>> ** Stability factor indicates degree of change in operating due to variation in temperature. **There are three variables which are temperature dependent. We can define three stability factors :

>> **Ideally, Stability factors should be zero to keep operating point stable and fixed.**

>> Practically, Stability factors should have the value as minimum as possible.

>> Effect of change in I_{CO}, is more dominant over change in β and V_{BE}. Hence S is calculated here.

**S = ∂I _{C} / ∂I_{CO} : Rate of change of collector current w.r.t reverse saturation current **

I_{C} = β I_{B} + (1+β) I_{CO}

1 = β (∂I_{B} / ∂I_{C}) + (1+β) ∂I_{CO} / ∂I_{C}

⇒ ∂I_{CO} / ∂I_{C} = ( 1 – β ∂I_{B }/ ∂I_{C} ) / (1+β)

⇒ **S = ∂I _{C} / ∂I_{CO} = (1+β) / (1 – β ∂I_{B} / ∂I_{C})**

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