Hey friends, welcome to the YouTube channel ALL ABOUT ELECTRONICS. So, in this video, we will learn about the common collector configuration of the BJT. So, in case of this common collector configuration, this collector terminal is common between the input and the output side. That means here, the input is applied between the base and the collector terminal and the output is measured between the emitter and the collector terminal. Now, here when we say that the collector is common between the input and the output side, then actually we are referring to the AC signal. But for the DC biasing perspective, the circuit would look like this. And here to use this BJT as an amplifier, it is biased in a such a way that this base-emitter junction gets forward biased and the collector-base junction gets reverse biased. And in fact, in terms of the DC biasing perspective, it looks quite similar to the common emitter configuration. But here this resistor is connected on this emitter side. But when we talk about the AC signals or during the AC analysis, all the DC voltage sources are considered as zero. That means during the AC analysis, this collector terminal gets grounded. Now, when we apply the AC signal at this base terminal, and when we measure the output at this emitter terminal, then we can say that the input is applied between this base and the collector terminal, and the output is measured between this emitter and the collector terminal. So, this is how the AC input signal is applied at the base terminal and the output is measured at the emitter terminal. And if required, the load can be connected over here. But for the simplicity, if we neglect this base resistor and the coupling capacitor then the equivalent circuit for the AC signal would look like this. But for the DC perspective, this is how the biasing voltages are applied. So, now if we see the direction of the currents, then the direction of the currents in the circuit would look like this. So, here this base current is the current on the input side, while this emitter current is the current on the output side. And similarly, if we see the DC voltage on the input side, then it is equal to Vcb and the DC voltage on the output side is equal to Vce. That means here, this Ib and the Vcb are the currents and voltage on the input side, while this Ie and Vce are the parameters of the output side. And similar to the other configurations, the behavior of the device can be described by the two characteristics. That means the input and the output characteristics. So, first of all, let's see the output characteristics. So, this output characteristic defines the relationship between this emitter current Ie and the voltage Vce. And if you look this different Ie vs Vce curves, then it is very similar to the common-emitter configuration. Because over here, this emitter current Ie can be given as alpha times Ic. And as the value of alpha is very close to unity, so we can say that this emitter current is approximately equal to collector current. That means with the change in this voltage Vce, the variation in this emitter current is similar to the collector current. And that is why in this configuration the output characteristics look similar to the common emitter configuration. Now, if we talk about the input characteristics, then it shows the relationship between this base current Ib and the voltage Vcb. And as you can see, for the fixed value of voltage Vce, as we increase this voltage Vcb, then this base current Ib reduces. Because, this voltage voltage Vce can be given as voltage Vcb plus voltage Vbe. That means for the fixed value of voltage Vce, as the voltage Vcb increases, the width of the depletion region increases. And due to that, the effective base width reduces. And as the effective base width reduces, the probability of the recombination in this base region reduces. And due to that, this base current Ib reduces. That means as the voltage Vcb increases, this base current Ib reduces. Now, for the fixed value of voltage Vcb, as we increase the voltage Vce, or in other words, as we increase this voltage Vbe, then the base current Ib will increase. Because as the voltage Vbe increases, more and more electrons will be pushed by this emitter terminal. And due to that, this base current Ib will increase. That means for the fixed value of voltage Vcb, as we increase this voltage Vce, then the base current Ib will increase. Or in other words, for the larger value of Vce, the entire curve will get shifted on the right-hand side. Now, as I said earlier, in this common collector configuration this emitter current is the current on the output side, while the base current is the current on the input side. And the ration of this emitter current to the base current is known as the current gain of this common collector configuration. And it is denoted by the symbol of gamma. Where gamma is equal to Ie / Ib. And this gamma can also be represented in terms of the α and β. So, now let's find the relationship between this α, β, and γ. Now we know that this emitter current Ie can be given as Ic + Ib. That means this γ is equal to (Ic +Ib) / Ib And if we neglect the reverse saturation current, then this collector current Ic can approximately be given as β Ib. That means γ is equal to Ic/Ib +1. And that is equal to β+1. That means γ is equal to β+1. Now, we also know that β can be given as α/ (1- α). That means β+1 is equal to α/ (1- α) +