- 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.
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 = α 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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