Electronic Circuits - AMIE

Electronic Circuits 

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 Transistors Bipolar Junction Transistor

Transistors are three terminal active devices made from different semiconductor materials that can act as either an insulator or a conductor by the application of a small signal voltage. The transistor's ability to change between these two states enables it to have two basic functions: "switching" (digital electronics) or "amplification" (analogue electronics). Then bipolar transistors have the ability to operate within three different regions:

Bipolar Transistor basic construction consists of two PN-junctions producing three connecting terminals with each terminal being given a name to identify it from the other two. These three terminals are known and labelled as the Emitter ( E ), the Base ( B ) and the Collector ( C ) respectively.

 

 Biasing technique 

Applying suitable DC voltage across the terminal of the transistor is called Biasing 

                                                                                                                Emitter Base       Collector Base

Operation Mode of BJT  are follows  
 1. Active Region   -   the transistor operates as an amplifier                       Forward               Reverse
 2. Saturation   -   the transistor is "fully-ON" operating as a switch             Forward               Forward
 3. Cut-off   -   the transistor is "fully-OFF" operating as a switch .              Reverse                Reverse

According to the  circuit arrangement the transistor can be mainly connected  in three configuration 
1. Common Base Configuration   -   has Voltage Gain but no Current Gain
2. Common Emitter Configuration   -   has both Current and Voltage Gain.  
3. Common Collector Configuration   -   has Current Gain but no Voltage Gain.

 1).Common Base Configuration

In this arrangement the Emitter Base junction is Froward biased ,and the base collector junction is reverse biased  

The emitter current IE flows as input current and the Collector current IC flows as output current .any change in the IE current makes a similar current changes in IC 

The ratio of collecter current to the emitter current  is called current amplification factor .(Alpha, α).

          IC                          Output 

α =  ---------  =            ------------------         

          IE                           Input
The Ic cosist of two current IC and IB ,comparatively  IB is too small current.