Transistor Configuration: Common Base, Collector, & Emitter

What is Transistor Configuration?

A bipolar junction transistor has three configurations based on the arrangement of the three terminals, especially from where the input is given, where the output is taken, and which terminal is grounded. So basically, three different types of transistor configurations can be made, which are:

Common Base (CB),
Common Emitter (CE) and
Common Collector (CC).

All three types of transistor configurations have applications based on the current gain, voltage gain, power gain, and input and output impedances, depending upon the usage with the particular requirement. Below, all the transistor configuration specifications are given in detail.

Common Base Transistor Configuration

The Base is a common terminal for input and output signals in this Transistor configuration. The input is applied between the base and emitter terminals, and the output is taken between the base and collector terminals with the base terminal grounded. Here the input parameters are base-emitter voltage and Emitter current, and output parameters are collector-base voltage and collector current.

The current gain is equal to or less than unity for the Common Base configuration. The amplifier circuit configuration of this type is a non-inverting amplifier circuit. It has a high voltage gain value. This transistor configuration has high output impedance and low input impedance. It has high resistance gain.

The voltage gain for this configuration is given below.

A_V = V_out/V_in = (I_C*R_L) / (I_E*R_in)

Current gain in common is given as

α = Output current/Input current = I_C/I_E

Input characteristics are obtained between Emitter current and Base Emitter voltage with constant output voltage, which is Collector Base voltage. The below figure shows the input characteristics of the Common Base configuration.

The output characteristics are obtained between the output current and output voltage with the emitter current constant. The collector current is recorded by varying the Collector Base voltage value for different points. The figure below shows the output characteristics of the Common Base configuration.

Common Collector Transistor Configuration

The Collector terminal is common for input and output signals in this transistor configuration. It is also known as emitter follower configuration because the emitter voltage follows the base voltage. It is mainly used as a buffer. They are widely used in impedance matching applications because of their high input impedance.

The input signal is applied between the base-collector region, and the output is taken from the emitter-collector region. The input parameters are Base Collector Voltage and Base Current, and the output parameters are Emitter Collector Voltage and Emitter Current. This has high input impedance and low output impedance.

A_i= output current/Input current

A_i= β + 1

The input characteristics of a common-collector configuration are obtained between the Base current and the Collector Base voltage keeping the Emitter Collector voltage constant. The below figure shows the output characteristics of the common collector configuration.

The output characteristics are obtained between the Emitter Collector voltage and emitter current keeping the base current constant. If the base current is zero, the emitter current also becomes zero, and no current flows through the circuit. The transistor operates in the active region and finally reaches the saturation region. The below figure shows the output characteristics of the common collector.

Common Emitter Transistor Configuration

This transistor configuration is known as the CE configuration of the transistor. In this Transistor configuration, the Emitter is a common terminal for input and output signals. It is an inverting amplifier circuit. This is transistor is connected in a common emitter configuration. The input is applied between the base-emitter region and the output between collector-emitter terminals. Collector current and Base current give the current gain beta. It is mostly used because it has medium input and output impedance. This Transistor configuration has medium current and voltage gains.

Current gain (α) = I_C/I_B
Current gain (β) = I_C/I_B
Collector current IC =α I_E = βI_B

The common emitter configuration input characteristics are obtained between Base current and Base Emitter voltage while keeping the Collector-Emitter voltage constant. The below figure shows the input characteristics of the common emitter configuration.

The output characteristics of the common-emitter configuration are obtained between the Collector current and Collector-Emitter voltage while keeping the base current constant.

The below figure shows the output characteristics of the common emitter configuration.

Comparison Between Transistor Configurations

The table below gives the major properties of the different transistor configurations. It compares transistor configurations concerning voltage gain, current gain, and input and output impedance.

Characteristics	Transistor Configuration
Characteristics	Common Base	Common Emitter	Common Collector
Current Gain	Low	High	High
Voltage Gain	High	High	Approximately 1
Power Gain	Moderate	High	Low
Input Impedance	Low	Medium	High
Output Impedance	High	High	Low

Important GATE Topics
Determinate And Indeterminate Structures	Astable Multivibrators
Bistable Multivibrator	Truss And Frame
Network Layer	Statically Determinate
Anomalies In Dbms	Eulers Equation Of Motion
Dalembert's Principle	Statically Indeterminate

FAQs on Transistor Configuration

What are the types of transistor configurations?

In the transistor, there are three terminals, in which one terminal is kept common. With this type of connection among terminals, there are three types of transistor configurations which are:-

Common Base Configuration
Common Emitter Configuration
Common Collector Configuration

Which transistor configuration is widely used?

There are three transistor configurations and among all those Common emitter transistors are used most widely because they provide high current gain, high voltage gain and high power gain, and have moderate input impedance and high output impedance.

What is the Q point in transistor configuration?

Q point is the operating point of a device or quiescent point. It is the steady-state DC voltage or current of the transistor with no input signal applied. It is an important factor in the designing of any circuit around a transistor since it determines the DC analysis, stability and voltage ranges over which we get the desired outputs. It determines the distortion ratio of the transistor action.

What is the need for biasing of transistors?

A transistor's steady state of operation depends on its base current, collector voltage, and collector current and for the transistor to operate as a linear amplifier, it must be properly biased to have a suitable operating point. The various types of biasing methods are:-

Fixed Bias
Collector base bias
Voltage divider bias

Which transistor configuration provides nearly unity current gain?

The common base has nearly unity current gain. In common base, alpha is the current gain

𝛂 = 𝛃/(𝛃+1)

So, it will always be nearly unity.

Which transistor configuration provides high input impedance?

Common collector configuration provides high input impedance, and because of this, they are used in impedance matching.

Which transistor configuration gives highest output impedance?

Common Base configuration gives the highest output impedance among all the three configurations.

Transistor Configuration