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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) +
1. That is equal to 1/ (1- α). That means we can say that this γ is equal
to β+1 = 1/ (1- α). so, this is the relationship between this
alpha, beta, and the gamma. Now, from this, we can say that γ is equal
to 1/ (1- α). And if we multiply both sides by α then we
can say that α.γ = α/ (1- α) And we know that α/ (1- α) is equal to β. That means we can say that α.γ = β. So, these are the relationship between this
alpha, beta, and the gamma. Now, this common collector configuration provides
very high input impedance and very low output impedance. And that is why it is often used for the impedance
matching in the amplifier circuit. Then if we talk about the current and the
voltage gain, then it provides the very high current gain and very low voltage gain. So, in this configuration, this current gain
is equal to β+1. And the voltage gain is close to unity. And if we talk about the power gain then this
configuration provides the low power gain. So, these are the basic properties of the
common collector configuration. And we will see in detail about all these
properties during the AC analysis of the BJT. So, I hope in this video, you understood the
basics of this common collector configuration. So, if you have any question or suggestion,
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