It is desirable and necessary to keep IC
constant in the face of variations of ICBO (sometimes represented as ICO). 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 IC
w.r.t. the collector leakage current *ICO at constant β
is called stability factor i.e.
at constant IB and β
The factor indicates the change in collector current IC
due to the change in collector
leakage current ICO. Thus a factor 50 of a circuit means that IC
changes 50 times as much as
any change in ICO. 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:
= β IB(β + 1) ICO
Differentiating above expression w.r.t. IC
, we get,
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