and they understand it well. So, initially let us assume that the V in is less than VB. Now, since V in is less than VB, base current will be 0, which means there is no collector current. Now, since there is no collector current, the output voltage V out here is nothing but VCC minus ICRC. So, there is no current flowing here. This voltage is same as VCC. So, that is something everybody understands, no problem. I have absolutely, people had absolutely no problems with this part. Cutoff is something very, very easy to understand. So, the transistor is not conducting. Therefore, I get full voltage out. Fine, no problem. Now, coming to the second case. This is the case where I define a voltage called VIH, whatever it is. Now, initially there is no need to understand what it is. But your VB is, V in is now greater than VB, but less than something. Now, since now V in is greater than VB, what will happen is now the BJT will turn on. As the BJT turns on, your IB is nothing but V in minus VB by R. So, I get some IB. Therefore, they will immediately give me some IC, which is beta times IB. Now, in the output loop, we know that the application of KVL will give you VCC is equal to IC into R2 or RC, whatever you use there, plus VCE. So, which means my V out, which is nothing but VCE is nothing but VCC minus IC R2. Since IC is increasing, output will keep on decrease, which is shown here. So, up to this particular point, there was up to what roughly 0.7, up to about 0.7. It is not conducting. So, I get full voltage. Now, it keeps on linearly decreasing. So, this is the active mode. I have cut off here. I have active mode here. Now, the question is, which I always post this question to students. Now, we will keep on decreasing, whatever we understand. Now, my question to them would be, how low V we all can become? And I get all kinds of answers. They say, equation has to be valid. KVL cannot be broken. Therefore, VO can take negative. Some people say, no, 0. And I always take a vote in the class. And that is very, very interesting. Do not ever tell the answers immediately. And that makes it very interesting. And then you tell them, no. V out cannot, because it is a physical device sitting there. V out can be negative. So, this is where they must understand that all equations and equations have domains. You cannot apply an equation, so it has to be valid. But in all my 17 years of my teaching, this is a question where students fumbled. At least 50 percent of the class fumbled. And they said, no. Equation has to be correct. Therefore, V out will be negative. And then you have to clarify. And this is where the biggest problem, biggest challenge is to convince them that it cannot be. And we will see how we can explain. And this being a physical device, and it is a junction there, you cannot have negative voltage. And anyway, when you tell them that, you are only applying a positive supply. How did you get negative? They understand. Something is fishy here. So, since V out is V c, it cannot go below V c sat. This is the minimum voltage possible. And another very important thing, we can tell them that V out cannot be negative. Anyway, two ways of explaining. One is, it is a physical device. It cannot have a negative voltage there. Second thing, you are not applying negative voltage. How would we get negative? You know, oscillators, this is only DC. So, you cannot get negative. That I think they appreciate. Most of the time, most students will be won by that argument. And this automatically gives you, sets a limit for the maximum possible value of I c. So, that value of I c is nothing but V cc minus V c sat by R, which means, at that particular point, there is a particular value of I b. And that would give us a particular value of I c. By the way, this was the book I was mentioning about. Microtronic circuit by Cedran Smith. And this has been my companion. I lost a few books also. Somebody would borrow and I never get it back. And I am happy. Somebody had refused it. So, I have bought at least four such books. And the fourth one is with me. This is another very, very good book, which we use here. Both the books we used here. This is by Electric Engineering by L S Bob Rowe. And this is also Oxford University Press and very easily available, all Indian books. And I am told, in fact, this book was recommended to us by the Oxford University Press fellow saying that this is a book used in bits planning for a long time. So, after using this book, we found it is a very good book. Good thing is, this particular book has the entire syllabus of most of the course we teach. May not be in a great detail, but one book has almost all the syllabus of a. So, this is a very good, all are cheap. Cheap books, typically 300 kind of price. But this is a book I love it. Extremely, but I do not like that book that much as much as this. We will talk about basic electronics. This book, students would love it because very simple explanation. At the same time, very accurate explanation. Keep it side by side with Milman and Halkas. And then you will see the difference. You will never touch Milman and Halkas again. Once you go through it. And the coverage is very good. And somebody was mentioning about measurements. This will tell you lots of ideas about actual values, even setups and so on. Very, very good book. I very strongly recommend this book. And fifth edition, that shows its popularity. And a very, very good book. So, we talked about maximum value of IC, which is possible in this scenario. Which is directly from KVL. Because mind you, I am facing very often 120 young adults looking at me, staring at me. They are trying to kill me. So, I will have to convince them. And they are all happy because it is KVL. KVL is sacred. So, they all agree. This is KVL. This has to be correct. So, that is no problem. And then, at this particular value of IC, there is a particular value of IB. So, that I can quite directly find from ICmax divided by beta as some, let me call it IB2. Now, since IB is equal to V in minus VB by R1, I will be very glad to email this to give this to anybody, everybody. Please, I think there is also will be made available to you. Very happy to give, no problem. And also talk later about actual materials available for you also freely. Now, this would give you a value of V in at that particular IC value as VB plus IB2 times R2, which I will call as VIH. Now, up to this particular point, the graph which we saw earlier would be linear. So, what we said is, we started here. At this particular point, it has reached the limit. At that point, it is still that point, it is active. And that point is what I will call as VIH. So, I have got 1.7 volts typically, which is typical of most transistors you will get around that voltage. Now, beyond this point, now we get into increasing the cell. I will come to that in a minute. Now, if I increase my V in beyond this, one thing is clear. What will happen to IB? Because again KVL cannot be broken. That is a very good argument, because input loop is V in is equal to IBRB plus VB, which is KVL cannot be broken. Therefore, IB will keep on increasing. But at the output side, again KVL cannot be broken. And KVL says that if Vc side is only 0.2, you cannot exceed, which means base current keeps on increasing, but the collector current is saturated. So, the word saturation, let us say, came from here. It is saturated, but the base current keeps on increasing. There is no limit. Keep on increasing base voltage rather V in, the base current will keep on increasing. So, the collector current is saturated. So, the word saturation actually came from there. Collector current is saturated. So, mind you, what is beta? Every input value of V in beyond that VAH, beta is changing. That is why I said careful about the word beta. Beta by definition is IC by IB. So, you must distinguish between beta DC and beta, which may not be very important in the first year level. It will make a mistake. It is okay. But when they get into electrical engineering students, they must understand the difference. Generally, the word beta DC is used to differentiate between the active region beta and the beta here. Of course, beta by definition is IC by IB. So, always remember that. We do not have the liberty to change the definition of IB. Beta, beta. So, beta keeps on changing. Rather, beta is now, what is happening? Decreases. But beta DC is some value, which we have defined for the active region. So, they have differentiated between these two. In this area, there is a graph of collector base voltage versus IC. And where they have, you have a starting line. Where they have segregated the saturation region and the saturation region. And in the saturation region, we have shown the collector difference in both these two. I really found the difference. I will come to that in a minute. In fact, I do not want to discuss that in a basic electronics course at all. In fact, I will sell them, discuss even in an analog electronics course. Now, the issue what you are talking about, I will just come to that in a minute. Because, see saturation is in my opinion, all these of my teaching. The most difficult thing to teach was saturation of a BJT. Op-Amp was very easy. Digital Tony was very easy. My challenge has always been teaching, what is saturation? So, that is why I do not even want to get in, but we will talk about that in a minute. But let us get over this. Now, because another one issue, we must make it clear. The word saturation means the collector current has saturated. That they can understand well. That is why the word saturation came from there. And therefore, it cannot increase any further, which means in my graph, even if I keep on increasing my V in, my output cannot increase. Now, this property is used in a BJT inverter. In a BJT inverter, you use only these two regions. You either operate less than VB or you operate greater than VIH. This is a region very interesting. The same circuit, we talk about different regions. In an amplifier, you want only this region. In a switching circuit, we avoid that region. We want only the nonlinear area. So, this again conceptually difficult for a student also. Why do you talk about sometimes active, sometimes active, sometimes cutoff saturation? This distinction should be brought very clearly. Linear circuits, which means amplifiers, you have to use active area. Switching circuits, you use nonlinear areas. So, this circuit has the beauty. It has all the three and that I think clarifies a lot. Now, let us come into the issue of saturation. Another big issue we have not solved. When we talk about features of saturation mode, we talk about IC less than beta DC times IB. That is why the word IC gets saturated, therefore saturated. Another very important thing. This is something I want to explain. We wrote right in the beginning, when we talk about mode of operation, the saturation both base emitter as well as base collector junctions get forward bias. This is again very difficult for a student to understand. The best way I found in explaining that is by this route. We started saying that VCE sat is equal to 0.2. The simple KVL again says VCE. I do not have the transistor here, but I appreciate you can follow this. VCE, that means you start from this voltage between two terminals. You can get around by adding this side also. So, VCE is same as VB plus VCB. That is what I have written there. Everybody understands no problem. And therefore, VCB is nothing but VC minus VBE. And VB we know is 0.7. VC side is 0.2. Therefore, we get VCB as minus 0.5. Which means base is more positive. So, this I found very, very, very, very easy to both for me. First of all for me and for others also to explain, very easy to explain. And now comes the issue. Now comes if somebody understood this step, then you will be in deep trouble. If they do not understand much, then they will stop it there. They understood well, then they will ask this question. Why all of a sudden, how did it jump so fast? Where did it jump? In fact, this I hope you appreciate my question. So, the issue is when you talk about a transistor and analyzing a circuit, there are many, many jargons we use. Once a jargon is VC side is equal to 0.2. Now, friend has raised this issue. Actually, when you, how do you define reverse bias? What is your basic definition of reverse bias? Let us go back to that. Then we will take up this issue. So, we should accept at least that VC side is equal to 0.2 is a jargon we use. We strictly speaking not really justified about the convenience we use. Let us use about convenience and that brings in lot of other problems, which actually our friend raised that he put the, you know exactly the same point. Now, let me get back to it. What we said is in an active mode, the base symmetry junction should be forward bias and the base collector should be reverse bias. Now, what is the definition of reverse bias? How long? No, what is the extreme limit? What is the limit of, what I mean is what should be the limiting value of VCB? VCB, till I can say that, now, yes, till this point it is reverse bias. What is the limiting point? That is the main issue. No, no, no. See, see, the base, what do you mean by forward bias? Let us take the, what do you mean by forward bias? Forward bias means like base symmetry junction. Base is more positive than emitter. That is forward bias agreed. Now, if, if base and emitter have the same voltage, what would you call it? Will you call it forward bias or reverse bias? Reverse bias is 100. Now, let me put the question back to you again. So, what is the limit? What is the limit of reverse bias or of the limit? VCB. What is the limit up to which I can go and say, still this point it is reverse bias. Is it greater than 30, greater than 30? Wait, good. That is the accurate answer. But the answer I would prefer first is 0. What is the correct answer? Let me take up that later. That brings in other problems. Now, see, let me take up VCB is equal to 0 at the limit, at the limit. VCB is equal to 0, which means VC is equal to VB, short circuit. In fact, if you want to make a diode out of a transistor, that is exactly how it is done. All ICs, diodes are made like this just by shorting the collector and the base. It is actually working as a transistor, very, very important to understand. That if you short a base and collector, it is still a transistor. It follows all the rules active mode. Fine. Now comes the issue of friend set. What will happen if I keep, you know, the forward bias, rather base voltage I want to increase slightly above 0, what will happen? Now, as you rightly said, as you keep increasing, you would see that, as you keep increasing, till it reaches around say about 0.5 volt or so, till that point you will see no current flow. So, I mean to be very precise, the VBC, VBC can be even up to 0.4 volt and still it will be reverse biased. If I confused you, please, I am sorry. But let me ask the point. Now, why it cannot take the same voltage as the base diameter junction? Let me repeat the question. For base diameter junction, we say forward bias means 0.7 volt. But when you talk about base to collector junction, we are saying now, do not exceed 0.4. Beyond that, you are in forward bias. Why do you say 0.4? In fact, you would see the most books. In fact, I would say Sir Thomas Smith, that way, it is a very good book. You show that your V, you know, it can come down. Your base to collector voltage can be as high as 0.4 and still it will be in the active mode. 0 at 100 percent, no problem. But you can even be positive up to 0.4 volt. Beyond that, if you go, it goes in a saturation. Now, the important thing to remember is, mind you, these are two different junctions. That is where the device comes into picture. The base to emitter junction is what? The emitter is heavily doped, whereas the collector is at a very different doping. Because of this, the way they operate, if you operate them as a diode, they are very different, very different. That is why we talk about different voltages here. For forward bias of the base to emitter, 0.7. For forward bias of the base to collector, we say 0.4, 0.5, so ok. That is why the distinction. But anyway, I would say, because of this, there are a lot of problems. So, Vc is equal to 0.2, let us stick to that. Basic codes stick to, Vc is equal to 0.2. And say there is a gray area, I do not know what will happen. That is the best. But if you want to be very precise, you can say, Vbc can be as high as 0.4 and still it will be 0.5. So, as I said, for this circuit, fortunately that did not come at all. And this is why, if you actually simulate this particular circuit, what we drew over was just the equations I plugged in and I plotted it. I cheated. But if you actually do a simulation of this, what do you think, how will you think if the graph look like? To do a simulation of this. Tomorrow process, Mahesh Patil will be showing SQL. And all this is a very wonderful tool. Again developed here, free. You should take it with you. And the wonderful tool. And if you do with any tool, can anybody say what will be the change in this circuit in this graph? This is exactly the equation we plugged in. But actually, if you do a simulation, what will be the difference? You will see the difference will be here. Here will be a very smooth variation you will find. And that is precisely because of this way we defined forward saturation is not clearly true. We can see that immediately. If you do a simulation compared with this, you will see that it goes in a saturation much slower. Not as fast as we said. So that, but why do we then use this model? Why do we use this? Simulation. Always remember that our job is not to teach perfect because there is tools available. But somebody should not use a tool before without seeing this. So he understands the limitation of the analysis and understands the limitation of a simulation tool. Ultimately an experiment is the right one. You will see that even after simulation go and do an experiment there will be variation. But important thing is to appreciate why? So try to cover gray areas. I would say cover slowly. Do not stress too much on them. But cover it slowly. So should I go over it again? But I found this extremely easy to do. Very liberating. Every class I can say let me do BJT inverter again. And then the class people are happy. And they understood all that. In saturation, I could explain very well from this. And that helped. You know the concept of what is saturation? In fact that is the biggest mind block. What is saturation? So current gets saturated. Collector current gets saturated. That is the word saturation. Second thing is the beta is less than beta DC. These two things and the third thing we said is the junctions get forward by DC. These three things they understand each one term. What it means in a circuit I think they will understand. And I mind you even for an electrician it is extremely important to understand what is saturation. I can tell you a complication otherwise what will happen? Then you maybe I do not know they have a common emitter amplifier experiment. In a common emitter amplifier let me put this question back to you. Now tell me the answer. Take a common emitter amplifier. Let us say gain of 10 or 20 or something. You give an input signal. Look at the output. And you say you have got a gain. Everything is working well. Now you keep increasing the input signal. What will happen? One side saturation. One side clipping or cutoff. If you keep increasing it will happen. But when you look at saturation what will happen? Just before saturation you do something happens to the signal. If you input is sine wave the output will be let us say in active mode what will happen? Output will be very phase difference. 180 degrees. 180 degrees agreed. But if you see when you increase the signal you will see something very funny. I have never seen it as you should see in the lab. You will see that when you increase the signal you will see that the positive half you will see not 180. Zero degrees. You will have no phase shift for one half at the top peak. You will see a sine wave repeating there. And the other one you will see TK. Did you follow me or shall I explain again? What I am saying is you design a BJT common emitter amplifier. We will do that in the lab. Do it. Works well. Kane you are measuring very well. Everything is fine. That is the input is 10 millivolt or 20 millivolt. In a 50 you are getting 1 volt. Everything is fine. Very good. Now keep increasing the input. As you keep increasing input what will happen is slowly the amplitude will increase. At some point you will get into saturation both ends. Saturation and cutoff. Depending on how good or bad you would design your biasing circuit. And I am assuming that it goes in the saturation. Not in the other one. First. Or it is your wave. Let us say equivalently it happens. If it goes in the saturation you will see that the output signal. So far it was you had a 180 degree phase shift. But you see that for the peak part it is not 180. It is no phase difference just for the peak peak part. That is the time it got in the saturation. Now my question is why? Any of you seen it? No. I think I should since I told you I think I should bring it up and show you. This is because of saturation. What happens saturation? This base to collector junction is forward biased. So what happens? The same signal will go through as it is. See so far your base was collector junction was reverse biased. So the current was not getting affected by anything happened in the base. So you had all the entire thing coming in the picture. So you had the VCC minus ICRC coming in the picture. But now once the diode gets forward biased the signal goes straight there. If base increases collector will also increase. Voltage will increase. So you will have zero phase shift for a small region. Very interesting to see. Again because of saturation. So saturation I am saying is a concept which is so complex. You can keep on doing it and you see things affect happening. We should see this in the lab. I think I will rig it up. So now what we will talk about is again as I said I am assuming that basically that by now students have understood active mode cutoff and saturation. Now they have to graduate from there. And I always found that those concepts are not clear. They are going to get stuck later. So those concepts should be made clear as much as possible. Don't worry about covering syllabus. But I think we have the liberty to do that here. We may not have the liberty. But I think these things I think they should very well understand. Because transistor is something which is very, very difficult to conceptually very difficult to understand. And these questions should be answered. That is why I think an experiment is something very, very, very, very important. Now the other before we take up in fact I like it. That is one of the reasons I like the transistor very well. All this presentation there is MOSFET or BJT. What he does is takes a device first talks about the characteristics as a device. Then he immediately gets into DC circuits. I like it very much. Now I believe if somebody taught me that we have to understood things better. In DC circuits what is the advantage? You have very simple equations. All with you get some number. But most books does the other way. They will talk about devices. Then they straight get into application amplifier. And you are lost. Then you talk about biasing. Lost. But if you get into DC circuits then you have a chance of trying different types of DC circuits. You can see some get into saturation, some get into cutoff. The issue of biasing becomes much simpler if they do the DC circuits first. Again you can use any DC circuit and all of us would definitely prefer that. Sometimes using this circuit. And again a simple circuit but a profound circuit. How complicated? And any amount of questions I can ask from here. And I always I can find out who is a good student, a discerning student and who is a bad student. Easily. Just a simple. How many times? I mean even though it is such a simple circuit. Now this is a circuit which we later would use for biasing. But very good circuit to teach biasing. Very good circuit. And also a very good circuit to teach for example, all especially active mode and saturation. Very very good circuit. Now this can be simplified by using Thevenin's theorem. You can simplify the base emitter circuit into a VBB, a Thevenin voltage and a resistance here. And we can write an equation here. And using Thevenin's theorem you can find VBB as VCC into R2 by R1 plus R2. And I can find RB as the parallel combination of R1 and R2. That is fine. Then I can write I can assume again and very very very important thing when we talk about transistor analysis whether it is MOSFET or transistor. You always assume active Y. In any DC circuit analysis you will always first assume active. Why do you do that? But I do not know. I do not even know. See finally I do not know. In fact just like we solve any simultaneous equation what do we do? We use an equation solve it then what do we do finally? We verify. The same thing has to be done here also. But my question is why do you assume? Why do not you first assume saturation? Let me ask the question. In this circuit why do not you first assume is saturated and proceed what is the problem? Why? Because of that. Very good. Because of that. No, no, what is the advantage I get? What do I advantage? No, no, KCL is always valid. Linear non-linear is secondary. KCL is always valid. That is not the reason. See that is I am saying as engineers you know I always tell students I am happy to be an engineer. I tell them you know these engineers who made fans run you know make cars run. Think at the same angle. Why do we do that? See if you follow this route you get a simple solution. That is the only reason. If I do not know the relationship with the one unknown I am able to take care immediately. Just by knowing IB I can find out all three currents, all the other two currents. What about saturation? I cannot. Only one equation I can use. IE is equal to IB plus IC. That is all I can say. I do not know what is IC. So that is the reason you always take active. I mean so that you would appreciate that only if you have a circuit in saturation then only will appreciate it. So that is why even for that you start with a active. Then prove that it is not active. Therefore it is saturated. Why not cut off? Why not cut off? What is cut off? Cut off means base symmetry junction if it is forward biased current will flow. So if it is branches is connected correctly which will be generally in a problem it cannot be cut off. It is either in saturation or active. Still you will take active because it gives you a very simple expression and at least you can verify whether it is saturated or not. If it is saturated then I know what to do next. What do I do next? No, no, no. But how do you solve? Very good. Very good. See the important thing to remember is you have many unknowns in a circuit. Sickly speaking you have six unknown, six unknown which are six unknown. Three voltages and three currents. VB we assume one is gone out of the window. The moment you take care of active you take care of another two. So you see systematically actually we did this to make sure that you have only one unknown at a time. So that one equation I can solve and then you say that no it cannot be therefore it is this. Now so here again that exactly the same strategy. If you find the assume it to be active therefore I can write this equation I can find VB as VBB minus VB by RB plus one plus V triple times RE. From here I can find IB then I get my IC I can get my current unknown. Now then I can find my VC, VC as VC minus IC RC, VE as IRE. VCE I can write as VC minus E. At this point I checked. That is my assumption I started was correct. VC is greater than VC sat then my assumption is correct. But if it is not it is in saturation. Now the moment you are in saturation then we need to write two equations. Because now I cannot use my assumption. So I have to write two equations. One equation for the base emitter loop one equation for the collector emitter loop. Two equations I have plus VC sat I know now very well. I can write that to be 0.2 volts. And then I have to use this equation. So I will have two unknowns two equations two unknowns. So this is the third equation. So we can write IC or IB in terms of the other one. And you get two equations. But this is very very difficult to do. Now the next issue is this again another very very very bottom. You can have any number of problems based on this. Basically how do you choose RC and RE? What is the role of that? Again there is lots of confusion lots of confusion. And especially about RC let me ask a question. So please listen to me carefully. I will give this circuit back to you. Now my question is here is the circuit. Now let us say I found this circuit to be active. And let us say now the RC is 1k. And my question is if I change my RC from 1k let us say to 1.5k. Will I change? Please carefully think and answer. No, why? See one very very very very very very common misconception. Very common is that the collector current depends on collectivation. No, this is a current source. In a current source you can say actually a current source rather a current sink here. But the important thing to remember is you have to assume that the RC is not too high. If it is too high what will happen? We will go into saturation. So in fact that is exactly what I was trying to discuss there. The RC let me ask another question. If you make RC is equal to 0 what will happen? Will the same IC will flow? Careful answer think and answer. The same circuit if I make RC is equal to 0 what will happen? Will the same IC flow or not? Good that I asked you to say. Think. In fact the biggest contribution you can do to your students is to help them to think. That is something missing today. I always tell students the motto of IBM. Do you know what is the motto of IBM? Think. That is the motto of IBM. So always you know think. What is the answer? If you make RC is equal to 0 with the same IC flow. Good. You have the guts to say that. Very good. The same flow. You see the problem? If you have problem think about how much problem your students will have. So IC does not depend on the resistance you put there. It should not be too high. The point is it can be as low as possible but not as high as possible. As you keep increasing RC what will happen? VC will keep decreasing. At some point VCE now will become VC sat. So the role of RC is to the role of RC is this that you provide an RC. Mind you we are not talking about amplifier now. We are only talking about DC circuit. If you keep changing RC you must be careful that it is not too large to make VCE equal to it goes into saturation. It is not gold. Which means you can reduce it as low as possible. Your transistor will always be inactive. But if it keeps increasing at some stage it will go into. So if you frame a question you can ask a question give a circuit active. You can say that find the maximum value of RC which will which you can keep to be keep in active mode. So this is something very very important very important very important and a very common concept. Let me ask about RE. What about RE in the previous circuit? What is the role of RE? What I am asking is if I keep increasing RE what will happen to IE? What will happen? See unlike IC this is a coupled unfortunately RE is coupled in all the equations. Whereas RC was independent. So that but RE is there in all your equations. So what will happen if we increase RE what will happen? All currents will decrease because ohms low ohms low. Automatically your IB will decrease IC will decrease. So now tell me what is the limit on IE then same you are it is working in active region. Now what if I do something how what can make it go into saturation by playing with RE. If you decrease RE what will happen? I will increase but some stage with the same value of RC what will happen? PC will come down. So the role of RE again here you talk about a minimum value below which you cannot come. You see the difference between RC and RE. So the same circuit you can have a question saying that what is the minimum value of RC in order to make sure that it stays in active mode. So this is very very important think about it just a simple calculator a few values you can plug in and you will see. Now this is extremely important for the students point of view. If they do not understand this biasing they will never understand. The role of RC and RE must be very clearly understood. Now you are not talking about amplifier at all. You are only talking about these components how they affect biasing. How they will drive somebody graduation to cut off. Now what I want to show is maybe at this stage I will switch over to the other one. We will show you transistor characteristics. Again let us working there let us hope we will work here also. Now when you talk about transistor again the most common configuration which we use is the common emitter configuration. Now in the common emitter configuration we talk about again being a three terminal device we said you need more than one characteristic. We talk about an input characteristic and an output characteristic. What is the input characteristic? You apply you change VB and see what happens to the IB that is the input characteristic. But being a three terminal device what you have to do? You have to do something at the output end. You have to keep the VCE common fixed. Now if you change VCE you will get different family of course. That is one characteristic that is the input characteristic. Now the output characteristic is what? You keep you have one value of IB and you keep changing VCE and see what happens to IC. So this is something where we can actually do the experiment. What we have here is basically what is already here then I will go to the next. We will actually do it. What we have shown here is basically this is the input characteristic. Let me first show you this. What you have in x axis you have VB and the y axis you have IB in microamps. What you have what is done is VCE is one case is kept at one volt other case is kept at five volt. What you see with what is what is one two observations? What observations can you say from here? What are the things you see from here? What is the what if I show you this graph? I hope you can see from there. Can you see two graphs here? White and a red one. What is the inference? Base current. Now one important thing is this is exactly like our diode characteristic. So this is exactly like a diode we are finding that is one important thing. Second thing you are finding is when you increase the VCE what happens? Base current. Base current. Increases. Increases. So this is a very important thing. Mind you for the same value of VBE you have now you can see this has come inside. For a smaller voltage you are having now same same value of the previous value. Why is it so? There is something again basic electronics should not know about it but later base width modulation. Once you apply larger and larger VCE the base width reduces because of its the current. So you can see that here nicely. But the important thing to remember here is this variation is very small. Very very small but you still see. Let me show the other one and after doing this we will run it again. This was just now done here but later we will do it again. This particular one you see here is the output characteristic where you have VCE on the x axis and you have IC on the y axis here and each of this family of curves is for a value of base current. And you can see a slower slope. Can you see? Smaller slope again due to early effect. Because early effect will come in the picture. If your VCE keeps changing the current keeps changing. That is all called base width modulation. And if you extend this curve it is supposed to meet the x axis at some point and 100 volt typically. So it will never meet. We are talking about 56 volts here. So we will have to calculate that slope and find out what is the early voltage. Now one important thing to notice here is can you see this region is what? This is the active region. What about this region? This is what he was talking about. This is the region we did not talk about. Mind you this saturation means VCE is equal to VTFAT 0.2 volt. It is not true. You are seeing that you can see this. If you reduce say somewhere from VCE if you reduce beyond 0.5 many things are happening. So that is why to be on the safe side we use 0.5. Because if it is 0.5 of VCE I mean for example VCE sat of 0.2. 0.2 will come here. 0.2 will come somewhere here. Still rough just barely but I am saying this is a kind of a figure we used you know with kind of now let us run the experiment. Let us run the experiment. It might take some time. What will happen is first the input characteristics will happen and then the output. So let us do this. We will do this. Let us hope it works again. What is happening in the top is the first iteration for the VCE value of 1 volt. And that has come here. Now the second iteration is going on. That is for the VCE value of 5 volts. It is an actual experiment. Now the other one will take some time. So this is over. This is exactly we are saying this getting the same curve as the previous one. Now you can see here again here is each curve will come here and that fellow will keep coming down. So you can see here the line here I mean basically at a particular voltage of VCE we can find out what is the kind of different base currents. Now what is something interesting here? Just looking at it what can you say about beta? What do you think? Looking at these graphs what do you think about beta of the transistor? Now this kind of an equipment if you buy this is called a curve tracer. Buy an equipment from some company charge you at least 5, 6 lakhs. The whole thing does not even cost us 30,000, 40,000 rupees. It can be even cheaper. We are trying to make it even cheaper. So you do not need a very costly equipment to do such anything. But if you buy an equipment it will cost you 4, 5 lakhs. A curve tracer is very costly. But this is exactly what we are doing curve tracer. So you can put this in a lab and very good. Then what are the tests of the transistor? Put it there, test and tell you. In fact you write little bit tell you beta also. But you look at the graphs what do you think about beta? Is it my question is if your beta DC fixed or is it increasing or decreasing or anything? Strictly speaking it is very difficult to say from here. Strictly speaking beta is a function of IC also. There is a value of IC for which the beta is maximum. And generally in a specification sheet the transistor is designed for a particular value like depending on the application of the transistor. If it is a power transistor you are talking about large values of IC. But you are talking about a small signal amplifier used for an amplifier kind of application. You are talking about a very small value of IC. Very small means 1 million. Typically if you are talking about a low noise amplifier kind of a transistor. Typically you will never talk about more than 1 million. Medium like SL100 they will say talk about all characteristics they will specify 10 million. If you talk about you know kind of power transistor 2N3055 you will talk everything about 1 amp. IC 1 amp will make all specifications. The point I am trying to make is there is a value of IC for which the transistor has been designed. So, it is optimized for that value. So, when you keep changing your IC your beta also will change generally not drastically. But you should not neither operate at a much larger current or a much smaller current. Especially lower currents will give you what? Higher beta or lower beta? Lower collector currents will give you higher beta or lower beta? Lower beta. Lower beta generally beta is supposed to be almost kind of linear. But it is not quite after sometime it kind of comes down. So, it is good that we saw a lot of things I have exactly 1230. So, I think I will stop right here. I mean very important you have an afternoon session also. Any quick question? Any question? I hope our time together has been meaningful. How will I check which terminal which terminal is if I do not know that the body is collector. So, using multimeter will check. So, in multimeter like some in diode portion is there if we turn the knob towards that portion and we check we find that the value comes in 3 digit value comes. So, whichever is higher we consider that that terminal is supposed to be like for emitter and for collector. So, I am I did not understand how we judge this one. Maybe you will be able to my understanding is generally you take a normal transistor try to measure the base emitter voltage that I would check mode check the base emitter voltage and the base collector voltage you would find that generally the voltage are almost similar. But when you measure the VCE we check try to check there in one direction I mean that is how you can find out this in the multimeters earlier multimeters the analog multimeters you would find that when you measure between collector and emitter you would find in one direction you will find you know current kind of slightly higher than the other one. That way you can uniquely say this is collector and this is emitter, but with modern maybe madam will be able to say. Can you distinguish between collector and emitter given a transistor just by check device very difficult. Yeah. Because one of the biggest problem is as you know in any semiconductor the variation of parameters in fact this one very important thing about a semiconductor discrete device the variation of parameter from one sample to another sample is huge sometimes can be as here 100 percent. So, even if you have from a good manufacturer originally also bought you might have a lot of variation. So, it is not a good idea too. So, best thing is that is why people use curve tracer you put a curve tracer you can immediately see the characteristics. In my opinion it is a good investment give us a project to one of a beta project. You can give it using a simple diode you do not need anything sophisticated like this you can make using counters and all you can do this. But it will be a good investment maybe a simple one graph you need one base current one collector current put and you said this is LCD displays are very easily available you can nicely get I would say good investment and your students will also feel happy you will also be happy and a curve tracer is the best any day because very often what happens is you do not know whether a device is working a simple diode test will not tell you good or bad. So, your simple curve tracer will tell you this is a beta you want to know what is that at a value of I C very important. So, have a simple circuit by which you can adjust the I C and measure the I B with the multimeter something simple curve tracer I would say give us a decent project good project for your college a simple diode test some simple such things nothing sophisticated simple you can make and it will be a good project for the lab let it be for the analog electronics lab put it there. Another important thing I want to say was we if you are interested we can make not that what we are doing is the best, but another very very important component of teaching is good assignment. So, this is assignment which we use in the last semester for transistor variety of problems the most of things I talking about are here if you want we can give you a full set of assignments which we used you are free to use it no problem. Now, similarly for diode source why I am saying is a problems are very important and the problems should be chosen such that they cover in a range of problems from very simple to you know reasonable complexity and all those problems should be available. We can make all this available to you no problem just one more thing before I close two major web resource are available to you. One thing is NPTEL I hope this will be a made available to you I think all this not so valuable. I have if you just put NPTEL if you just go to the NPTEL side type basic electronics huge IIT Roorkee that is very good both web codes and video codes look at the web codes the entire syllabus is covered and very well simple cover coverage is there. Another one is for example Professor Mahesh Patil tomorrow he will be taking lecture his entire codes and more is put on his website tomorrow he will give I think he should not let him very good I have been also using it you are free to download. In fact, we believe in just taking it because we have got lot from the nation we have to give it back to the nation and the best way to give it back to you because you are you know serving many more students than us. So, please feel free to take it and use it. So, there again tomorrow he will if you go to his website he will tell you our E department go to Mahesh Patil you will get the entire thing can download take it just use it and the entire course is covered and more lot more and we defined it basically for the basic electronics course which we have here first year for all students. So, you feel free to use that. Thank you very much.