Common Emitter (CE) Configuration

Look at the figure 1(a),  input is applied to the base terminal and output is obtained from collector. Whereas emitter is a common terminal to both the input and output signal. This is common emitter configuration. The common emitter configuration has medium input impedance and medium output impedance. Both the current and voltage gains are high. 

To understand the complete behaviour of a transistor, we need to analyse the input and output characteristics. For this analysis, the transistor must be in active region.

Biasing:

How to design or bias a CE transistor configuration?

Here are a few steps to follow.

• Indicate all the directions of current (consider non transistor)
• Introduce supply voltage. The polarities of the supply should support the directions of IB and IC
• Here we consider electron current (the electron current flow from negative to positive terminal)
• Repeat the same procedure for pnp transistor. For pnp transistor all the polarities are reversed

So, look at the complete circuit shown in figure 1(a) and
1(b)

Figure: Common Emitter Configuration - Input and output characteristics and biasing 

Supply voltages are such that they support the direction of transistor currents. (Means the direction of current from supply is in the same direction as that of transistor currents).  Positive terminal of the supply is connected to the base, makes the base-emitter junction forward biased. While the positive terminal of the supply is connected to collector, makes the base-collector junction reverse biased. 

In common emitter configuration:

• Input voltage is base-emitter voltage
• input current is IB
• Output voltage is collector-emitter voltage
• output current is IC

Input Characteristics:

The graph plotted between base current IB and the base emitter voltage VBE at constant collector emitter voltage VCE is called input characteristics. Base current IB  is taken along y- axis (dependent variable) and base emitter voltage VBE along x-axis. 

From figure 1(c) we can conclude that: 

• The input characteristics curve of CE configuration resembles the diode characteristics curve
• After threshold voltage, the small change in VBE,  IB increases rapidly
• Small dynamic input resistance. It is the ratio of change in VBE to the resulting change in IB , for a given value of collector emitter voltage. 

ri = ∆VBE / ∆IB | VCE = constant

Output Characteristics:

The graph plotted between collector current IC and collector emitter voltage VCE at constant values of base current IB is called output characteristics. Collector current IC is taken along y-axis and collector emitter voltage VCE is taken along x-axis.

From figure 1(d) we can conclude that:

• The value of βdc is simply calculated by the ratio of IC to IB. For βac we take the ratio of small change in IC (∆IC) to the small change in IB (∆IB). It means dynamic output resistance of CE configuration is high  

βac = ∆IC / ∆IB | ∆VCE = 0

• From the graph you can observe, the change in VCE  will cause little change in IC for constant IB. It means dynamic output resistance of CE configuration is high

rO = ∆VCE / ∆IC | IB = constant

• The output characteristics curve has all three regions, active, saturation and cut-off

Current Amplification:

As you know the current amplification factor is defined by the ratio of output current to the input current. Here input current is IB and output current is IC. Hence the current amplification factor is 

β = IC / IB

Since IB is very small, so the value of β is quite high. 

Input Resistance:

Input resistance of CE is the ratio of base to emitter voltage VBE to the base current IB. 

Ri = VBE / IB

IB is negligibly small and VBE is also a small value (VBE =0.7V). So, the input resistance is moderately high.

Output Resistance:

In this configuration, output resistance is the ratio of base to collector voltage VCE to the output current IC.

RO = VCE / IC

Voltage Gain:

Voltage gain is the ratio of output voltage to the input voltage. Input voltage is the small forward base emitter voltage (VBE = 0.7V). Output voltage is the voltage across collector terminal and it is quite high. So, the voltage gain is high for CE configuration.

AV = vce / vbe = ic*Rc

AV = β*ib*RL / ib Ri

Av = β*RL / Ri

Power Gain:

Power gain is the ratio of output power to the input power. 

Instantaneous input power Pi = i2bRi

Instantaneous output power Po  = i2cRL

AP = Po/Pi

AP = β2*i2b*RL / i2bRi

AP = β2 *RL / Ri

Learn more about other configurations

Common Collector Configuration
Common Base Configuration

Understand The Biasing Effect On BJT

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.

• Fixed bias or base bias
• Emitter bias
• Collector feedback bias
• Voltage divider bias

Bipolar Junction Transistors

BJT - Bipolar Junction Transistors - Introduction, Types, Cross-Sectional View / Structure

You studied a two-terminal electronic device that is diodes. Now it's time to study a three-terminal electronic device that is a transistor. There are many different types of transistors but we stick to the most basic type of transistors. In this post, I am going to introduce Bipolar Junction Transistors.

Outline:

• Introduction to BJT
• Types
• Circuit symbol
• Current directions
• Cross-Sectional View/ Structure

Introduction To BJT?

The term bipolar refers to the fact that current through the transistor constitutes from both minority and majority carriers that is holes and electrons.

It is a three-terminal or three-layer semiconductor device. These three layers are connected back to back. The left layer is called the emitter, the middle layer is called the base and the right layer is called the collector.

Types Of BJT:

Figure 1: Types of BJT and their circuit symbols

First of all, I would like to explain the simplified structure of BJT. Figure 2 shows the simplified structure of the device.

Look at figure 1(a), the transistor consists of three semiconductor regions: the emitter (n-type), the base (p-type) and the collector (n-type). This is an NPN transistor. An NPN transistor consists of two layers made from n-type semiconductors separated by a p-type semiconductor. So, two pn junctions exist in a single transistor. Look at the edges of the emitter and base. This junction is called emitter-base junction (or emitter diode). Now, look at the edges of the base and collector. This junction is called the collector base junction (or collector diode).

Similarly, look at figure 1(b), there is a PNP transistor. It is a dual of an NPN transistor. It consists of three semiconductor regions: the emitter (p-type), the base (n-type) and the collector (p-type).

a PNP transistor consists of two layers made from p-type semiconductors separated by an n-type semiconductor. So, two pn junctions exist in a single transistor. That is emitter-base junction and collector-base junction.

In both types of transistors, the emitter region is heavily doped. The base region is thin and lightly doped as compared to the emitter and collector. The collector region is moderately doped.

Circuit Symbol & Current Directions:

The direction of arrows shows the direction of the current. It will be a little bit confusing as a beginner to understand the current directions. Have a look at the emitter, there is an arrow on it. This because the practical BJT is not symmetrical. The arrow shows on the emitter terminal describe the conventional current directions. When we apply the KVL equation to the transistor circuit, we make use of conventional current directions.

I would like to write some basic equations. These will help you in learning BJT theory.

Voltage equations

VCB = VC - VB

VBE = VB - VE

VCE = VC - VE

Current equations

IC = β*IB

IE = IC + IB

Cross-Sectional View:

Figure 2: Simplified cross-sectional view

All three semiconductor regions are differently doped. The base is always in the middle, lightly doped and highly resistive material. From the cross-sectional view, it is clear that the collector base junction has a much larger area than the emitter base junction. The emitter is heavily doped because the emitter should capable of injecting/emitting electrons (for NPN transistor) and holes (for PNP transistor) into the base. The lightly doped base is used for isolation in between collector and emitter. The surface area of the collector is large and moderately doped. When the emitter-base junction is forward biased and the collector base junction is reverse biased, the collector will collect all electrons emitted from the emitter.

The base terminal is also used to adjust the base-emitter voltage. Any change in base-emitter voltage will change the current between the emitter and collector significantly.

Diffusion & The Barrier Potential | The Unbiased Transistor:

Figure 3

Look at figure 3(a). It shows a transistor before diffusion. As I discussed above, there are two back to back diodes. The emitter diode and the collector diode.

Of course, there are negatively charged electrons in the emitter region trying to recombine with positively charged holes in the base region.

Similarly, there are negatively charged electrons in the collector region as well. And these negatively charged electrons are also trying to recombine with positively charged holes in the base region.

Of course, there will be two depletion regions are formed just like in diodes. For each of these depletion regions, the barrier potential is 0.7V ( standard value for silicon devices). Figure 3(b) shows these two depletion regions.

Effect Of Biasing On Barrier Potential On BJT | Modes Of Operations:

When an external voltage is applied to the transistor, then it is called a biased transistor. There are many different biasing techniques available. You will learn more about biasing in later posts.

Why do we need biasing? With the help of biasing we establish the desired voltage and current conditions for the transistor (also termed as Q point). Whenever we want to design a circuit, biasing is necessary for the correct operation of the transistor. A beginner needs to learn how to bias. How to apply voltage if you want to derive a transistor as an amplifier? We can not obtain proper AC amplification without proper DC biasing.

BJT Configurations:

BJT has three terminals. Based on these terminals the transistor can be contacted into three different configurations. Each configuration has its characteristics. 

• Common emitter
• Common base
• Common collector

In each configuration, one terminal is an input, the second terminal is output and the third terminal is common in between input and output.

Out of these three configurations, a common emitter is extensively used. As you know, to drive a transistor in the active region, the base-emitter junction is forward biased while the base-collector junction is reversed biased. This condition is valid for all three configurations. 

Key Terms:

Alpha: The ratio of collector current IC to the emitter current IE is called alpha. It is always less than 1 (unity).

Beta: The ratio of collector current IC to the base current IB is called beta. Its value ranges from 20 to 200 or even higher.

Biasing: The proper application form of DC voltage, to derive a transistor into a suitable mode.

Frequently Asked Questions:

What will happen if collector and emitter are interchanged?

As I discussed above, BJT is not a symmetrical device. As you interchange the collector and emitter, the transistor will change its mode. Now the transistor works in reverse active mode. In this condition, alpha and beta are much smaller, because the device is optimised to work in forward mode.

BJT is not a symmetrical device. Explain.

As I discussed above, the emitter region is heavily doped than the collector region. The non-symmetrical behaviour is due to different doping ratios.