Stability Factors

Stability Factors

$\displaystyle \Rightarrow$ Stability factor indicates the degree of change in operating due to variation in temperature. There are three variables which are temperature dependent. We can define three stability factors :

$\displaystyle S = \frac {\partial I_{C}}{\partial I_{CO}} \rightarrow \beta, V_{BE}$ Constant

$\displaystyle S^{'} = \frac {\partial I_{C}}{\partial V_{BE}} \rightarrow \beta, I_{CO}$ Constant

$\displaystyle S^{''} = \frac {\partial I_{C}}{\partial \beta} \rightarrow I_{CO}, V_{BE}$ Constant

$\displaystyle \Rightarrow$ Ideally, Stability factors should be zero to keep operating point stable and fixed.

$\displaystyle \Rightarrow$ Practically, Stability factors should have the value as minimum as possible.

$\displaystyle \Rightarrow$ Effect of change in $\displaystyle I_{CO}$ , is more dominant over the change in $\displaystyle \beta$ and $\displaystyle V_{BE}$ . Hence $\displaystyle S$ is calculated here.

$\displaystyle S = \frac {\partial I_{C}}{\partial I_{CO}}$

$\displaystyle \Rightarrow$ Rate of change of collector current w.r.t reverse saturation current

$\displaystyle I_{C} = \beta I_{B} + (1+\beta) I_{CO}$