Characteristics of 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.

Transistor Characteristics is the basis that represents the relationship between the electric current and electric voltage of a circuit. Basically 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 due to 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 with respect 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:

  • Common Emitter Transistor

  • Common Base Transistor

  • Common Collector Transistor (emitter follower).

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 in order 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. In this case, the input is applied in the region of base-emitter and the output is obtained from between the terminals of collector and emitter.

In this case,

VBE is the input voltage,

Iis 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.


IE  = IC + IB

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This equation is transistor equations 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 base current gives current gain beta in the common-emitter configuration.

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, which means that 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 inputs 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 input resistance Rin.

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  1. 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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FAQ (Frequently Asked Questions)

1. What is the Difference Between the Transistors NPN and PNP, and What is the Usefulness of each?

Ans- The PNP and NPN transistors are made up of different materials, and hence the current flow to 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 emitter.

2. What is the Working Principle of PNP and NPN Transistors?

Ans- The concept behind the working principle of NPN transistors is that, when the supply of current to the base terminal is increased, the transistor turns ON and conducts the supply of current from the collector to the emitter. When the supply of current is low or reduced, the transistor stops conduction and shuts OFF.

The concept behind the working principle of a PNP transistor is that, when the electric current is passed through the base terminal of the transistor, the transistor shuts OFF. When the flow of current at the base terminal of the PNP transistor is stopped, then the transistor turns ON.

3. What are the Features of Transistors?

Ans- Transistors can change the output of the device when the input signal is varied, and hence they are used as amplifiers. Transistors are mainly used in electric devices like desktop, smartphone, laptop, tablet, etc for performing logical operations.