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Current time:0:00Total duration:6:28

Video transcript

in this video let's look at the circuit symbol of a transistor we've already seen that there are two kinds of transistors one is the NPN where a p-type is sandwiched between two n types and we also have a PNP transistor where a P an n-type is sandwiched between two P types and in the previous video we even saw what their names are the different regions have their own different names we call the heavily doped one as the emitter the very lightly doped thin region is called the base and the biggest region of the transistor which is usually moderately doped or even very lightly doped is called the collector now let's look at the circuit symbol of this the circuit symbol of a transistor looks somewhat like this this way this way like this this and like this tada there you have it this is how a transistor is drawn in any circuit so you can see there are three regions there will be three terminals for for a transistor and these are the three terminals so this one is the base that's easily identifiable this is the base and one of these two is the collector and the other one would be the emitter now our symbol has to be a little bit better because we should be able to identify which is the emitter and which is the collector because you've seen they're not the same things and we should also be able to identify whether we're dealing with an NPN transistor or a PNP transistor so the way we do this is we put an arrow mark on the emitter head all the time so if this is the emitter we're gonna put an arrow mark over here if this is the emitter we're going to put the arrow mark over there so that's how we can identify which is the emitter and the direction of the arrow mark is going to tell us whether it's an NPN transistor or a PNP transistor so here's how we do it if you look at an NPN transistor when it's working like an amplifier these mo these electrons get emitted from n to P right and as a result there is a current from P to n right because electrons are negative charged particles so that results in a current let me use some color okay let me is this green itself so that uses a current from P to n and so if I were to draw for a NPN transistor and let's say this was the emitter what I would do is let me just write that this would be n this would be P this would be P this would be n again and if this is the emitter I'm going to draw this P to n symbol over here so I would just put an arrow mark over here tada there you have it so the arrow mark is telling me that this is the emitter and if this is the emitter this is the base this has to be the collector and by looking at the arrow mark one can understand who this is the p-type and this is the n-type therefore it's a NPN transistor so similarly if we were to draw a PNP transistor the circuit symbol would look exactly the same I mean I mean pretty much the same okay let's see okay this would be the base let's say I want to make this the emitter and this has to be collector now if I want this to be P and P this would be P this would be P and this would be n again if we were to make this work as an amplifier then notice the holes will get injected from P to n the working is exactly the same and as a result we will now get a current from P to n and so I'm going to represent that on the emitter lead from P to n so our diagram will look like this and so that's how we can show the circuit symbol all right let's take a couple of practice examples so we have two transistor symbols just just pause the video and see if you can identify whether it's an NPN or a PNP transistor and whether you can identify which is the emitter collector and base alright let's do it let's start with this one the moment I see an arrow mark over here I understand this is the emitter this is the base base is easy to identify right it is the base and here is the collector and then I always remember that the current is always from P to n can you see the current from the emitter to base is always from P to n P to N and P to n so the first thing I'll do is I I know that this is P and this is n once I do that well this is P and this is n this has to be P and so I understand ah this is a PNP transistor alright let's do this one the moment I see this is an arrow over here I'm gonna call this as the emitter this is the base this is the collector again I remember the current from the in the emitter base is always from P to n so it's going this way so this has to be P this is P and this is n and so NP and yay NPN transistor and one last thing is that I was always curious as to what was the motivation behind this symbol for the transistor and this picture might give us some answers this is the picture of the world's first transistor ever built or at least it's replica and this transistor looks a little bit different than what we have today and we don't have to worry about how it works these are absolute devices we don't use them today but over here you can see this sort of like a triangle thing over here well this metallic wire was the emitter this is a metallic wire this was the emitter back then this another metallic wire you can see was the collector and this emitter and collector these two wires were pressing on you can see some some kind of a grayish thing over here this was the germanium that they used this was the semiconductor and we called they call them as the base and the semiconductor is lying on top of a copper copper slab or something but anyways you can sort of see we don't have to worry about how this thing works okay don't worry but you can sort of see where this symbol comes from right the emitter and collector are literally sticking to the base like like what we have drawn over here and this also tells us why the base was called the base I mean if you look at this modern transistors like once we use today it doesn't make sense why would we call a sandwich layer base but over here it makes perfect sense that was literally the base on which the emitter and the collector was stuck so I think they devised the names and the symbol when this transistor were built but once we improve the transistor the name just got stuck the symbol got stuck and we thought we'll just keep them