 Now, we will talk about the output characteristic which is lot more interesting, so we will quickly go there, write down output characteristic, so what is the variable as a current, what is the output current here? This IC is the output current, okay, so this is IC which is measured as milliampere and then on the X-axis we have VCE, the output voltage, so output voltage, output current, fine and I am changing the output voltage, this is 2, 4, 6, 8, 10 and like that and the current is changed like this, 2, 4, 6, 8, you know like that, now tell me, if you change collector emitter, voltage, what effect the current will have if you are keeping all the input variables constant? Not much, it is like constant, it should be because reverse bias, see here again you referred to input characteristic, input characteristic will tell you how much current should change and that current is a function of what is base emitter voltage, fine, so if you change the output voltage, okay, output voltage will increase the reverse bias, okay, so if you increase the reverse bias, it is favorable for current to flow or not, see refer this, when you have base collector, reverse bias, is it favorable for current to flow or not? Yes sir. It is favorable, right, but then it will not create its own charge, what I am trying to say is that if 5 electron crosses the forward bias, the reverse bias can catch only 5 electrons, no matter how strong the reverse bias is, okay, so initially it will favor and then would get saturated, because if 5 electrons are coming from this side, it will catch only 5 electrons, getting a point, fine, so initially it will try to increase but then quickly it will saturate, fine, here you are keeping the base current constant, you are keeping the base current as 10 micro ampere, let us say, alright, now if I increase the base current, if I increase the base current from 10 micro ampere to 20 micro ampere, what should happen, any guesses? Graph will shift up, graph will shift up, graph will shift up, because there are now more number of electrons but then again it will catch only those many that are coming, it will not catch more than what are coming in, so you will see graph like this, this is for 20 micro ampere base current, this is let us say 30 micro ampere base current, alright, okay, and then you can just keep on plotting like that, now one very, very interesting and important thing, you can notice here is that if you increase the base current by this much, this much is how much, from 10 to 20, how much is the change, change in base current is 10 micro ampere, all of you agree, 20 minus 10 micro ampere, fine, now change in collector voltage is what, roughly 2 milli ampere, yes or no? Yes sir. This is a change in collector current corresponding to change in the base current, okay, now if I do this, change in collector current divided by change in base current, how much you are getting, quickly tell me? 200. 200? Yes sir. What this tells you, if you change the base current by one unit, the collector current changes by 200 units, all of you understand the implication? Yes sir. Now why I am not stressing so much on this fact, is this, this particular thing which you have just learned, that change in collector current divided by change in base current is roughly 200 in this case, this can be utilized as an amplifier, okay, amplifier, level of amplifier is almost every electronic device has an amplifier, okay, like for example your mobile phone, your loud speaker, microphone, TV, radio, everywhere there is an amplifier where what happens you know, when you speak a mechanical wave gets created, okay, let's say this is a mechanical wave, very irregular shape and size, okay, this mechanical wave gets converted into an electrical signal corresponding to this sound wave, an electrical signal gets created, this is your electrical signal, this is, this is a sound and this is corresponding electrical signal, let's say this is, you know, a current gets generated corresponding to your sound, now this current can be fed as base current, you can feed this current as a base current. What happens is when this is fed as base current inside this transistor, what will happen, the base current is changing, right, so this is a change, it increases this much and then it decreases, it remains constant here and then it increases, it decreases suddenly and then it increases suddenly, so there is a continuous variation of the base current, fine, so you can see delta IB is there everywhere, okay, now this, this entire thing, if you pass it as base current then this graph will get amplified, okay, this will get amplified and this will come out as collector current, this base current will get amplified 200 times, okay and you will get it as a collector current, so without changing the shape and without changing the shape of the signal or you are retaining the integrity of the signal but you are just increasing the amplitude out of it, you are getting it, this is possible only when the change is amplified, the change should be amplified, delta should be amplified, okay, it's not same thing as adding 100 ampere on let's say 2 milliampere, okay, it's not like that, if you do that let's say this is your weak signal and if you add let's say 10 ampere onto it, okay, if you add 10 ampere current the signal will just, it will, it was like this, this line correspond to 0 ampere then this entire graph will just shift upwards, okay, this signal was not amplified, it just that it got shifted by 10 ampere supports, okay, and also when the noise or any signal is there, it's not a very nice looking sine curve which you can estimate and then you can do some mathematical analysis and then you amplify by superimposing the sine curve of the current, so it's all the signals are very, very irregular shape and size like this, that so if the change is getting amplified which is the case with the transistor then you don't need to worry about shape of the signal, it will just stretch it and you get an amplified signal, alright, so are you able to appreciate the functioning of the transistor as an amplifier? So, any doubts, anything? Okay, no doubts, alright, so this was just a small glimpse of how useful the transistor can be, okay, so we have still to learn systematically on how it will be getting utilized, okay, now before we get into further discussion about the usefulness of it, we need to define some parameters that will be used to talk in greater detail about the transistor, okay, the first parameter is input resistance, okay, input resistance is small r i, okay, now more or less all the time the transistor is used as an amplifier, okay, so we are more bothered about the resistance against the change rather than resistance for a particular instant what is current and voltage, we are going to estimate how much resistance it offers for the change in voltage, okay, so if that is a case then the way we define input resistance is change in input voltage which is v delta vce divided by change in input current, writing it and this you are keeping the output voltage constant, alright, this is how the input resistance is defined, second parameter is output resistance, so vce is constant and delta vce is 0, yeah, no sorry the output voltage is vce, the input voltage is vbe, sorry, this is vbe, yeah, so input voltage change divided by input current change, now write down output resistance, can you guess what it should be, vce by ic, delta vce by delta ic, correct and what you should take constant, vb, no that's the trick here, now IB, IB you will take constant because IB is, you know, you have to understand that IB has a special place, okay, so when you say something is constant from the input side, the input side is more or less represented by the IB current, so we will mention IB rather than vbe, although there is no great logic behind it, just the notation, this is IB, okay, so this is how you define input voltage, sorry, input resistance and output resistance, okay, now we are going to discuss about the current amplification, the third parameter, current amplification is represented by a letter beta, okay, what do you think beta should be equal to? Delta ic by delta i, correct, anything to be constant, here you will have output voltage constant, fine, so when you are amplifying a signal, you pass the current, okay, you just connect an output source, let's say you connect 100 volt as output source, okay, so it will take power from the output voltage and amplify delta ic, sorry, amplify delta IB, okay, this is by the way beta ac, okay, similar expression for beta dc is defined, beta dc is simply a ratio of ic and ib at a particular instant and what is constant here, nothing needs to be a constant, nothing needs to be a constant over here because it is at a particular instant, you know, it's at a particular instant, okay, so it's not a process that something should be constant and only you will be able to measure it, so there is nothing to worry about here.