Question

What is the role of emitter resistance in the transistor amplifying circuit?

(a) To prevent thermal runaway

(b) To prevent increase in gain

(c) To lower the output impedance

(d) To increase gain

This question was posed to me during an online interview.

I'm obligated to ask this question of Characteristics of Amplifier topic in chapter Transistor Biasing and Low Frequencies of Analog Circuits

Answer 1

The correct answer is (a) To prevent thermal runaway

Explanation: Thermal runaway is the increase in the collector current without an increase in input due to heating of semiconductor material which in turn reduce the resistance thus increases current. The emitter resistor decreases effective input voltage decrease when collector current increases and thus it reduces collector current itself.