23 Jul 2018

Modes of Operation of BJT (Active Mode, Cutoff Mode, Saturation Mode, Reverse Active Mode of Transistor)/Bipolar Junction Transistor

Here we will learn the basic concepts of Bipolar Junction Transistor (BJT) and how to operate BJT transistor in different operating modes.

The image shown below shows the schematic symbols for two types of transistors- NPN transistor and PNP transistor


Symbols of NPN and PNP Transistors (BJT)

Symbols of NPN and PNP Transistors, Symbols of BJT Transistors, Representation of NPN and PNP BJT Transistors
Symbols of NPN and PNP BJT Transistors 

In the Image you can easily see that both types of transistors (NPN and PNP Transistors) have 3 terminals- emitter terminal, base terminal and collector terminal.
So why we call these transistor types as NPN or PNP?
Actually the pure form of semiconductor is known as intrinsic semiconductor, but when we add impurities externally in very small amount, then it is called as doping. This doping is done to increase the conductivity of the pure form of semiconductor. The pure form of semiconductor before adding external impurities is called as intrinsic semiconductor but after adding impurities it is known as extrinsic semiconductor.
The extrinsic semiconductors can be of two types- N-Type and P-Type semiconductors.  If electrons are present in majority after doping then it is known as n type of semiconductor but if holes are present in majority then it is called as p type of semiconductor.
So now you can understand that, the NPN transistor contains two N type regions and one P-type region while the PNP transistor contains two P-type regions and one N-type region.

Now see the schematic symbol carefully. Observe the direction of arrow in both types of transistors. In both of these types, the direction of arrow is in P to N direction. The direction of arrow shows the emitter current direction.

The image given below is another representation of BJT transistor
Symbols of NPN and PNP BJT Transistors, NPN and PNP BJT Transistors
Symbolic Representation of NPN and PNP BJT Transistors 

In this image also you can observe the three terminals - emitter, base and collector along with the N and P regions.
In NPN transistor electrons are in majority while holes are in minority but in case of PNP transistor holes are in majority and electrons are in minority.

For transistor to work properly we want that maximum number of electrons or holes (charge carriers) emitted by the emitter region reach to the collector region. 
So because of this the emitter region is doped highest so that it can emit a large number of charge carriers while the base region is doped lightest and its size is made smallest to reduce the recombination of charge carriers (electrons and holes) in the base region.

We can operate the bipolar junction transistor (BJT) in 4 different operating modes.These four modes of operation are-


Operation Modes of Bipolar Junction Transistor (BJT)

*Active mode
*Saturation mode
*Cutoff mode 
*Reverse active mode


Active Mode Operation of BJT Transistor (Bipolar Junction Transistor)

BJT (NPN Transistor) in active mode is shown in the image given below-


 Active Mode Operation of BJT Transistor (Bipolar Junction Transistor),  Active Mode Operation of BJT,  Active Mode Operation of Transistor,  Active Mode Operation of Bipolar Junction Transistor, Active Mode Operation of BJT   Transistor (NPN BJT)
Active Mode Operation of
BJT Transistor(NPN BJT)

In active mode, the emitter-base Junction is forward biased and the collector-base Junction is reverse biased. This mode of operation is also known as active region (forward active region). In active mode the bipolar junction transistor works as an amplifier.
You can see in the image that the emitter base Junction is forward biased. Here we have taken the example of NPN transistor to show the active mode. Observe the emitter and base region - the P-type (base region) is at Higher potential than the N-type (emitter region). Therefore this junction is forward biased. 
But the collector-base junction is reverse biased as N-type region (collector) is at higher potential in comparison to P-type region (base). Now observe this image very carefully. As we know that electrons are majority charge carriers in case of N type semiconductor while holes are in majority in P type semiconductor. As we know that the direction of electric field is from higher potential to lower potential, therefore in this case according to the image, the direction of electric field is from base to emitter at the emitter-base Junction while its direction is from Collector to base at the collector-base Junction.
Since we want, the large amount of charge Carriers that are emitted by the emitter to reach to the collector region. Therefore, the majority charge Carriers (electrons) that are emitted by this N type of emitter, feel force in the direction that is opposite to the direction of electric field (as electrons have negative charge). Therefore the electrons emitted by the emitter are pushed into the base region. Again these electrons experience force towards the collector region because of the direction of electric field (collector to base). Therefore the maximum Number of electrons that are emitted by the emitter reach to the collector region. So this was the operation of NPN transistor in active mode. 
Now we will discuss the same active mode operation in case of PNP transistor. 
Look at the image given below-


Active Mode Operation of PNP BJT, Active Mode Operation of BJT, Active Mode Operation of transistor
Active Mode Operation of PNP BJT

As discussed earlier, The basic condition for active mode operation of the transistor is - the emitter base junction should be forward biased while the collector base junction reverse biased. So in this case of PNP transistor also you can observe the polarities of these two junctions. Here observe the direction of electric field (from higher potential to lower potential). In case of PNP transistor, the emitter emits holes (emitter is of P type having holes as majority charge carriers) and these holes experience force in the direction of electric field as holes are positively charged carriers. Positive charge carriers experience electric force in the direction of the electric field. So as indicated in the diagram, the holes experience force towards base region and reach to the base region from the emitter region. Again these holes feel force towards the collector region when they are in the base region because of the direction of electric field. Hence majority charge carriers (holes) emitted by the emitter (P type) reach to the collector region. So this is also the active mode of operation.


Now let's discuss the cutoff mode of BJT transistor-


Cutoff Mode Operation of BJT (Bipolar Junction Transistor)

In cutoff mode both the junctions of the bipolar junction transistor (emitter-base junction and collector-base Junction) are reverse biased. Because of this, transistor in cut off mode works as an open circuit (OC) switch. It is also called in 'off state' or '0 state'
Look at the image shown here-


 Cutoff Mode Operation of BJT (Bipolar Junction Transistor), Cutoff Mode Operation of BJT (Transistor working as Open Switch),  Cutoff Mode Operation of BJT
Cutoff Mode Operation of BJT (Transistor working as Open Switch)

You can see in this image that no collector current flows (collector current is zero) in the cutoff mode. The image also shows another representation of the cut off mode. This image shows the symbol of an open circuit as no current flows through an open circuit. 

Note-The transistor acts as a switch (open switch) in the cutoff mode. The image given below depicts the cutoff mode. You can observe here that both junctions are reverse biased. This image also indicates the direction of electric field.


 Cutoff Mode Operation of BJT (Bipolar Junction Transistor),  Cutoff Mode Operation of BJT,  Cutoff Mode Operation of Bipolar Junction Transistor
Cut off Mode Operation of BJT
(Bipolar Junction Transistor)

Now we will discuss the saturation mode.


Saturation Mode Operation of BJT (Bipolar Junction Transistor)

The image given below shows the BJT transistor in saturation mode. In saturation mode of the transistor, both the junctions (emitter-base junction and collector-base Junction) are forward biased. You can also observe this biasing in this image. The direction of electric field is also depicted in the diagram.

 Saturation Mode Operation of BJT (Bipolar Junction Transistor),  Saturation Mode Operation of BJT,  Saturation Mode Operation of Bipolar Junction Transistor
Saturation Mode Operation of BJT
(Bipolar Junction Transistor)

The another image given below shows- that in saturation mode, the transistor acts as a Switch (Closed Switch) or short circuit. As clear by the image that in saturation mode maximum collector current flows. The transistor in saturation mode is also said to be in '1 state' or 'ON state'


Saturation Mode Operation of BJT (Transistor working as a Closed Switch), Saturation Mode Operation of BJT, Saturation Mode Operation of Transistor working as a Closed Switch
Saturation Mode Operation of BJT
(Transistor working as a Closed Switch)

In saturation mode the transistor can be under one of the two modes-
#Forward saturation region
#Reverse saturation region

If the Emitter junction voltage > collector junction voltage
Then the transistor is called under forward saturation region

But if collector junction voltage > emitter junction voltage
Then the transistor is under reverse saturation region.

Now we will discuss the reverse active mode


Reverse Active Mode Operation of BJT (Bipolar Junction Transistor)

The reverse active mode is just opposite to the active mode because in this mode of operation the emitter-base junction is reverse biased while the collector-base junction is forward biased.
As the gain is negligible in this mode of operation, therefore the transistors in never operated in reverse active mode. Because it is just opposite to the transistor action that we desire.
To understand the reverse active mode you can see this image-
Reverse Active Mode Operation of BJT
(Bipolar Junction Transistor)

This image clearly shows that the emitter-base junction is reverse biased and the collector-base junction is forward biased. This biasing of junctions is exactly opposite to the active mode of operation of the transistor. Therefore this mode of operation is called as reverse active mode.
So this was all about different types of operating modes of Bipolar Junction Transistor (BJT).

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