>> 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 ICO, is more dominant over change in β and VBE. Hence S is calculated here.
S = ∂IC / ∂ICO : Rate of change of collector current w.r.t reverse saturation current
IC = β IB + (1+β) ICO
1 = β (∂IB / ∂IC) + (1+β) ∂ICO / ∂IC
⇒ ∂ICO / ∂IC = ( 1 – β ∂IB / ∂IC ) / (1+β)
⇒ S = ∂IC / ∂ICO = (1+β) / (1 – β ∂IB / ∂IC)