Question

Which of the following biasing techniques are prone to thermal runaway?

(a) self bias

(b) collector to base bias

(c) fixed bias

(d) the biasing technique is identified by temperature effect

I had been asked this question in final exam.

My question is from Thermal Runaway in chapter Transistor Biasing and Thermal Stabilization of Electronic Devices & Circuits

Answer 1

Right choice is (c) fixed bias

Explanation: The collector current of a fixed bias transistor is IC= β(VCC-VBE)/RB. When the temperature is increased, the reverse saturation increases. The collector current also increases. This in turn increases the current again which leads to damage of transistor.