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Characteristics of a Transistor

Definition

A transistor is a semiconducting device that amplifies or switches electronic signals and electrical power. They are the building blocks of modern electronic gadgets and technology. They regulate current by amplifying the input current to a more significant output current. Various applications of a transistor include-

1. Usage in logic gates, flip-flops circuit
2. The memory card in smartphones
3. Transistors act as switches for various modern gadgets like smartwatches, computers, laptops, etc.
4. Microcontrollers and microprocessors
5. Register circuits

A transistor consists of a p and n diode – positive and negative. Thus, Moore’s law is followed, or we can say Moore’s law is the rule of the transistor. Moore’s law observes that the number of transistors in a dense integrated circuit (IC) doubles every two years.

Whatever be the configuration, transistor characteristics are represented by the graph of current vs. voltage of a transistor. These configuration circuits, if are two-port network, can be analyzed using the following characteristic curves-

  • Input characteristics – The input characteristics curves describe the changes in input current concerning the input voltage by keeping the output voltage constant.
  • Output characteristics – The output characteristic curve is obtained by output current vs. output voltage graph by keeping the input current constant.
  • Current Transfer characteristics – Current transfer characteristics curve shows the variation in output current vs. input current graph by keeping the output voltage constant.

Configuration of a transistor

A transistor can be placed on the electrical circuit in three configurations – common emitter transistor, common base transistor, and common collector transistor, also known as an emitter follower. The connection of the transistor terminal determines these three configurations.

1. Common emitter (CE) configuration of the transistor

We place a common emitter terminal transistor between the input and output terminals in the common-emitter configuration.
 

Transistor characteristics

Definition 

Formula/expression 

Input characteristics 

The variation of emitter current (IB) with base-emitter voltage (VBE), keeping Collector emitter voltage (VCE) constant.

Rin = VBE / IB

VCE = constant 

Output characteristics 

The variation of collector current (IC) with collector-emitter voltage (VCE), keeping the base current (IB) constant.

Rout = VCE / IC

IB = constant

Current transfer characteristics 

The variation of collector current (IC) with the base current (IB), keeping collector-emitter voltage (VCE) constant. The resulting current gain has a value greater than 1.

α = Ic / IB

VCB = constant 


2. Common base (CB) configuration of a transistor

In the common base configuration, the common base terminal of the transistor is connected between the input and output terminals.
 

Transistor characteristics

Definition 

Formula/expression 

Input characteristics 

The variation of emitter current (IE) with base-emitter voltage (VBE), keeping collector base voltage (VCB) constant. 

Rin = VBE / IE

VCB = constant 

Output characteristics 

The variation of collector current (IC) with collector-base voltage (VCB), keeping the emitter current (IE) constant.

Rout = VCB / IB

IE = constant

Current transfer characteristics 

The variation of collector current (IC) with the emitter current (IE), keeping collector base voltage (VCB) constant. The resulting current gain has a value less than 1.

α = Ic / IB

VCB = constant 


3. Common collector (CC) configuration of a transistor

In common collector configuration, the common collector terminal of the transistor is connected between the input and output terminals.
 

Transistor characteristics

Definition 

Input characteristics 

The variation of emitter current (IB) with collector-base voltage (VCB), keeping Collector Base voltage (VCB) constant.

Output characteristics 

The variation of emitter current (IE) with collector-emitter voltage (VCE), keeping the base current (IB) constant.

Current transfer characteristics 

The variation of emitter current (IE) with the base current (IB), keeping collector-emitter voltage (VCE) constant.


Frequently asked questions on transistors

Ques 1: What is the difference between transistors NPN and PNP? What are the applications of these transistors?

Ans: NPN and PNP transistors are made up of various materials. Therefore, the current passing through these transistors is             also different.
1. In a PNP transistor, the current flows from the emitter to the collector, whereas in a transistor, the current flows from the      collector to the emitter.
2. In a PNP, there are two layers of P-type material and one layer of N-type material. In an NPN transistor, there are two          layers of N-type material and one layer of P-type material.
3. In a PNP transistor, a positive voltage is applied across the emitter terminal to produce the flow of electric current from        the emitter to the collector. In an NPN transistor, the positive voltage is applied across the collector terminal to produce        the flow of electric current from the collector to the emitter.

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