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Current time:0:00Total duration:7:21

Video transcript

in previous videos you've seen that if you take an NPN transistor and make the connections in such a way that the emitter base is forward biased and the collector base is reverse biased then the transistor acts like an amplifier but so far we had kept these voltages a constant we didn't chain them much because you wanted to focus more on the working of a transistor without complicating things but to get a full understanding of how does a transistor behave in various circumstances we need to understand how changing these voltages are going to affect these currents and the best way to do that is by drawing a graph of voltage versus current in this video we're gonna see how changing this voltage is going to affect this current so let's do that this voltage is called the input voltage and this current is called the input current and the reason for that is whatever voltage or current that you want to amplify will be connected over here will be seen over here and so the graph that you draw for this voltage versus this current is called the input characteristics and before a plot that we've just given name for this you see this voltage is the voltage across the base and so usually we will call this as the base voltage but since this voltage is with respect to the ground all the all these voltages are with respect to the ground and the ground is the one that's connected to the emitter we should like to call this as the base emitter voltage so we be e and similarly this voltage which we are not going to be concerned with too much over here but anyways this voltage is called we see because it's the voltage of the collector and again this is the +5 volt with respect to ground it's five volts higher than the ground and the ground is the emitter so we call this as V C II so our graph our graph over here the input characteristics is going to be vbe that's the input voltage versus IB versus IB and by the way usually when you plot this graph we like to keep this output voltage a constant and the reason for that is this output voltage might affect this input current we'll see a little bit later how that happens but we don't want this over voltage to meddle with our experiment so usually we keep VCE VCE constant over the entire experiment all right so let's quickly go ahead and clot that graph so we have rebe versus IV what does the graph look like well notice that this graph is actually the graph of a PN Junction under forward bias right that's all that graph is going to be it's a forward biased graph and we've seen what a forward biased graph looks like right when this voltage is very low let's say about point 1 world or point 2 world then there hardly be any electrons will that will be injected over here and the reason for that is because the depletion region will still be there as a diffusion will be very low and so the amount of current that we get over here will be extremely tiny but as you increase that voltage and eventually you get to 0.7 volt that's when the depletion region vanishes and that's when you know a lot of electrons can get injected and that's where the current will starts skyrocketing and so the graph that you would get over here from the base 4 for the base over here would be something like this would be something like this okay this is not supposed to be squiggly over here let me try one more time that part all right much better much better so this voltage at which this current starts skyrocketing that's about 0.7 volt that is about point seven volt and by the way even though this current is skyrocketing considerably this current is pretty low compared to say the emitter current or the collector current this current by the way is in micro amperes and the reason for that is because there are very low amount of recombination happening in the base and so very less number of electrons will be pulled out from over here but anyways this is what the graph looks like now this is the graph at which VCE value for the entire experiment kept the VCE value about 5 volt now here's the question what if we repeat this entire experiment and let's say a much higher voltage of we see let's say about 10 volts of we see what would happen I want you just think for a while and see how it would affect the current well what is that if you increase this voltage then the reverse bias increases and over here the depletion region widens and as a result the electrons that were injected into the base are more likely to get swept across because of the higher depletion region because more the depletion region is the one that has the electric field remember and as a result the recombination chances decreases and so you would expect a slightly smaller base current for the same voltage so if you were to plot this graph for a higher voltage of VCE you get a pretty identical graph but the graph change is a little bit it might look somewhat like this now yeah something like this okay so this would be the graph that we might get about 10 volts when your VCE this is the value of VC okay 10 volts and so you can pretty much see that for the same voltage is as before you're getting a little bit less current so for example at point 7 volt you see you're getting about this much current rather than what used to get over here I hope that makes sense again because now the recombination rate has decreased because the depletion width has widened and as it is more electrons are getting connected and by the way these two graphs are highly exaggerated over here it turns out that in reality if you do the experiment the difference will be very minuscule so we usually like to say that the output voltage the VC hardly affects the input current it does affect ever so slightly now the major takeaway that you can get from this input characteristic graph is that if the voltage of the base the base emitter voltage if that voltage is somewhere over here let's say about I don't know maybe point 2 volt or something like that notice that the input current is pretty much zero IB is pretty much zero which means hardly and electrons are getting injected and as a result hardly any electrons will get collected which means IC would also be zero so if you want your transistor to work as an amplifier make sure that your base emitter voltage is at least at around point seven volt because once you hit point seven volt then the current will be considerably high and the amplification will so that's one that's the key takeaway over here so let me just write that down if we ve if we a is less than point seven would that it's not doesn't have to be exact point seven point six two point seven pretty much then we could say that the IB would be zero and I will make IC IC also zero so your transistor won't work over here at all which means below point seven world our transistor is not going to amplify anything so this is something that engineers have to take care of when they're using their transistors as amplifiers