Question

A transistor in a series voltage regulator acts like a variable resistor. The value of its resistance is determined by _______

(a) emitter current

(b) base current

(c) collector current

(d) it is not controlled by the transistor terminals

I had been asked this question at a job interview.

Query is from Voltage Regulation Using Zener Diode in section Application of Diodes of Electronic Devices & Circuits

Answer 1

Right choice is (b) base current

For explanation I would say: The principle is based on the fact that a large fraction of the increase in input voltage appears across the transistor so that the output voltage remains to be constant. When input voltage is increased, the output voltage also increases which biases the transistor towards less current.