BJT Biasing and Stability Questions and Answers

Answer: C

The emitter resistor is there to provide thermal stability.

If temperature rises, the collector current tends to rise with it, because leakage current and beta increase. The extra current raises the voltage across the emitter resistor, which lowers the base-emitter voltage and pulls the current back. This negative feedback holds the Q-point steady and prevents thermal runaway.

An unbypassed emitter resistor actually reduces gain and raises the input impedance, so 'increase gain' and 'reduce input impedance' are the reverse of what happens. It does not significantly reduce the output impedance.

Answer: D

Biasing in class A mode fixes the operating point near the middle of the load line so that the device conducts for the full cycle. The key accepts both the improvement of stability and the reduction in the number of sources: a properly designed bias network, such as the voltage divider with an emitter resistor, keeps the operating point steady against changes in temperature and in transistor parameters, and it lets a single supply provide both the base bias and the collector supply, which removes the need for separate sources. Saving power is not the aim, because a class A stage draws current throughout the cycle and is the least efficient class. So both (B) and (C) hold, which is the last option. The answer is both (B) and (C).