Q.On what factors does stabilization of a transistor depend on?

Stabilization is the process of keeping a transistor's operating point — chiefly its collector current $I_C$ — fixed against the influences that tend to shift it. The operating point drifts mainly because of temperature changes, and because a replacement transistor of the same type may have different parameters.

The drift traces back to three temperature-dependent parameters:

• the reverse saturation (leakage) current $I_{CBO}$, which roughly doubles for every 10 °C rise and enters $I_C = \beta I_B + (1 + \beta)I_{CBO}$;
• the base-emitter voltage $V_{BE}$, which falls as temperature rises (about 2.5 mV per °C);
• the current gain $\beta$, which increases with temperature.

Each of these changes pushes $I_C$ upward, so a good biasing circuit must resist their effect. How well it does so is expressed by the stability factor

$S = \frac{\Delta I_C}{\Delta I_{CO}} \quad (\beta,\ V_{BE}\ \text{constant}),$ …