Electronics Engineering And Circuit Design

BJT Biasing | Active Mode | Cut-off Mode | Saturation Mode

Modes Of Operations:

Outline:

Discuss different modes of operation in BJT

Active mode
Cut-off mode
Saturation mode
Reverse Active mode

Transistor operation

As I talked over, there are two types of BJT (NPN and PNP). The operation of both types of transistors is the same except for the direction of current or polarities. So, there is no need to explain both types in detail here. If you can understand the operation of the npn transistor, you will understand the operation of PNP as well. So I am going to discuss the operation of npn only.

A BJT can be used in different ways like signal amplification, digital logic circuits. In signal amplification, it will remain in active mode. While in the digital logic circuit it will work in both saturation and cut-off modes. Now the question is, how do you derive a transistor in different modes? This is what an engineer has to do. It is all about how to bias a transistor. If you understand how to bias a transistor, you can easily design an amplifier or digital logic circuit.

There are four possible modes of BJT, as shown in the table below. We can get different modes with the help of biasing. Biasing means applying voltage. There are two junctions emitter-base junction (EBJ) and collector-base junction (CBJ). We can bias these two junctions in four different ways and get four different modes of operation.

Modes	Emitter - Base Junction (EBJ)	Collector - Base Junction (CBJ)	Use
Cut off	Reverse biased	Reverse biased	Open switch
Saturation	Forward biased	Forward biased	Closed switch
Forward Active	Forward biased	Reverse biased	Normal amplifier
Reverse Active	Reverse biased	Forward biased	Low gain amplifier

So, the basic principle involves:

The voltage between two terminals controls the current in the third terminal
So it is a three-terminal voltage-controlled current source

Active Mode

To turn on the transistor, EBJ must forward biased. EBJ is not forward biased until

VBE = Vγ = 0.7 V

BJT is in active mode if and only if it satisfies the condition below

VBE ⩾ 0

VB - VE ⩾ 0

and

VCB ⩾ 0 or VBC ≤ 0

VC - VB ⩾ 0

VC > VB > VE

EBJ is forward biased and CBJ is reverse biased. The purpose of the emitter is to emit or inject electrons into the base. The base is lightly doped and very thin, with only a limited number of holes available. The recombination process occurs between electrons from the emitter and holes from the base. But the base has only a few holes, so only a small number of holes recombined with electrons. The collector has collected almost all of these electrons. In this mode change in the collector, the voltage does not affect the collector current.

Fig 1: collector current Vs collector voltage q

Saturation Mode:

In saturation mode both the junctions (EBJ and CBJ) are forward biased. Saturation and cut-off modes are used in switching applications. In this mode, the transistor conducts heavily and acts as a closed switch.

VBE > 0

VB - VE > 0

VB > VE

The base should at a higher potential than the emitter. When the base junction is at a higher potential than the base-emitter junction becomes forward biased.

Similarly

VBC > 0

VCE = VCE(sat) = 0.2V

Cut-Off Mode:

In a cut-off mode both the junctions (EBJ and CBJ) are reverse biased. When both junctions are reverse biased, it means there is no flow of electrons. That is

VBE < 0

VBC< 0

And hence under this condition

IB = 0

But there is a small leakage current called collector leakage current (ICEO).

In this condition, the transistor behaves as an open circuit or open switch. Leakage currents are neglected in most cases.

Reverse Active Mode:

Just as the cut-off mode is exactly the opposite of saturation mode, similarly reverse active mode is exactly the opposite of active mode.

So, you can easily evaluate, in reverse active mode EBJ is reversed biased and CBJ is forward biased.

If EBJ is reversed biased then:

VBE ≤ 0

VB - VE < 0

Similarly, if CBJ is forward biased, then it satisfies the condition below:

VCB ≤ 0

VC - VB < 0

The quadrant graph of shows the above concepts.

Fig 2: Quadrant graph

Conclusion:

Now, you are familiar with all four modes. The next step is to learn about biasing. Biasing means the application of proper DC voltage to the different terminals of the transistor. The desired mode of operation is achieved with the help of proper biasing. There are many different biasing methods. I will discuss the methods below.