Stability Factor- The rate of change of collector current I
_{C}with respect to collector leakage current I_{CBO}/ I_{CO}( In common base configuration ) at constant current amplification factor β and base current is known as stability factor ( S ).

Stability Factor ( S ) = dI

_{C}/ dI_{CO}( at constant β and I_{B})…….. ( 1 )

**Significance of Stability Factor**- The stability factor indicates change in collector current with respect to change in collector leakage current.
- If the stability factor is 20, it means that the collector current changes 20 times due to change in collector leakage current.
- It is desirable that the value of stability factor should be as low as possible. The ideal value of stability factor is unity which is never possible.

The collector current in the common emitter configuration is given by

I

_{C}= βI_{B}+ ( 1 + β ) I_{CO}……….. ( 2 )
Differentiate above equation ( 2 ) with respect to I

_{C}
1 = β ( dI

_{B }/ dI_{C}) + ( 1 + β ) ( dI_{CO}/ dI_{C})
1 = β ( dI

_{B }/ dI_{C}) + ( 1 + β ) / S ( As stability factor S = dI_{C}/ dI_{CO})
1 – β ( dI

_{B }/ dI_{C}) = ( 1 + β ) / S
S = ( 1 + β ) / 1 – β
( dI

_{B }/ dI_{C})
The equation of stability factor is

**S = ( 1 + β ) / 1 – β ( d**

**I**

_{B }/ dI_{C})- If the value of stability factor greater than 25, it will result in unsatisfactory performance of transistor amplifier.

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