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Active, saturation, & cutoff state of NPN transistor

Video transcript

an NPN transistor connected in such a way behaves as an amplifier but it doesn't always amplify there are certain conditions for that right so in this example we'll explore this in great detail we'll understand under what conditions our transistor acts as an amplifier and under what condition it doesn't so we'll explore the complete behavior of a transistor in this video now I think the best way to understand these behaviors is to look at a practical circuit and the difference between this circuit and a practical one is that a real circuit would have some resistors over here we include the resistors to limit the current so that the current won't increase a lot and our transistors won't blow up and so let me introduce a resistor over here in this in this wire over here and I'm also going to replace this schematic figure with the circuit symbol of a transistor so let's quickly do that so let me bring in the circuit symbol of our transistor so remember this is the emitter this part remote resembles the base and this would be the collector the arrow mark tells us what direction the emitter base current flows in so we can get rid of this schematic now over here and instead of directly connecting the collector to the five volt supply let's introduce a resistor in between and I'm also going to change the supply voltage instead of making it five would let me do it let me make it three worse it's only so that it's gonna become easier for the numbers that I've chosen for this particular example now what this resistor does is that notice because of the addition of this resistance when there is a current flowing IC is flowing over here there's going to be a difference in the potential between this point and this point so the voltage at this point is no longer going to be 3 volts at at all times it will depend on this current and so what we have done by adding that resistor is that our VCE is no longer at this point this voltage or we see our output is now this voltage at this point this is going to be our VCE and that is going to be our output voltage all right now we are going to start playing with this supply voltage and see how the circuit starts behaving the first case we're going to talk about first case is when this voltage vbe Wiebe e is less than 0.7 volt now in reality there's going to be another resistor over here as well but that's not going to affect our understanding so I've just negated that resistance all right so if we B is less than point seven volt what's going to happen well we've already seen that before in input characteristics that if this voltage is less than point seven volt then the base emitter is not properly forward biased and as a result if we go back to our schematic this depletion region doesn't reduce electrons get hardly injected into the base and as a result the base current is almost zero and since hardly an electrons are getting injected hardly any electrons are getting collected as well and so even the collector current goes to zero so let's write that down so we're here the base current is pretty much zero it's approximately zero it's very sign it's very tiny which can be approximate to be zero and even the collector current is approximately zero now notice as a result because the collector current is zero there is no voltage drop over here because Ohm's law no current means no voltage drop no potential difference therefore this voltage now is going to be exactly the same as this voltage and therefore VCE is going to be 3 volts so in this case VCE is going to be +3 wolves and we will see that this is the maximum voltage ever possible ok now this is the behavior in which notice our transistor is not conducting any current there are no currents at all and this behavior of a transistor or this state of a transistor is called the cutoff state of the transistor so we say our transistor is in cutoff so note is in cutoff our output current is zero but our output voltage is maximum all right our next possibility is this vbe vbe is more than or equal to 0.7 volt now we've seen that once you hit that points in world the depletion region vanishes and after that even for tiny changes in the voltages the current was going to change a lot so for large ranges of the input current the IB value the voltage pretty much stays at point seven volt and it's for that reason we're going to pretty much assume that this voltage is not going to increase more than 0.7 volt so we can also approximate the second case as VB is approximately 0.7 volt what's going to happen in this case well let's explore to do that let's give some value for this resistance let's say that resistance is 1 kilo ohm 1 kilo ohm all right let's look at some different scenarios now there's going to be some current considerable current over here let's say the current over here is I don't know about 10 micro amperes to keep numbers very simple what will happen over here oh we've already seen that the output current once the transition starts conducting our output current is going to be the amplified version of the input current let's say the amplification factor is a hundred we call that as current gain beta let's say that beta value for our transistor is a hundred it this just means that the output current is hundred times more than the input current so our output current is going to be a thousand micro amperes one hundred times more or a milli ampere let me just write that down so in this example it's going to be a milli ampere all right now think about this because this current is a milli ampere 1 milli ampere flowing there's going to be some voltage drop across this can you see that and we can calculate that voltage drop using Ohm's law that voltage drop V equals IR so how much voltage drop comes over here well I is 1 milli R is 1 kilo so if you multiply the mêlée and the kilo cancel and so the voltage drop becomes 1 hold so there's a voltage drop of one world here and as a result notice out of three words that is supplied 1 volt gets dropped here that means our output remaining now becomes 2 volts so this would become two words so you can see the effect of an input current the output current increased because initially it was zero but now notice the output voltage decreased this is now working as an amplifier because the output current is amplified version of the input current all right now let's increase this voltage just a little bit more and let's say as a result the current changes to 20 grampers what will happen to the output current well output current will be 100 times more than this so Alfred going to be 2 milli amperes now I want you to pause the video and calculate what is the output voltage B all right let's see well since this is now 2 milli amperes the potential drop over here would be tors Ohm's law so this would be two words and notice as a result the voltage over here well this is three you lost two and as a result the output voltage becomes one world one world it's still acting like an amplifier good for us but what if we increase this voltage even further and let's say the current now goes to 30 micro amperes I'm pretty sure you can do your calculations now this would be hundred times more so there will be three milli amperes three milli amperes oops three milli amperes this as a result would become 3 volts has to be right because 1 kilo ohm 3 volts and as a result notice now the output voltage has become zero and this is a critical state for our transistor as you'll see zero world it's still acting like an amplifier but is barely acting like an amplifier let's see why if we were to increase this current even further let's say we got it to 40 micro amperes as an example then what will happen to the output current will it be 4 milli amperes well it seems like that but it can't be and the reason it can't be 4 milli amperes is because then this voltage drop would be 4 volts but that can't be because the supply voltage itself is 3 words you cannot have a drop more than three words so what will happen well the output current will not increase anymore it's gonna stay at 3 milli amperes so even if you increase the current beyond this the output current is just gonna stay 3 milli amperes this is the critical state you cannot possibly get more than 3 milli amperes output current over here I hope that makes sense so as a result we have now two conditions when VB has hit point seven volt so they were right that runs two conditions all right so let's say the first case in this I'm gonna call that as case number two because first case is already done over here in this the first case would be IB when IB is about zero so IB is above zero but below 30 micro amperes let's say below or equal to whatever okay 30 micro amperes in this region our amp our transistor works as an amplifier in this region we could say IC equals beta times IB that's what we found right hundred times IB and our output voltage our output voltage that is VCE that was swinging between zero volts and three words we saw that over here right this is the region where the output current is linearly changing with the input current notice and therefore this region is called as the linear state of our transistor or the linear region of our transistor it's also called as the active region because transistor is it's acting like an amplifier it's behaving like it's supposed to do we would say the transistor is active but there is another possibility that I'm gonna write then over here there's another possibility and then this possibly what happens is I mean this happens when IB goes beyond 30 micro amperes beyond 30 micro amperes this is when our output current has been saturated our output current now IC cannot go about 30 micro 30-million 333 milli amperes it is now become maximum 3 milli amperes it has become maximum so even if you have to increase that current to I don't know maybe 40 micro amperes if you're doing is a 40 50 60 whatever you do our output current will not go just not go beyond this point and our voltage now the upper volta CH has gone to zero or it has become minimum this state where the output current is saturated it cannot go beyond that number beyond that particular value we call this as the saturation state of our transistor saturation and hope you can see that the saturation state is exactly opposite of the cutoff state in the cutoff the transistor is not conducting at all and as a result the output voltage is maximum in the saturation State the transistor is fully conducting can you see that that's the maximum current you can ever get over here and the output voltage as a result has become minimum transistor is saturated so the key takeaway over here is that if you want your transistor to work as an amplifier then our input current has to be within some range that range is decided by many factors like this number over here the voltage supply that you have used the resistor that you're going to use we don't have to worry too much about that that's going into the design and electronics but it has to be within some range and if you go beyond that range we are asking for trouble because our transistor will either act like cutoff or saturation let's end with the curious question you see this cutoff region and the saturation region do you think there's any use for that and when they're not used for amplification but can they be used for something else I mean if you look at this carefully in the cutoff region since there is no current flowing from here to here even though there is a voltage difference even though there is a voltage it's as if the transistor is acting like an open switch right it's as if the transistor is open and that's why the current is not flowing and in the saturation since maximum current is flowing over here and since there is no voltage drop across this it's as if this transistor is acting like a short circuit right so can you think of any application for that