 Now, this same transistor can be connected in multiple ways, right? In our syllabus, we have a special kind of connection, which is discussed in your NCIT, that is common emitter configuration, okay? So we'll talk about common emitter configuration and while we discuss common emitter, you will understand how and why there can be many other type of connections. So write down common emitter. Common emitter, I have written it as CE, common emitter type configuration, okay? Now, we know that a transistor will be like this, you draw with me, okay? So let's say this is emitter, this is collector and this is base. Now why it is called common emitter? Because in this configuration, this is output between collector and emitter. You have output and between emitter and base, you have input, getting it? So this is input side and this is output side. And what is common between input and output? Emitter. Emitter is common. So that's why it is called common emitter. Similarly, there can be common base, common collector, okay? But that's not in our syllabus, so we'll not get into details of it, okay? So now, just like when we discussed about diodes, we talked about its, you know, how it behaves as in the plot between voltage and current. Similarly, in the case of transistor also, for common emitter configuration, we are going to discuss how voltage and currents are related, all right? Now, since we are going to find out how voltage and currents are related, we need to connect voltmeter and emitters, okay? And also we need to connect a voltage source which will vary the different voltages, okay? So that's why there is a small modification to this. So I need to measure which current I need to measure. I need to know all these three currents, right? Collector current, base current, and emitter current, but do I need to measure all of these three? No, sir. Any two is enough. Any two is enough, okay? So what I'm going to do is that I am going to measure collector current and I am going to put a milli-emitter over here, fine? It's a small doubt, sir. So since there is no connection between the collector and base, where will we place the cell? There is no connection. So don't you want to put a voltage supply between the base and collector and base and emitter? No, they are connected internally. You can't connect a source between base and collector and base and emitter, okay? But then, yeah, you can connect a voltage source between these two points, or between these two points. But you can't connect here. So I mean externally between B and C. Yeah, externally you have to connect. Then only it will work. It's like I am drawing a diode like this, okay? This diagram is right now like this only, okay? Unless you connect this somewhere, it will not be functioning. But then this is a diode, okay? Similarly, this is a common emitter configuration, okay? Now in order to understand how it works, I am connecting different measuring devices. So I am going to measure the base current. Why I am very interested in measuring base current because usually the input current is passed as a base current, okay? You will understand how it is later on. So this is collector current. This one is the base current, okay? Now base current is very small. I am going to use micro emitter over here, fine? More or less all the electrons that are passing through the emitter will get collected by the collector. So from emitter, all the electron that goes like this, more or less collector will catch them. Few electrons only will pass through as a base current, okay? So that is why the base current usually is very small. Now, so what I will do, I will earth emitter. Okay? And then I am going to measure the voltage between collector and emitter, fine? This voltage, I am going to call it as VCE, fine? This is a voltmeter connected, fine? And then we are going to connect a resistance over here, okay? Also a voltage source over there. I need to make sure that, you know, emitter and base is forward bias and base and collector is reverse bias. So that is ensured only when you connect external batteries. This is VCC, okay? And similarly on this, this is output side, entire of this is output side. Now I am going to build the input side. In the input side also, I am going to measure this voltage difference between base and emitter, okay? So this voltage source and this voltage difference, I am going to call it as VBE, okay? This one is R1 resistance at the output side and at the input side, we have R2. This is R2 and there is this voltage source, okay? The arrow represents that I can change it as in I can vary that voltage, all right? So this is the complete setup to understand the behavior of common emitter configuration. Similar kind of setup we have drawn for the diode, but the diode circuit was relatively simpler, okay? But then here you can understand here that, you know, you have to measure two out of three currents and you have to measure the output voltage as VCE and input voltage as VBE. So I think it is logical. So that is how you build this circuit and now let us talk about its characteristics. Any doubt on this? Anyone of you? No, sir. No, sir. Fine. So here you can appreciate one thing is that you have two sets of voltage and current. You have input voltage and input current and you have output voltage and output current, fine? So you have two characteristics to draw. One characteristic is called input characteristic that is a plot between input voltage and input current and there is a plot for the output characteristic that is the plot between output voltage and output current, fine? And when you plot and when you have a plot between input current and input voltage, you make sure that the variables at the output sides are constant, otherwise there will be factors at the output side which will affect the input current and input voltage. You don't want that. You just want to see how input voltage is related with input current. Similarly, when you plot for output voltage and output current, you want to make sure that input variables are not touched, okay? So we'll keep that in mind and we'll see how the variation happens. So the first one, the first one is called the input characteristic, okay, right down input characteristic. So like I said, input characteristic is a plot between input current and input voltage. Can you tell me which one is input voltage and input current? VVB is input voltage, right? No. What is actually driving the circuit, input side? VBB. VBE. VBE. Okay. This is the input voltage, all right? Input current is what? IB. IB. See, you need to see what is inputted and outputted to the, to this, okay? You are only bothered about this. What is going in, inside this, what is coming out from this, right? That is why VBE is the input. You don't care about whether R2 is connected far away or VBB is there. Currently, what this transistor is feeling as input is VBE, nothing else. And this is IB, okay? Now VBE, is it forward bias or reverse bias? Forward bias, okay? So can you guess what kind of plot you will get? Soft point curving. Okay. Let me just, this is suppose 0.2, 0.4, 0.6, 0.8. What will be the current when voltage is 0.4? Hmm? Will there be current or not? So yes, sir. No. Forward. Forward bias will have a barrier of around 0.7 volt. Remember that. Till you reach 0.7 volt, even forward bias will not be functioning. Oh. Fine. So there will not be any current till the voltage goes to 0.7. And after that, it will behave like a piece of metal. And that's how the characteristic will be, okay? All right? Let's say this characteristic is for the output voltage of 10, 10 volts. Fine. Now, if I make output voltage, if I increase output voltage to 20 volt, will the characteristic change or not? Will the graph change? What I'm saying is that right now this is a plot for a fixed output voltage of 10 volts. Now, if I change that output voltage from 10 to 20 volt, will the plot changes? Yes, sir. No, sir. But we said that we won't take into account output. Sir, but he said it will change, no. Others? Will it change or not? All of you. It will not change, okay? It will not change because see ideally it should, you know, it should have gone changed because you are wearing the output conditions. But in this case, it will not change because what I said is once you are crossed 0.7 volt, it behaves like a piece of metal. So piece of metal offers zero resistance path. It doesn't matter what happens at the output side, okay? So it will continue to flow large amount of current, okay? So that is VBE, right? That is voltage base emitter. But you said we are changing voltage collector emitter. Right. So but the 0.7 barrier is only for base emitter. How will it affect collector emitter, sir? It will not affect. See, what I am saying is that this plot is a plot between VBE and IB, okay? This plot is plotted by keeping VCE at 10 volts and you got a plot. Now I am changing VCE. I am making it 20 volts. What I am saying here is that if you change VCE, that is immaterial here. It will not change the nature of the plot. Because from the input side, base emitter offers a zero resistance path after 0.7 volts, okay? Okay. Okay. Fine. So that is the input characteristic.