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Output characteristics of NPN transistor

Let's explore the behaviour of output current (collector current Ic) as the output voltage (Vce) is changed in an NPN transistor.  Created by Mahesh Shenoy.

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  • piceratops ultimate style avatar for user Aditya Sriram Bhaskara
    Shouldn't we also consider breakdown voltage when drawing the graph for Ic vs Vce (at a constant Ib). Because then,when we increase the voltage the current shoots up but the voltage remains same?
    (5 votes)
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    • blobby green style avatar for user uniqxlel
      This was actually mentioned in the introductory video for transistors. Essentially, this has already been accounted for by the fact that the Collector part of the transistor is 1. Bigger than the Base and Emitter and 2. Lightly/moderately doped, as seen in the Common Emitter figure.

      That said, this is a valid concern since it's a reverse bias. Perhaps the reason breakdown voltage hasn't been mentioned is it's actually more complicated than simple reverse bias? Or maybe he'll mention it in later video/s.
      (1 vote)
  • leafers seedling style avatar for user srsamartha
    At , if we put more voltage, then won't there be more electrons going into the collector region as it is more positive there?
    In other words, shouldn't Ic increase when Vce is increased?
    (2 votes)
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    • blobby green style avatar for user mousumitirumo022717
      We have studied in the previous video that increasing the external voltage across the PN junction will not increase the current across it, it will only only produce the same number of charge carrier but number of charge carrier produced remain same. We know current is number of charges per unit time. Thus number of electrons diffusing should be increased to increase the current in the circuit. Only when the circuit is run in breakdown region the current will shoot up but that to because of high acceleration and knocking of electrons from the bonded atoms.
      (2 votes)
  • blobby green style avatar for user Agnirup Chakraborty
    Nah like shouldn't it be like this that when we increase VCE, the electrons that have reached the base from the emitter have more tendency to be swept across the depletion layer between B and C rather than being out as Ib (as we know reverse bias favours movement of minority carrier across).....and hence increasing Ic?
    (2 votes)
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  • blobby green style avatar for user Jmath
    Why doesn't the Vce affect the diffusion current in the forward biased BE junction?
    (1 vote)
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Video transcript

let's explore the output characteristics of an NPN transistor we were already seen how we can make our NPN transistor to work as an amplifier and we've seen these different currents and everything if your emitter base has to be forward biased and the collector base has to be reverse biased and we also put the names for these voltages this voltage which is connected to the base with respect to the emitter we call it as the vbe and we call this as VCE in a previous video we saw how changing vbe how changing this voltage affects this current we call it as the input characteristics so in the output characteristics what we can do is we're going to change this voltage this is the output voltage and we'll see how this affects the output current IC so let's do that so we're going to plot a graph of IC IC versus VCE and one thing we need to be careful about is remember that IC can also change due to IB that's the whole idea behind amplification right when I will be doubles IC also doubles if IB were to triple IC would also triple but we don't want our IC to change with IB we want to see how IC behaves when VCE changes so we don't want I be to metal to changes in IB to metal with our experiment and for that reason when we perform this experiment we have to make sure that IB is a constant so IB must be constant alright let's look at the plot now so here it is what would you think this graph looks like well what is this is a graph where the PN Junction so here's the PM is the n where the PN Junction is reverse biased right because the then the n-type is going to more positive than the p-type so this should be a graph of the reverse bias and we've seen what the real grass graph looks like in that the current is usually independent of the voltage and the same thing when I find over here the current that we get over here is pretty much independent of this voltage and the reason for that is because this current only depends on how many electrons get injected over here for example if hundred electrons get injected one electron gets collected or one electron comes out of the base then regardless of what voltage you what ninety-nine will get swept across over here so if you were to see this graph you much pretty much see a constant value so this is what you would see here it is voila there it is this is the part that I was saying that this is a constant value and this may be for a particular value of IB I don't know maybe that is for so let's say that the IB for this entire experiment was fixed at 10 micro amperes as an example and so you can see that IC value is pretty much fixed alright and this IC value oh it's it's it's amplified compared to IB maybe in this transistor it is hundred times more amplified so then this IC value would be about hundred times more than ten micro amperes and a hundred times more than ten would be about 1 milliampere oops so it's that so this value let's say is about 1 milliampere that's why I see is in milli amperes now you may be wondering what's going on over here well what is over here even with the current being input current being 10 micro ampere here the the output current is changing over here so this is the part where the output current is is changing with respect to the output voltage can you see that this is the part so why is that happening well the thing is as you decrease the output voltage says you go from plus Phi 2 plus 4 and then plus 3 then maybe plus 2 in plus 1 and so on we're still fine nothing happens but once you decrease the voltage below 0.7 volt let me just write that down so let's say this vc value went below point 7 volt I don't know maybe maybe it goes to point 3 world point 3 would now notice that even though the collector is connected to a positive supply the base is now more positive than the collector because base is at 0.7 and base is P and your collector is N and since base is more positive that means now our PN Junction has been forward biased can you see that our base is having a point 4 volts more than the collector so this is an effective point for world forward biasing and because of that two things happen one is that the depletion width decreases in the forward bias and as a result it becomes harder for these electrons to get swept across but they do get swept across with a lot of electrons are reaching over here so they do get but becomes harder that's one reason why the current drops and the second reason is now under forward bias these electrons can start diffusing into the base region you see when it's a reverse bias these electrons don't play any role but due to forward bias these electrons start moving down and as a result the net flow that you are getting from from the emitter to collector that starts decreasing so it's these two subtle effects which is the reason for for this current dropping and making this current dependent on this voltage anyways we don't have to worry too much about this in detail all we remember is that if the VC value drops too low below point seven volt then this will be forward biased and due to that forward bias you will see that the the collector current starts dropping so we need to be careful when we are making our transistor work as an amplifier because if we somehow forward bias this collector base Junction then we are in trouble because notice we're not getting the amplified output that we are expecting so for amplification to what we would expect to be in this part of the graph not in this part of the graph alright now what we could do is we could repeat this entire experiment for a new value of IB so over here we had kept the IB as 10 micro amperes say we repeat the entire experiment and we keep IB as 20 micro amperes can you now predict what this graph is going to look like I just want you to pause the video and just think about what this graph is going to look like all right let's think about this since IB is now 20 micro amperes that means we have doubled the value of IB V where else we would expect the IC value to also double as long as we're in this region as long as this collector base Junction is reverse bias so we expect the graph to jump now to double the value we would expect the graph to be now at 2 milli amperes over here in this region and of course if we decrease the value of VCE the same effect will happen and the graph would know eventually the current would die out so here's what it would look like there it is and similarly if you could say triple it maybe if you go to 30 micro amperes well the same thing is going to continue and you could keep on going keep on going keep on going and this is what you will end up getting so long story short what we can understand from this is that as long as VCE is high enough to reverse bias the collector base Junction we are getting this region where we are or our transistors work as an amplifier but if we see goes to low we are now in this region over here and again let's not wait too much about why this you know you're getting this kind of graph the the effects that we discussed over here are very subtle and a little bit complicated will not worry too much about that we'll just say that okay this is the region where you know the collector current is not behaving like an amplified version of the base current so as an engineer if you want to use this in your circuit we have to make sure that our transistor if you're using it for an amplifier for example we want to make sure that we are in this part of the graph we are not in this part of the graph and one last tiny tiny detail a small detail that I have skipped for quite a while now is that this connection that we have done that we've been using for such a long time now if we give a name - it's called the common emitter connection common emitter connection or we just people start from is C II more or cee connection and the reason we call that is because notice that if you look at this complete circuit then you have a circuit connecting base and emitter and you also have a circle connecting the collector and the emitter which means the emitter is common to both the input and the output circuit and that's why it's called the common emitter connection and so these graphs the output and the input characteristics we have seen are called the common emitter output in the input characteristics there are other ways to connect your transistors as well you can make base common or you can make a collector common but the good news is we're not gonna worry about these other modes all right those are only in the engineering domains we don't have to worry about them at all the only thing that we'll be talking about is always common emitter more and the reason for that it turns out that it's superior as an amplifier common emitter mode is superior to all the other modes and again we're not gonna try to understand why that is true that's into the engineering part of it all right