Q.Explain i) Stabilization of Q point. ii) Selection of Q points on the load line. iii) Thermal runaway. iv) Heat sink. v) Leakage currents.

This question covers five operating-point and temperature topics; each is defined below with its role in transistor biasing. …

[!TLDR]

Stabilization fixes the Q point; the Q point is selected by application (switching vs amplification); thermal runaway is cumulative heating; a heat sink removes heat; leakage currents are the reverse minority-carrier currents $I_{CBO}$ and $I_{CEO}$.

i) Stabilization of Q point: the process of keeping the operating point (mainly the collector current $I_C$) constant against variations due to temperature, $\beta$ and transistor replacement. In voltage divider bias, if $I_C$ rises, the drop across $R_E$ increases, $V_{BE}$ falls, and $I_B$ and $I_C$ reduce — restoring the Q point. Because $I_{CQ} = (V_2 - V_{BE})/R_E$ has no $\beta$, stabilization is excellent.

ii) Selection of Q points on the load line: the position of the Q point on the load line is chosen by the application. For switching, the operating points are chosen in the saturation region (closed switch) and the cutoff region (open switch). For amplification, the Q point is chosen at the centre of the active region (middle of the load line) so the output can swing symmetrically for faithful amplification without entering saturation or cutoff.

iii) Thermal runaway: since $I_C = \beta I_B + (1+\beta)I_{CBO}$, a rise in $I_C$ raises the junction temperature, increasing $I_{CBO}$, which further raises $I_C$ and temperature cumulatively. Uncontrolled, this shifts the Q point to saturation and can burn out the junctions. This self-destruction due to rising temperature and leakage current is thermal runaway.

iv) Heat sink: a device (usually a shaped copper or metal conductor) attached to the transistor that absorbs the unnecessary heat generated and radiates it to the surroundings. It takes temperature out of the device, raising its power rating and safeguarding it against thermal runaway. …