Question

Which type of connection/s is/are rarely employed for audio-frequency circuits due to assortments in input & output resistances in addition to current gain less than unity?

$\left( a \right)$ Common collector
$\left( b \right)$ Common base
$\left( c \right)$ Common emitter
$\left( d \right)$ None of the above

Answer 1

Hint: In common base circuit: no current, but voltage gain will be there, and in common collector circuit there will be the current gain, but no voltage gain. So by using these statements we will be able to answer this question easily.

Complete step by step answer: We know that the transistor is a two junction, three layered and three terminal semiconductor device and these terminals are base, collector, and emitter. It is composed of p- type and n- type materials; therefore it is available in PNP and NPN configurations.
And we know that in common base transistors, base is common for input and output circuits. It has the low value of impedance at input and high value of impedance at output. The current gain is less than unity. It is used in audio and radio frequency circuits.

Therefore, the option $\left( b \right)$ is correct.

Additional information: There are basically three configurations of transistors namely Common collector, common emitter and common base. All three configurations have different characteristics and applications.
Characteristics of common collector:
In this transistor, collector is common for input and output circuits. It has the high value of impedance at input and low value of impedance at output. The current gain is ninety nine percent. And the power and voltage gain is close or less than unity. The application of a common collector is for impedance matching.
Characteristics of common emitter:
In this transistor, an emitter is common for input and output circuits. It has the high value of impedance at input and low value of impedance at output. The current gain is ninety eight percent. It is used in high frequency circuits.

Note: Common collector is commonly known as emitter follower because the input applied at the base is followed very near at the emitter having voltage gain near to unity. So from this question we can say that the common base is mainly used for providing the high frequency applications.