15/01/2020

Common Collector ( CC ) Transistor Configuration & Its Application

  • As the collector terminal is common to both input and output of the transistor, it is called as Common Collector Configuration
  • The input voltage VCE is applied between base and collector terminal whereas the output is taken from collector and emitter terminals. 
  • The common collector configuration for NPN transistor is shown in the Figure.

common-collector-transistor-configuration


Current Amplification Factor
  • It is defined as the ratio of the change in emitter current to the change in base current.
                      γ = dIE / dIB …… ( 1 )
Expression of Collector Current
IC = IE + ICBO ………… ( 2 )
Now
IE = IB + IC……………( 3 )
From Equation ( 2 ) and ( 3 )
IE = IB +  α IE + ICBO
IE – α IE = IB + ICBO
IE ( 1 – α ) = IB + ICBO
IE = IB / ( 1 – α ) + ICBO / ( 1 – α )
As the base current is very small IE ≈ IC
IC = IB / ( 1 – α ) + ICBO / ( 1 – α )

As α = β / ( 1 + β )
( 1 – α )  = 1 – [ β / ( 1 + β ) ]
              = 1 / ( 1 + β )
Therefore 1 / ( 1 – α )  = ( 1 + β ) 
IC = IB / ( 1 – α ) + ICBO / ( 1 – α )
IC = ( 1 + β )  IB  + ( 1 + β )  ICBO

Relation between α and γ
As we know that
                      α = dIC / dIE…….. ( 4 )
IE = IC + IB
dIE = dIC + dIB
dIB = dIE – dIC………………. ( 5 )
From equation ( 1 ) and ( 5 )
γ = dIE / ( dIE – dIC )
Divide numerator and denominator by dIE
γ = [ dIE / dIE ] / [ ( dIE – dIC ) / dIE ]
γ  = 1 / ( 1 – α )
       γ  = 1 / ( 1 – α )

Application
  • As the input resistance of CC transistor configuration is very high and output resistance is very low, the voltage gain is always less than unity.
  • The CC transistor configuration is used for impedance matching.
Give Reason : The Common Collector ( CC ) transistor configuration is used for impedance matching.
                                             OR
The voltage gain of the Common Collector ( CC ) transistor configuration is less than unity.
  • Because it has high input impedance and low output impedance

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