Transistor Configuration

By Mohit Uniyal|Updated : June 20th, 2022

A BJT is a three-terminal device. There are three Transistor configurations based on the transistor connections. For connections, we need four terminals, two terminals for input and two terminals for output, but only three are available, so we use one terminal as a common terminal. By doing so there are Transistor configurations which are Common Base, Common Emitter and Common Collector. 

In this article you will get to know about the three Transistor configuration, their input and output characteristics, their current and voltage gain, and a comparison between all the three configurations to have a better understanding of real-life applications.

Table of Content

Common Base Transistor Configuration

In this Transistor configuration base is used as a common terminal for both input and output signals. The input is applied between the base and emitter terminals and 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 the 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.

AV = Vout/Vin = (IC*RL) / (IE*Rin)

Current gain in common is given as

α = Output current/Input current = IC/IE

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. By varying the Collector Base voltage value for different points, the collector current is recorded. The figure below shows the output characteristics of the common base configuration. 

Common Collector Transistor Configuration 

In this Transistor configuration collector terminal act as common for both input and output signals. It is also known as emitter follower configuration because the emitter voltage follows the base voltage. It is mostly 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.

Ai = output current/Input current 

Ai = 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. In this, if the base current is zero then 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

In this Transistor configuration, the Emitter is used as a common terminal for both input and output signals. It is an inverting amplifier circuit. The input is applied between the base-emitter region and the output is taken 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) = IC/IB
  • Current gain (β) = IC/IB
  • Collector current IC =α IE = βIB

The input characteristics of the common emitter configuration 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 shows the comparison between transistor configurations with respect to voltage gain, current gain, and input and output impedance. 

Characteristics

Transistor Configuration

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

Comments

write a comment

FAQs on Transistor Configuration

  • 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
  • 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.

  • A load line is drawn when DC biasing is done to the transistor and no input signal is applied then such a load line is called a DC load line. The DC load line is the load line of the DC equivalent circuit, which is defined by reducing the reactive components to zero.

    byjusexamprep

  • 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.

  • 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

Follow us for latest updates