Question

Which among the below specified conditions is applicable to prevent the occurrence of thermal runaway in voltage divider bias circuit?

(a) VCE < VCC / 2

(b) VCE = VCC / 2

(c) VCE > VCC / 2

(d) None of the mentioned

This question was addressed to me in an internship interview.

I need to ask this question from Frequency Compensation in section Frequency Response of Electronic Devices & Circuits

Answer 1

Right answer is (a) VCE < VCC / 2

To explain I would say: Thermal runaway is a growing process which can ultimately harm the transistor due to unnecessary internal heating or increase in ambient temperature. It occurs due to increase in collector current ahead of the maximum specified value. Additionally, it also occurs due to excessive internal power dissipation above the maximum allowed value. Thermal runaway can be disallowed by maintaining the provision of thermal stability in addition to the treatment of heat sink.

According to thermal stability condition, VCE should be less than Vcc/2 results in the Q-point operation at a safe level. On the contrary, if the location of Q-point is at VCE > Vcc/2, then the transistor is more likely to get damaged due to thermal runaway.