Question:

For a single-stage BJT common base amplifier:

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Common base → current gain<1, voltage gain>1 → good for high-frequency applications.
Updated On: Jun 12, 2025
  • Current gain and voltage gain can be unity
  • Current gain can be>1, voltage gain<1
  • Current gain<1, voltage gain>1
  • Both current and voltage gains<1
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The Correct Option is C

Solution and Explanation

A common base amplifier is a type of transistor amplifier where the base terminal is common to both input and output. In analyzing the gains of a common base amplifier, consider the following:
Current Gain (α): In a common base configuration, the current gain (α) is defined as the ratio of the output current (collector current, Ic) to the input current (emitter current, Ie). Mathematically, it is given by:
α = Ic / Ie
The value of α is typically slightly less than 1, but it is always less than 1 because some part of the emitter current is lost due to minority carrier recombination in the base region.
Voltage Gain (Av): The voltage gain of a common base amplifier is defined as the ratio of the output voltage change (at the collector) to the input voltage change (at the emitter). It is given by the formula:
Av = gmRc
Where gm is the transconductance and Rc is the load resistor at the collector. The voltage gain can be greater than 1, especially when a high resistance Rc is used.
The correct answer for the given options related to a single-stage BJT common base amplifier is:
Current gain < 1, voltage gain > 1
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