Stability Factor

It is desirable and necessary to keep I_C
constant in the face of variations of I_CBO (sometimes represented as I_CO). The extent to which a biasing circuit is successful in achieving this goal is measured
by stability factor S. It is defined as under :
The rate of change of collector current I_C
w.r.t. the collector leakage current *I_CO at constant β
and I_B
is called stability factor i.e.
Stability factor,

S =

at constant I_B and β

The factor indicates the change in collector current I_C
due to the change in collector
leakage current I_CO. Thus a factor 50 of a circuit means that I_C
changes 50 times as much as
any change in I_CO. In order to achieve greater thermal stability, it is desirable to have as low stability
factor as possible. The ideal value of S is 1 but it is never possible to achieve it in practice. Experience shows that values of S exceeding 25 result in unsatisfactory performance.
The general expression of factor for a C.E. configuration can be obtain as under:

I_C
= β I_B(β + 1) I_CO

Differentiating above expression w.r.t. I_C
, we get,

1 =

S =

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