BJT : Common Base Configuration Explained


Hey friends, welcome to the YouTube channel
ALL ABOUT ELECTRONICS. So, in this video, we will learn about the
common base configuration of the BJT. Now, if you see, the most common application
of the BJT is to use it as an amplifier. That means if we apply some input signal to
this BJT, let’s say some sinewave then it will amplify that signal. And whenever we use this BJT as an amplifier,
then there are different configurations in which it can be configured. Like, a common emitter, common base, and the
common collector. So, in each configuration, one terminal of
the BJT is common to both input and the output side. For example, this is the diagram of the common-emitter
configuration of the BJT. And as you can see, here the emitter terminal
is common between the input and the output side. So, in this particular video, we will learn
about the common base configuration. So, in case of the common base configuration,
this base terminal is common between the input and the output side. That means here, the input is applied between
the base and the emitter terminal, and the output is measured between the collector and
the base terminal. And as I said in the previous video, whenever
we use this BJT as an amplifier then it is used in the active region. That means the base-emitter junction is forward
biased and the collector-base junction of this BJT is reverse biased. So, for the NPN transistor, the applied voltage
should be such that, this base-emitter junction gets forward biased and the collector-base
junction gets reverse biased. And for the NPN transistor, if you see the
direction of the currents then it will look like this. That means here, the emitter current will
go away from the transistor, while the base current and the collector current will enter
into the transistor. So, if we talk about the PNP transistor, then
in case of the PNP transistor, the direction of the currents will get reversed. That means now, the emitter current will enter
into the transistor while the base and the collector current will go away from the transistor. But in any case, if you see, the emitter current
is the summation of the base current and the collector current. So, in case of this PNP transistor, the emitter-base
junction is forward biased and the base-collector junction is reverse biased. Now, in this common base configuration, the
behavior of the device can be described by the two characteristics. One is the input characteristics, and the
second is output characteristics. And these characteristics are similar to the
V-I characteristics which we had seen for the diode. And these characteristics show the behavior
of the device when the voltage and the current through the device get changed. So, in this case, if you see, this emitter
current is the input current and the voltage between this base and the emitter terminal,
that means this voltage Vbe is the input voltage. So, the input characteristics define the relationship
between this base-emitter voltage and the emitter current for the fixed value of the
Vcb. And throughout our discussion, we will talk
about the NPN transistor. so, if we see the input characteristics of
this common base configuration then it will look like this. So, here there are three curves for the different
values of Vcb. That means for drawing each curve, the value
of Vcb has been kept constant. So, if you look at these curves, these curves
look similar to the forward characteristics of the PN junction diode. Because indeed in this case if you see, the
PN junction on this input side has been forward biased. And due to that, this characteristic looks
similar to the forward characteristics of the PN junction diode. Now, one more thing if you observe in this
curve, as the value of Vcb increases, the curve slightly shifts towards the left-hand
side. And at the same time, this emitter current
also increases. And the reason is that, as the collector-base
junction gets more and more reverse biased, the width of the depletion region will increase. And due to that, this base region will get
narrower. So now, the less voltage is required to forward
bias this emitter-base junction. And due to that, the curve shifts a little
bit on the left-hand side. Now, from the input characteristics, we can
find the input impedance of the device in this particular configuration. So, if we find the slope of this curve, then
we can find the input impedance of the device. That means we can say that the input impedance
or the input resistance is equal to delta Vbe divide by delta Ie. And if you see over here, even if there is
a small change in the Vbe, there will be a huge change in the emitter current. That means from this input characteristic
we can say that in this configuration, the input impedance of the device is very low. And in fact, that is expected because, in
the forward bias condition, the resistance of the PN junction diode is very small. And typically it used to be in the range of
ohms. That means we can say that in this common
base configuration, the input impedance of the device is very low. So, similarly, let’s talk about the output
characteristics. So, here this collector current is the output
current. And the voltage between the collector and
the base terminal is the output voltage. So, this output characteristic defines the
relationship between this collector current and collector to the base voltage when the
input parameters are kept constant. That means the output characteristics shows
for the fixed value of the emitter current, if we change the collector to base voltage,
then how the collector current will change. So, here the different curves are shown for
the different value of the emitter current. And as you can see, as the emitter current
increases, the collector current also increases linearly. Now, in this curve, there are three regions. The active region, the cut-off region, and
the saturation region. So, on by one, let’s talk about all these
three different regions of operation. So, as you can see, in case of the active
region of operation, even if we increase the collector to base voltage, then also the collector
almost remains constant. And in this region, the relationship between
the collector current and the emitter current can be given as Ic=α * Ie
Now, here the emitter current is the input current and the collector current is the output
current. So, the ratio of this collector current to
the emitter current gives the current gain of the BJT in this common base configuration. That means in this configuration, even if
we increase this reverse bias voltage, then also there is a marginal increase in this
collector current. So, we can say that in this region of operation,
the collector current behaves like a constant current source. And for the fixed value of emitter current,
it is almost independent of the collector to base voltage. And in fact, due to this, this region is used
for the amplification. Because in this region, the collector current
only changes with the change in the input current. Then if we talk about the second region, then
the second region of operation is the saturation region. So, in this region, as we reduce the value
of Vcb, that means as the collector to base voltage goes negative, then this collector
current starts reducing. Because, as we remove this reverse bias between
the collector to the base junction then the electrons which have entered into the base
region from the emitter will not able to cross this collector junction. And due to that, this collector current starts
reducing. So, whenever, both emitter-base junction and
the collector-base junctions are forward biased then the BJTwill operate in this saturation
region. Then the third region of operation is the
cut-off region. That means whenever, the emitter current is
zero, at that time even if we increase this collector to base voltage, then also the collector
current almost remains zero. That means whenever we remove the voltage
between the emitter and the base terminal, then the emitter current Ie will become zero. And under this condition, the collector current
Ic will be almost equal to zero. And in this condition, the only collector
current which exists is due to the minority charge carriers. And in other words, it is only due to the
reverse saturation current. Because we had seen that the total collector
current Ict can be given as Ic +Ico. Where Ico is the reverse saturation current. Now, in the common base configuration when
we open this emitter terminal, the reverse saturation current which is flowing between
the collector to base terminal is known as the Icbo. And this Icbo, is similar to the reverse saturation
current which we had seen for the diode. So, due to the improvement in the construction
techniques, this reverse saturation current is typically in the nono-ampere range, but
for the high power transistors, it is used to be in the range of micro-amperes. On the other end, if you see the collector
current Ic, then it used to be in the range of mA. And due to that, in this cut-off region, if
you see this graph, the collector current almost appears as a zero. Now, similar to the diode, this reverse saturation
current is sensitive to the temperature. That means as the temperature increases, this
reverse saturation current will also increase. So, here the total collector current Ict can
be given as Ic + Icbo. Or we if we represent it in terms of the emitter
current then we can say that it is equal to α* Ie +Icbo. So, this will be the total collector current
which is flowing through the circuit. Alright, so now let’s understand briefly how
the signal is getting amplified in this common base configuration. And here for the simplicity, the DC biasing
voltages are not shown. So, let’s say, Vi is the input voltage which
we want to amplify and the RL is the load resistor. Now, as I said earlier, in this configuration,
the input impedance is very low. And typically it used to be in the range of
ohms. So, let’s say, here the input impedance or
the input resistance is equal to 10 ohm. And let’s say the input voltage is equal to
5 mV. So, we can say that the input current is equal
to 5 mV / 10 ohm. That is equal to 0.5 mA. Now, for a moment, if we assume that α is
equal to 1, in that case, this output current is the same as the input current. Because we know that the collector current
Ic can be given as α times Ie. And for a moment is we assume this α is equal
to 1, in that case, this Ic and Ie will be equal. So, under this condition, the output current
will be same as the input current. That means the same current will also flow
in the output circuitry. Now, for the common base, the output impedance
Ro is very high. And in fact, that is also evident from the
output characteristics. Because if you see over here, in the active
region of operation, even if we increase this collector to base voltage, then also the collector
current almost remains constant. That means the output resistance, that is
delta Vcb divided by delta Ic will be very high. And typically, it used to be in the range
of hundreds of kilo-ohm. So, let’s say, here the output impedance is
equal to 100 kilo-ohms. And here the value of the load is equal to
1 kilo-ohm. So, if we want to find the output voltage
Vo then the Vo can be given as Io times RL. That is equal to 0.5 mA * 1 kΩ. That means the output voltage vo is equal
to 0.5 V. Now, here the input voltage is 5 mV, while
the output voltage is equal to 500 mV. That means the input signal gets amplified
by the factor of 100. Or we can say that here the voltage gain is
equal to 100. So, in this way, the signal is getting amplified
in this common base configuration. But if you see the current gain, that is the
ratio of this collector current to the emitter current then it is less than 1. That means the current gain for this common
base configuration is less than 1, but it provides the voltage gain. And typically, the voltage gain in this configuration
varies from 50 to 300. Now, one more thing if you notice over here,
the same current is transferred from the input side towards the output side. That means there is a transfer of current
from the low resistance circuit to the high resistance circuit. And that is why the BJT was named as a transistor. So, that’s it for this video. And I hope in this video, you understood the
basics of this common base configuration. And in the next video, we will talk about
the common-emitter configuration. So, if you have any question or suggestion,
do let me know here in the comment section below. If you like this video, hit the like button
and subscribe to the channel for more such videos.

10 thoughts on “BJT : Common Base Configuration Explained

  1. I might've missed it, but introducing the shottkey equation would help. That way, base emitter resistance can be introduced without hindrance, not to mention even that output curve.

  2. The timestamps for the different topics covered in the video:

    1:11 What is Common Base Configuration

    2:55 Input Characteristics of the Common Base Configuration

    5:45 Output Characteristics of the Common Base Configuration

    6:33 Three Different regions of operation (Active, Saturation, and Cut-off)

    10:52 Signal Amplification in CB Configuration

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