Let's explore the output characteristics of an NPN transistor. We've already seen how we can make our NPN transistor to work as an amplifier and we've seen these different currents and everything. 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 that 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 IB 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 IB to meddle, the changes in IB to meddle with our experiment. And for that reason, when we perform this experiment, we're going to, 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, notice this is a graph where the pn junction so here's the pn is the n where the pn junction is reverse biased, right? Because the the n type is going to do more positive than the p type. So this should be a graph of the reverse bias. And we've seen what the reverse bias graph looks like in that the current is usually independent of the voltage.
And the same thing we're gonna 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 100 electrons get injected and one electron gets collected or one electron comes out of the base then regardless of what voltage you put 99 will get swept across over here. So if you were to see this graph you must 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 microamperes as an example.
And so you can see that IC value is pretty much fixed. All right. And this IC value, well, it's amplified compared to IB.
Maybe in this transistor, it is 100 times more amplified. So then this IC value would be about 100 times more than 10 microamperes. And 100 times more than 10 would be about 1 milliampere. Oops. So let's do that.
So this value, let's say, is about 1 milliampere. That's why IC is in milliampere. Now you may be wondering what's going on over here. Well, notice over here, even with the input current being 10 microampere, here the output current is changing over here. So this is the part where the output current 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, say as you go from plus five to plus four, and then plus three, then maybe plus two and plus one, and so on, we're still fine, nothing happens.
But once you decrease the voltage below point seven volt, let me just write that down. So let's say this VC value Went below 0.7 volt. I don't know maybe maybe it goes to 0.3 volt 0.3 volt 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 four volts more than the collector? So this is an effective 0.4 volt 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, because a lot of electrons are reaching over here, so they do get, but it becomes harder. That's one reason why the current drops.
And the second reason is now under the 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 the emitter to collector, that starts decreasing.
So it's these two subtle effects which is the reason 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 0.7 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 are not getting the amplified output that we are expecting So for amplification to work 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 microamperes.
Say we repeat the entire experiment and we keep IB as 20 microamperes. 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. Alright, let's think about this. Since IB is now 20 microamperes, that means we have double the value of IB.
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 reversed by us. So we expect the graph to jump now to double the value. We would expect the graph to be now at 2 mA over here in this region. And of course, if we decrease the value of VCE, the same effect will happen and the graph would, you know, eventually the current would die out. So here's what it would look like.
There it is. Similarly, if you could say triple it, maybe if you go to 30 microamperes. Well, the same thing is going to continue.
And we could keep on going, keep on going, keep on going, and this is what you'll 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, our transistor is working as an amplifier. But if VCE goes too low, we are now in this region over here. And again, let's not worry too much about why this, you know, you're getting this kind of graph.
The effects that we discussed over here are very subtle and a little bit complicated. We'll 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 to it, it's called the common emitter connection. Common emitter connection. Or we just, short form is CE mode, or CE 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 circuit 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've seen are called the common emitter output and 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 going to worry about these other modes, 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 mode. 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 going to try and understand why that is true. That's into the engineering part of it. All right.