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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.

I was sufferd from this concept in my yesterday class .

It is very helpful for me

Thank you bro

Thank you

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.

so helpful

thank you sir

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

Sir may I know best and simple reference book for BASIC ELECTRONICS …VTU BOARD

Very good 👌

Great video

sir, please explain state machines