In this article, we are going to learn about Bipolar Junction Transistor Configurations, Common Emitter, Common Base and Common Collector. So far we have covered Introduction of Bipolar Junction Transistor.

We will learn in detail about different configurations of Bipolar Junction Transistor in this post, as we know that Bipolar Junction Transistor is a three terminal device, hence it can be configured in three different ways and we will cover these three different transistor configurations with respect to their circuit diagram, working, and advantages and disadvantages.

Introduction of Bipolar Junction Transistor Configuration:

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There are three terminals in BJT transistor named as Emitter, Base, and Collector. However, when we connect the transistor to any circuit we require four (4) terminals, two for input and two for output, to do that we make one terminal common to both input and output circuit.

The input is applied to that common terminal and one of the two terminal of BJT, and output is taken from the common terminal and remaining one terminal of BJT, Accordingly.

A Bipolar Junction transistor can be connected in a circuit in this three different ways.

• Common Emitter Configuration –   It is the BJT configuration in which it behaves like an amplifier (it has both current and voltage gain).
• Common Base Configuration –   It is the BJT configuration in which (it has Voltage Gain but no Current Gain).
• Common Collector Configuration –   it is the BJT configuration in which it behaves like voltage buffer (It has Current Gain but no Voltage Gain).

These all (Bipolar junction Transistor configurations) have different advantages and disadvantage, and in this post, we will cover them one by one.

Common Emitter Configuration:

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The term common-Emitter is derived from the fact that the emitter terminal is common to both the input and output sides of the configuration. This is the one famous configuration of BJT transistor, we mostly operate BJT transistor to this configuration because it allows us the operation of Amplification.

• We apply input in the Base-Emitter terminal and we take an output from Emitter-Collector Terminal.
• Here the input parameters are VBE, IB and output parameters are VCE, IE.

Current Amplification factor in Common-Emitter Configuration is represented by Beta (β).

The equation of β is:

β=IC/IB

This is a very important factor, the greater the beta β the greater the ability of transistor to amplify.

Few features of this configuration are:

• It provides High input impedance and low output impedance.
• It provides medium current gain and voltage gain.
• It provides the 180* shift. (input and output is inverted at 180* phase).

Circuit diagram of Common Emitter configuration:

 

Input Characteristics Curve of Common Emitter Configuration:

It is the function of base current IB and VBE (base-emitter voltages) at constant collector-emitter voltage VCE.

• Input characteristics curve can be obtained by keeping the output voltage VCE constant and varying the input voltage VBE and then record the value.
• Now using these values we can draw a graph between the value of IB and VBE at VCE constant.

The equation for calculating the internal resistance at the input side is:

Ri= ΔVBE/ΔIB, (at VCE constant)

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Output Characteristics Curve of Common Emitter Configuration:

It is the function of IC collector current and VCE collector-emitter voltage at making IB Base current constant.

• Output characteristics curve can be obtained by keeping the input current IB constant and varying the output voltage VCE at different value and then record those values.
• Now using those values we can draw a graph between the value of IC and VCE at IB constant.

The equation for calculating the internal resistance at the output side is:

Ro= ΔVCE/ΔIC, (at IB constant)

Also Read: Common Emitter Amplifier Working, Circuit and Advantages

Common Base Configuration:

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The term Common-Base is derived from the fact that the base terminal is common to both the input and output sides of the configuration hence the name is given to this configuration is Common Base. One of the major uses of this configuration is to be used in cascade amplifiers, such as microphone pre-amplifiers or power amplifiers because of their high-frequency response.

• We apply the input in the Base-Emitter terminal and we take an output from Base-Collector Terminal.
• Here the input parameters are VBE, IE and the output parameters are VCB, IC.

The amplification factor in Common-base Configuration is represented by Alpha (α).

The equation of alpha α is:

α=IC/IE

Few important features of Common Base configuration are:

• • It provides High input impedance and low output impedance.
  • It provides low current gain and high voltage gain.
  • It provides the 0* phase shift. (input and output waveforms are identical in nature).

Circuit diagram of Common Base configuration:

Input Characteristics Curve of Common Base Configuration:

It is the function of input current IE and input voltage VBE (base-emitter voltages) at constant collector-base voltage VCB.

• Input characteristics curve can be obtained by keeping the output voltage VBE constant and varying the input voltage VBE and then record the value.
• Now using these values we can draw a graph between the value of IE and VBE at VCB constant.

The equation for calculating the internal resistance at the input side is:

Ri= Δ VBE/Δ IE, (at VCB constant)

Output Characteristics Curve of Common Base Configuration:

It is the function of the output current IC and out voltage VCB and making input current IE constant.

• Output characteristics curve can be obtained by keeping the input current IE constant and varying the output voltage VCB at different value and then record those values.
• Now using those values we can draw a graph between the value of IC and VCB at IE constant.

The equation for calculating the internal resistance at the output side is:

Ro= ΔVCB/Δ IC, (at IE constant)

Also Read: Common Base Amplifier circuit, Working, and Advantages.

Common Collector Configuration:

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The term Common-Collector is derived from the fact that the Collector terminal is common to both the input and output sides of the configuration hence the name is given to this configuration is Common Collector. One of the major uses of this configuration is to be used as a voltage follower or voltage buffer. This configuration allows you to handle large loads. And it is also widely used for impedance matching application because of their high input impedance property.

• We apply input in the Base-Collector terminal and we take an output from Emitter-Collector Terminal.
• Here the input parameters are VBC, IB and output parameters are VCE, IE.

The amplification factor in Common-base Configuration is represented by Gamma (γ).

The equation of gamma γ is:

γ= IE/IB.

Few important features of this configuration are:

• It provides Very High input impedance and low output impedance.
• It provides a high current gain and low voltage gain.
• It provides the 0* phase shift. (input and output waveforms are identical in nature).

Input Characteristics Curve of Common Collector Configuration:

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It is the function of input current IB and input voltage VCB (collector-emitter voltages) at constant collector-emitter voltage VCE.

• Input characteristics curve of the common collector can be obtained by keeping the output voltage VCE constant and varying the input voltage and input current VCB, IB and then record the value.
• Now using these values we can draw a graph between the value of IB and VBC at VCE constant.

The equation for calculating the internal resistance at the input side is:

Ri= ΔVCB/ΔIB, (at VCE constant)

Output Characteristics Curve of Common Collector Configuration:

It is the function of the output current IC and out voltage VCE and making input current IB constant.

• Output characteristics curve can be obtained by keeping the input current IB constant and varying the output voltage VCE at the different value and then record those values.
• Now using those values we can draw a graph between the value of IE and VCE at IB constant.

The equation for calculating the internal resistance at the output side is:

Ro= Δ VCE/Δ IC, (at IB constant)

Also Read: Common Collector Amplifier circuit, Working and Advantages

Comparison between Common Emitter, Common Base and Common Collector configuration of a transistor:

Summary of Input and Output Parameters of Different Transistor Configurations: 

How to Determine the Transistor Configurations?

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one must be able to identify whether a given transistor is connected as a common emitter configuration, common collector or common base configuration. There is an easy way to identify it. Just find out the terminals where the input a.c. the signal is applied, and where the a.c output is taken from the transistor. The remaining third terminal is the common terminal.

For Example, if the a.c input is applied to the base terminal and the a.c output is taken from the collector terminal, then common terminal must be the emitter terminal. Hence the transistor is connected in common emitter configuration. Likewise, you can verify the rest of the configuration using this concept. Let’s say,

If the input is applied to the base terminal and output is taken from the emitter, then common terminal must be the collector terminal. Therefore, we can say the transistor is connected in the common collector configuration.

Conclusion:

Most Commonly Used Transistor Connection:

From all of three BJT transistor connections, the most commonly used transistor connection is a common emitter circuit, because it is the most efficient one. It is used in about 80-95 percent of all transistor applications.

The main reasons for the high use of this circuit arrangement are:

1. High Current Gain
2. High Voltage and Power Gain
3. Moderate output to the input impedance ratio.

This is all about Transistor Configurations Common Emitter, Common Base, and Common Collector Configuration, we learned about working of different transistor configurations and their circuit diagrams. if you liked the article then comment below and share this information with your classmates and friends.

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