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Transistor as a switch

Let's see how we can exploit the cut off and saturation behaviour of a transistor and build an incredibly efficient switch.  Created by Mahesh Shenoy.

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Video transcript

in a previous video by taking this example circuit we saw the complete behavior of a transistor the key takeaway was when the input current is within a particular range the transistor acts as an amplifier but outside those range the transistor has these two other characteristics which is called the cutoff region where the output voltage is very high but the in current is zero and the saturation region where the current is very high but the upper voltage is zero and if this thing seems new to you or if you need a refresher then we've spoken a lot about this in a previous video so it would be a great idea to watch that video and then come back over here but in this video we're gonna focus on these two regions of the transistor the cutoff and the saturation region and see if we can do something useful with it all right let's get rid of all the unwanted stuff and only keep the things that we need cutoff state and the saturation State and let's do a quick brief summary of what we need over here so that you know we are in the same page in the cutoff stage the input current is zero the output current is also zero and as a result there is no difference in the voltage over here and so this output voltage is literally 3 volts because this is the same as this point so you get high voltage output and when you are in the saturation region what's happening is a maximum current is flowing over here and so all that voltage of 3 volt gets dropped here and nothing comes across this across the transistor so the output voltage goes to zero and it's called saturation because further increase of input current if you increase beyond 30 micro amperes that's what we got in the previous example if you further increase it the output current cannot further increase at all alright so that's why it's called a saturation now let's see what a transistor is behaving like in in these two in these two cases you see in the cutoff region regardless of what voltage you put over here whether you put 3 volt or 4 volt or whatever voltage you put over here the transistor is not allowing any charges to flow from here to here cut off and as a result can you see that it is behaving like an open circuit just concentrate on this part it's behaving as if it's an open circuit between this part so we're gonna say the transistor behaves like a logic off okay I read it out it behaves like an off it's as if it's a switch which we have oft we switched it off sort of like that right so it's an off state but when when you are in the saturation region when you input current goes high in the saturation region notice that it allows the maximum current to flow from here to here the output voltage over here is zero as if that means it's acting like a short-circuit right that's when you get the potential difference to be zero short-circuit which means it's allowing maximum transistor flow in other words it's like it's acting like on you see so the transistor is allowing the charges to flow from here to here so we could say it's acting like on so we can summarize the two behaviors by representing this transistor as a switch now so notice in the saturation state when the current is more than 30 micro amperes in the saturation State our transistor acts like a closed circuit short-circuit arm but if the current goes to zero but if the input current goes to zero off on off on off so this means our transistor is behaving literally like a switch it can switch on and switch off things all right let's it has a tiny bulb over here and see how we can switch on a switch off things so let's let me go to the let me go to the on state over here and let's put an LED over here all right when our transistor is in the saturation or the on state notice that all the charges will flow over here because this is a short circuit whenever you have a short circuit all the charges flow or there may be your tiny amount of current might flow over here so in significantly tiny that our bug will not glow at all so although our transistor is in the on state notice because it's supplying zero voltage over here our LED will not turn on at all so this is the in this state it will be there will be no glow so let me write that down here there'll be no glow however if we were to now turn our transistor into the cutoff the off state see what would happen all the LED would grow now because now there are no charges flowing from here to here so the only part allowed for the charges to flow is from here to here because now the entire three worlds is coming across the LED and so the LED starts conducting and therefore in this region the LED is glowing and there you have it you can switch on and off this LED just by using a tiny current I mean just look at the current needed to turn on and turn off this LED isn't that amazing that's why this is an amazing switch so let's look at the on/off again well we are in the saturation State the transistor is on but the LED is off now we are in the cutoff state where the transistor is off but the LEDs on oh you may have one question as to why are we using our transistor and doing all these things as a switch why can't we just use a mechanical switch well there are many advantages for this one is that notice that a mechanical switch has a moving part whereas this switch has no moving parts remember which is representing it as a mechanical switch in reality is just a transistor with absolutely no moving parts and as a result what happens is you see with the moving parts there is a limit to how fast you can switch it on and off things but since this has no moving parts its switching speed is incredibly high the second thing is look at the power consumption the electricity it uses is so tiny so it makes it extremely low power device and house is extreme it's extremely efficient and another important thing is that today's transistors can be made extremely tiny which means you can fit a lot of these switches in a very small space so the advantages are they are extremely tiny they are extremely fast and they're extremely low-power and the radical thing is by using these switches lots and lots of these switches together you can make operators which can do addition which can do subtraction and all of these mathematical computing things and eventually build a computer that's right that's what's inside a computer lots and lots of switches like these and so what's mind-boggling for me is that whether you're playing a computer game or watching a Khan Academy video what's really allowing this experience for you is lots and lots of tiny transistor switches turning on and off incredibly fast