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Physics

Common Emitter Configuration

Common Emitter Configuration

Reviewed by:

Gaurang Shah

Common Emitter Configuration: An Introduction

Known as a common emitter layout, this arrangement places the emitter between the collector and the base. The emitter and base form the connection for the input circuit, while the collector and emitter provide the connection for the output circuit. Because the emitter is shared by the input and output circuits, the setup is known as having a common emitter. The NPN and PNP transistor's shared emitter configuration.

Common Emitter Configuration

Since the emitter terminal is grounded in a common emitter configuration, this design is often referred to as a grounded emitter configuration. The terms common emitter configuration, common emitter amplifier, and CE amplifier are also frequently used. The most popular transistor arrangement is known as a common emitter (CE). When a significant current gain is required, common emitter amplifiers are utilised.

The base and emitter terminals are used for the input signal, while the collector and emitter terminals are used for the output signal. As a result, a transistor's emitter terminal serves as both an input and an output terminal, earning the term "common emitter configuration."

Common emitter configuration

VBE stands for the supply voltage between the base and the emitter, while VCE stands for the supply voltage between the collector and the emitter. In a common emitter arrangement, input current (also known as base current or IB) and output current (also known as collector current or IC) are written as letters.

Base Current Amplification Factor

The Base current amplification factor of common emitter configuration is given by the difference between the change in collector current (IC) and the change in base current (IB). It is indicated by $\beta$ .

$\beta = \dfrac{{\Delta {I_C}}}{{\Delta {I_B}}}$

Collector current and Current Gain

Collector Current

In common emitter configuration the collector current is given by using the following equation. The equation relates the input current IB with the output current IC in the CE configuration.

${I_E} = {I_C} + {I_B}$

${I_C} = \alpha {I_E} + {I_{CBO}}$

By using the above two equations and solving them we get:

${I_C} = \dfrac{\alpha }{{1 - \alpha }}{I_B} + {I_{CEO}}$

Where,

IC - Collector current

IE- Emitter current

$\alpha$ - Current amplification factor

IB - Base current

ICEO - When the base is open then the collector-emitter current

ICBO - When the output is open then the collector base current

Current Gain

A current gain in common emitter configuration can be written as:

$\beta = \dfrac{{\Delta {I_C}}}{{\Delta {I_B}}}$

Where $\beta$ is current gain and IC change in collector current and IB change in base current. $\Delta {I_C}$ is greater than $\Delta {I_B}$ . $\beta$ is greater than 1, in common emitter configuration.

Input and Output Characteristics

Input Characteristics

The connection between the input current, or base current (IB), and the input voltage, or base-emitter voltage (VBE)., is described by the input characteristics of the common emitter configuration.

Input and Output Circuit Diagram

Diagram of the circuit used to analyse the input and output characteristics of a transistor's common emitter arrangement.

Draw two lines first—one vertical and one horizontal. The vertical line represents the y-axis and the x-axis by the horizontal line. The input voltage (VBE) is taken along the x-axis (horizontal line), and the input current or base current (IB), is taken along the y-axis (vertical line).

Input characteristic of CE configuration

The input voltage VBE is raised from zero volts to various voltage levels while the output voltage VCE is maintained at zero volts. The matching input current (IB) for each voltage level of the input voltage (VBE) is kept track of.

Output Characteristics

The connection between output current (IC) and output voltage (VCE) is described by the output characteristics of the common emitter configuration.

Draw two lines first—one vertical and one horizontal. The y-axis is represented by the vertical line, and the x-axis by the horizontal line. The output voltage (VCE) is taken along the x-axis, while the output current (IC), also known as the collector current, is taken along the y-axis (vertical line).

The output characteristic in CE configuration

The output voltage VCE is raised from zero volts to various voltage levels to establish the output characteristics, while the input current, or base current, IB is maintained constant at 0 A. The associated output current (IC) for each level of the output voltage is kept track of.

Interesting Facts

Chips are intricate structures composed of layers of circuitry linking the components and millions, or even billions, of transistors and other components. Chips have gotten stronger over time by combining more microscopic components and cramming them closer together.
Specialized manufacturing facilities known as fabs are used to create semiconductors. Cleanrooms, which are enclosed spaces with stringent controls for airborne contaminants, humidity, and temperature, are located inside these fabs.

Solved Problems

1. Calculate the emitter current in a transistor that has $\beta$ =40, and the base current is 0.03 mA.

Sol.

Given:

IB = 0.03 mA and $\beta$ =40

We know that,

IC = $\beta$ IB

IC = 40 x 0.03 = 1.2 mA.

We know that,

${I_E} = {I_C} + {I_B}$

IE = 1.2 + 0.03 = 1.23 mA

The emitter current comes out to be 1.23 mA.

2. Find the value of $\beta$ when $\alpha$ is given 0.96.

Sol.

Given:

$\alpha$ = 0.96

We know that

$\beta = \dfrac{\alpha }{{1 - \alpha }}$

$\beta = \dfrac{{0.96}}{{1 - 0.96}}$ = 24

The value of $\beta$ is 24 after solving the question.

Summary

We have seen how common emitter configuration works and how it amplifies the output. The other configuration is also important to get the full knowledge of this working and the relation which are set up between them. The base amplification factor is also related to the amplification factor which will help in solving the problems of physics.

List of Related Articles

FAQs on Common Emitter Configuration

1. What is a common emitter (CE) configuration in a transistor?

A common emitter (CE) configuration is a type of transistor circuit where the emitter terminal is common to both the input and output circuits. The input signal is applied between the base and emitter terminals, while the output is taken from between the collector and emitter terminals. Because the emitter is grounded or shared, this setup is also known as a grounded-emitter configuration.

2. Why is the common emitter configuration the most widely used transistor amplifier?

The common emitter configuration is the most popular choice for amplifiers primarily because it provides significant gains for both current and voltage. Its key advantages include:

High Current Gain (β): It amplifies the input current significantly.
High Voltage Gain: It also provides a substantial increase in signal voltage.
High Power Gain: The combination of high current and voltage gain results in a very high power gain.
Moderate Impedance: Its input and output impedances are moderate, making it easy to connect with other circuit stages.

3. What are the key formulas for current gain in a CE configuration?

In a common emitter configuration, the most important formula is for the current amplification factor (β), also known as the current gain. It is the ratio of the change in collector current (ΔIC) to the change in base current (ΔIB).
The formula is: β = ΔI_C / ΔI_B.
Additionally, the relationship between the three transistor currents is always: I_E = I_C + I_B, where I_E is the emitter current, I_C is the collector current, and I_B is the base current.

4. How are the input and output characteristics of a CE configuration determined and what do they show?

The characteristics are determined by plotting graphs of current versus voltage:

Input Characteristics: This is a graph of the input current (Base Current, I_B) versus the input voltage (Base-Emitter Voltage, V_BE), while keeping the output voltage (V_CE) constant. It resembles the forward-bias characteristic of a p-n junction diode.
Output Characteristics: This is a graph of the output current (Collector Current, I_C) versus the output voltage (Collector-Emitter Voltage, V_CE), while keeping the input current (I_B) constant. This graph shows how the collector current is largely controlled by the base current and is almost independent of the output voltage in the active region.

5. What is the difference between the common emitter (CE), common base (CB), and common collector (CC) configurations?

The main difference lies in which of the three transistor terminals (Emitter, Base, Collector) is made common to both the input and output circuits:

In Common Emitter (CE), the emitter is common. The input is at the base, and the output is at the collector. It provides high voltage and current gain.
In Common Base (CB), the base is common. The input is at the emitter, and the output is at the collector. It has a voltage gain but no current gain (current gain is less than 1).
In Common Collector (CC), the collector is common. The input is at the base, and the output is at the emitter. It has a high current gain but no voltage gain (voltage gain is less than 1).

6. Why does a common emitter amplifier produce a 180-degree phase shift between the input and output signals?

The 180-degree phase shift is a fundamental characteristic of the CE amplifier. When the positive half-cycle of the input AC signal is applied to the base, it increases the forward bias. This leads to a large increase in the collector current (I_C). This increased current flows through the collector load resistor (R_C), causing a larger voltage drop across it. As the output voltage (V_CE) is measured as V_CC - I_CR_C, an increase in I_CR_C causes V_CE to decrease. Therefore, a positive input results in a negative-going output, creating an inversion or a 180-degree phase difference.

7. What is the relationship between the current amplification factors alpha (α) and beta (β)?

Alpha (α) is the current gain for a common base configuration (α = I_C/I_E), while beta (β) is the current gain for a common emitter configuration (β = I_C/I_B). The relationship between them is fundamental to understanding transistor operation. The formulas are:

β = α / (1 - α)
α = β / (1 + β)

Since α is always slightly less than 1 (typically 0.95 to 0.99), the denominator (1 - α) becomes very small. This results in the value of β being very large (typically 20 to 200), which explains why the common emitter configuration provides a much higher current gain than the common base configuration.