A BJT is configured as a common-base amplifier; which of the following statements is INCORRECT?

Concept of Common-Base (CB) Configuration:

In a common-base BJT amplifier:

• Input signal is applied to the emitter

• Output is taken from the collector

• Base is common to both input and output (grounded for AC signals)

 Additional Information

1) Suitable for high-frequency applications

• Correct: The CB configuration has excellent high-frequency response due to:

  • Low input impedance

  • No Miller effect (capacitance multiplication)

  • Better bandwidth compared to CE configuration

2) Voltage gain is very high

• Correct: While current gain (α) is slightly less than 1, the CB amplifier provides high voltage gain (comparable to CE).

3) Operates as an amplifier in saturation region

• Incorrect (Answer):

  • BJT acts as an amplifier only in the active region (where IC = βIB holds).

  • In saturation, the transistor acts as a closed switch (not for amplification).

4) Works as an off switch when both junctions are reverse-biased

• Correct: This describes the cutoff region (transistor acts as an open switch).

Conclusion:

The incorrect statement is Option 3, because a CB amplifier must operate in the active region for proper amplification, not saturation.

Final Answer: 3) The CB configuration operates as an amplifier when the transistor is in the saturation region.