Question

For a single-stage BJT common base amplifier:

• A. Current gain and voltage gain can be unity
• B. Current gain can be>1, voltage gain<1
• C. Current gain<1, voltage gain>1
• D. Both current and voltage gains<1

Hint:

Common base → current gain<1, voltage gain>1 → good for high-frequency applications.

Answer 1

The Correct Option is C

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, I_c) to the input current (emitter current, I_e). Mathematically, it is given by:

α = I_c / I_e

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 (A_v): 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:

A_v = g_mR_c

Where g_m is the transconductance and R_c is the load resistor at the collector. The voltage gain can be greater than 1, especially when a high resistance R_c 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