[Physics Class Notes] on Characteristics of a Transistor Pdf for Exam

A Transistor is a type of semiconductor device; that has many functions, such as switching, amplifying, detecting, signal modulation, and more.  In almost all modern electronics, Transistors are the most important active components. As a result, many people consider the Transistor to be one of the most important innovations of the twentieth century. In this article, you will understand the Transistor, its Input, Output, and Current transfer characteristics. So, with no further ado, let us start by understanding a Transistor. 

 

What is a Transistor?

A Transistor is an Electric device that regulates the flow of Electric Current and Voltage. It acts as a switch or gate of Electric signals. A Transistor is usually composed of three layers of semiconductor components that carry Current. Most of the Transistors are composed of pure silicon, with some made of germanium however, sometimes other semiconductor materials are used.

 

Transistors can be used for a wide range of digital and analogue functions, including amplifiers, switches, Voltage stabilizers, signal modulation, and oscillators, because of their high response and accuracy. Transistors can be packaged individually or in a tiny space, allowing for integrating up to 100 million Transistor integrated circuits.

 

Parts of Transistor

A Transistor is made up of three layers of semiconductor materials, or terminals that help to link the Transistor to an external circuit and carry Current. The Current that is applied through one pair of terminals of a Transistor is controlled by the Current applied to any other pair of terminals of the Transistor. For a Transistor, there are three terminals. They are as follows:

1. Base: The base is used to activate the Transistor.

2. Collector: The Transistor’s positive lead is known as the collector.

3. Emitter: The Transistor’s emitter is the negative lead.

Characteristics of Transistor

Transistor Characteristics is the basis that represents the relationship between the Electric Current and Electric Voltage of a circuit. There are three types of Transistor characteristic curves based on the configuration of the circuit.

1. Input Characteristic – The Input characteristics describe any changes that occur in the Input Current because of the variation of the Input Voltage by keeping the Output Voltage constant.

2. Output Characteristic – This is a graph of Output Current on one axis and Output Voltage on another, at a constant Input Current.

3. Current Transfer Characteristic – This is a characteristic curve that points to the fluctuation of the Output Current to that of the Input Current. Here, the Output Voltage is kept constant.

Transistor Configuration

Any type of Transistor circuit can be designed by using the above mentioned three Transistor characteristics. The configuration of the Transistors is based on the Transistor terminals. There are three types of Transistor circuit configuration, these are:

Each circuit configuration has a different characteristic curve. Based on the requirement of the circuit, the Transistor configuration is chosen accordingly.

 

Few things are considered while using the correct Transistor for the circuit. These are the maximum Voltage rating between the emitter and the collector (UCEmax), maximum power to build a circuit, and maximum collector Current (ICEmax). An Electric circuit must not exceed these maximum values to function properly. Permanent damage to the circuit may occur if it exceeds the value. It is also important to maintain proper Current amplification and frequency.

 

Common Emitter Configuration

In this kind of configuration, an Emitter is used as a common terminal for both Input and Output. It works as an Inverting Amplifier Circuit. Here, the Input is applied in the region of the base Emitter and the Output is obtained from between the Terminals of the Collector and Emitter.

In this case,

VBE is the Input Voltage,

IB  is the Input Current,

VCE  is the Output Voltage, and

IC is the Output Current.

 

The Common-Emitter Configuration is usually based on Transistor-Based Amplifiers. Under this condition, the Emitter Current is equivalent to the sum of base Current and collector Current.

Hence,

IE  = IC + IB

 

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This equation is the Transistor equation for the CE configuration. The ratio of the collector Current to that of emitter Current gives Current gain alpha in the Common Base Configuration. Similarly, the ratio of the Collector Current to that of the base Current gives Current gain beta in the Common-Emitter Configuration.

 

The Relationship Between the Two Current Gains is:

Current gain (α) = IC/IE

Current gain (β) = IC/IB

Collector Current IC =αIE = βIB

This configuration uses one of the three circuit configurations. It has average Input and Output values of impedance. It also has an average Current and Voltage gains. The Output signal of this configuration has a phase shift of 180⁰, so the Input and the Output are inversely proportional to each other.

 

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Input and Output Characteristics of Common-Emitter Configuration

1. Input Characteristics of Transistor

The Input characteristic of a Transistor is obtained between the Input’s Current IB and the Input Voltage VB by having a constant Output Voltage VCE. By keeping the Output Voltage VCE constant and changing the Input Voltage VBE of different points, we can examine the values of the Input Current of each of the points. Now, using the values obtained from different points, a graph is drawn by plotting the values of IB and VBE at constant VCE.

Rin = VBE/IB (at a constant VCE)

This is the required equation to calculate the In
put resistance Rin.

 

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2. Output Characteristics

The Output characteristic of a common emitter is obtained between the Output Voltage VCE and Output Current IC at a constant Input Current IB. By keeping the base Current IB constant and changing the value of Output Voltage VCE at different points, we can calculate the value of collector IC for each point. Now, if we plot a graph between IC and VCE, we get the Output characteristics of a common-emitter configuration.

Rout = VCE/IC (at a constant IB)

This is the equation to calculate Output resistance.

 

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The Output characteristics can be divided into three regions:

• The active region of the Transistor

• Saturation region of the Transistor

• Cut-off region of the Transistor

 

The Active Region of the Transistor

The active region of the Transistor is the area on the Output curve where the Output Current is almost constant and independent of the Output Voltage. The Transistor operates in the Active region if the base resistance is greater than the maximum value allowed. A Transistor can only be used as an amplifier if it is in the active region. In addition, the emitter junction should be in forwarding bias, and the collector junction should be in reverse bias for operation in the active region.

Saturation Region of the Transistor

The saturation region of the Transistor is the area where the collector Current rapidly increases with a little increase in Output Voltage. The base resistance should be less than the maximum allowed value to run the Transistor in the saturation region. Both emitter and collector junctions should be in forwarding bias for operation in the saturation region. The Transistor works like the ON stage of a switch in the saturation zone.

Cut-off Region of the Transistor

The base Current is effectively zero in the Cut-Off region. As a result, even at higher Output Voltage, collector Current becomes zero. To operate a Transistor in the cut-off region, both the emitter and collector junctions must be in reverse-biased condition. A Transistor operates like the OFF stage of a switch in the cut-off region.