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Current time:0:00Total duration:11:30

Video transcript

when you sing a song into a mic it converts the changes in the air pressure into changes in electric current now if you attach this to a speaker then it converts these changes in the current back to sound but you will probably hear nothing and the reason for that is because this current produced by the microphone is very tiny causing extremely tiny vibrations and as a result you will hear nothing but if you could somehow increase the strength of the current over here and keep the pattern exactly the same and now since the current has increased the speaker will vibrate very nicely and since the pattern is exactly the same you'll pretty much hear his voice and his song so in order to make this thing successfully work we need a device in between that increases the strength of the current but at the same time keeps the pattern exactly the same such devices which do that are called amplifiers and usually these amplifiers are found inside the speaker's themselves now earlier amplifiers were big and bulky which made our speakers our devices big and bulky so in order to build let's say a pocket radio or maybe to build headphones we need to make tiny amplifiers and it was this trio Shockley Brattain and Bardeen working at Bell Labs realized the key to building a very tiny amplifier was using doped semiconductors and after a lot of research and experiments they finally rented the first semiconductor amplifier in around 1947 1948 so let's find out what they did their idea was to use an n-type semiconductor with a lot of electrons and sandwich in between a p-type semiconductor with very few holes like this or or another thing that you could do is take a p-type semiconductor with a lot of holes and sandwich in between an n-type semiconductor with very few electrons they call it the transistor the transistor and we'll see the reason behind this name in the future videos but since this material has n over here then it has a P and it has an N type again we call this as the NPN transistor and similarly if you look at this one you have P this is the p-type then the n-type then again the p-type we call this is the PNP transistor and the key to working of this is that this middle region that is this that's the key actually this middle region that we have the p-type here and the n-type here it's your satisfy two conditions one it has to be very thin and we'll see why in a minute it has to be very thin and two it has to be very lightly doped very lightly doped as you can see the doping here is much smaller than what you have over here same thing over here we'll see now that under such circumstances these things will act like an amplifier so we can look at either the NPN or the PNP let's let's look at NPN and see how it can act as an amplifier so here is our NPN transistor let's begin by attaching a power supply across its ends so let's put some metallic contacts and let's say we attach the positive of the power supply here let's say about five words so plus five world and the negative which is usually the ground we're gonna connect that over here this is the ground and by the way I'm not showing the power supply if it was a real circuit a practical circuit would be like this you would have a positive of the power supply here the negative the power supply right over there and that would be connected over here but I'm just ignoring this part of the circuit it's there of course but I'm ignoring that so we can focus more on the transistor action so what do you think is going to happen just pause the video and think about this well since we have a positive over here we might expect the electrons to get pulled out like this and you might have a current over here electrons flowing like this but in order for that to happen the electrons must continuously flow from this region into this region as well right we need electron flow everywhere but can the electrons from this region whoa into this region the answer is no the reason is remember that at every PN Junction there is a depletion region which acts like a barrier for the flow of majority charge carriers the electrons are the majority over here they really like to flow from here to here due to diffusion but the barrier prevents them and as a result since these electrons can flow from here to here due to the barrier these electrons can't get pulled across and there will be no current in the circuit and regardless of what voltage you put even if you put a ten volts or a fifteen volts over here you can't really don't expect any current so it'll be nothing now if you really want the current you know what we could do you could attach another terminal over here you could attach another terminal and put another circuit over here say we apply a positive to this a pause again I'm not gonna draw the entire circuit I'm just gonna draw the positive through this let's say we put about a positive point seven world over here and we'll see in a while I am choosing points on an old what do you think is going to happen I can pause the video and think about this well now if you look carefully notice the P is connected to more positive than this n this n is grounded and as a result for forward biasing this Junction so if you take this this Junction is being forward biased and similarly if you look at this Junction notice the N is connected to more positive than P and as a result this is reverse biased this is reverse biased and now remember that in a when a PN Junction is reverse biased it doesn't allow the flow of majority charge carriers and so these electrons and holes can't flow across but this Junction is forward biased under forward bias the majority charge carriers can flow across they can't diffuse into each other and guess what if you remember for silicon if you hit point seven volt that's when the depletion region vanishes and as a result these electrons and holes can now easily diffuse into each other so let me show that so these electrons will start diffusing into the P and of course these holes who also diffuse over here but I'm going to neglect the holes because they're a very tiny number anyway now the question is what's going to happen to these electrons and now we're reaching the climax of the transistor action alright notice that these electrons can either be pulled out from here because of the positive or they can be pulled out from here because of the positive here what's going to happen well remember that whenever we have a forward bias PN Junction in order for those electrons to get pulled out they have to undergo recombination we have spoken a lot about this in previous videos so if you need more clarity it would be great to watch that but anyways in order for the electrons to get pulled out they have to undergo recombination and the recombination chances over here in the transistor is very very small for two reasons one is because this P region is very lightly doped so there are a small number of holes to begin with and as a result the recombination chances are small but the second reason is that this is very thin and as a result most of the electrons that get injected over here will find themselves already at this end and as a result they can now be posed by this voltage you can also think of it this way you see when they are reaching this this Junction because it is reverse biased remember a reverse bias doesn't allow a majority charge carriers to flow through but they accelerate my knowledge charge carriers right and these electrons in the P region are the minority charge carriers so because of the electric field they get accelerated and they get collected over here and then this they can you know flow through the terminal over here so what's happening over here is that most of the electrons which get injected will just come out from here and only a fraction of them will get recombined and as a result they get pulled out from here so if you were to put some numbers if you were to put some numbers we could say about a hundred electrons are being injected for a second hundred electrons are injected for a second and maybe due to recombination your electron is being pulled out from this terminal so one electron per second and as a result about ninety nine electrons get pulled out from here ninety nine electrons get pulled out from here which means the in this wire is about 99 times the current in this wire and you may be like whoa what's the big deal well the big deal is let's think of it this way you see in order to pull one electron from here when we try to pull one electron from here about 99 electrons gets pulled out from here right that's the way we can think about this well I imagine if we increase this voltage and try to pull more electrons from here let's say I've tried to pull two electrons from here what will happen well we have the statistics here we've seen that almost out of 100 one gets pulled out so in order to remove to about 200 will get injected they have to get injected right you're increasing the forward biased voltage more will get injected about 200 get injected to comes out from here that means 199 would 198 will get collected over here that means notice when you double this this also has doubled and if this were a triple this would be triple if this were to half this would be half and so on in other words if the current in this wire fluctuates the current in this wire would fluctuate in exactly the same manner however the current over here will always be 99 times more than the current over here in other words this is this the current in this while is the amplified version of the current in this wire and it's exactly what we needed and so if you want to use this to amplify your sound then you can connect your microphone wire over here now the microphone will be the one that will provide the voltage over here needed to forward bias this Junction and the voltage provided by the microphone will depend upon this sound if your sound is very loud this voltage will be high if the sound is very low like you're whispering the voltage will be very low over here of course the engineers will make sure that the voltage over here will never go below point seven world and all of that stuff but don't worry this voltage will fluctuate pretty much depending upon your sound and as a result the current in this wire will also fluctuate depending on your sound now guess what the current in this wire will fluctuate in this exactly the same manner as the current fluctuates over here that's what we saw but it is 99 times more so it's amplified so if you feed this now to a speaker then in the speaker the sound generated will be much louder than the sound that you're producing over here but the pattern of that sound will be exactly the same because the fluctuations are exactly the same which you mean that sound will be exactly like your voice but will be much louder and that's how a transistor can be used as an amplifier