question_answer

Direction for: In each of the following questions a statement of Assertion is given followed by a corresponding statement of Reason just below it. Of the statements mark the correct answer as

Assertion: A transistor amplifier in common emitter configuration has a low input impedance. Reason: The base to emitter region is forward biased.

A) If both assertion and reason are true and reason is the correct explanation of assertion.

B) If both assertion and reason are true but reason is not the correct explanation of assertion.                

C) If assertion is true but reason is false.

D) If both assertion and reason are false.

Correct Answer: B

Solution :

In transistor the emitter-base junction is forward bias while the base-collector junction is given a large reverse bias. Under forward bias the charge carriers move towards base causing the flow of current. The following shows the circuit of a common-emitter amplifier -circuit using an n-p-n transistor. The input (base-emitter) circuit is forward biased by a low voltage battery \[{{V}_{BE}}\], so that the resistance of the input circuit is small. The output (collector-emitter) circuit is reverse biased by means of a high voltage battery so that resistance of output is high. The weak in put AC signal is applied across the base-emitter circuit and the amplified output signal is obtained across the collector-emitter circuit. Input impedance of common-emitter configuration \[\left( \frac{\Delta {{V}_{BE}}}{\Delta {{I}_{B}}} \right){{V}_{CE}}=constant\] Here, \[\Delta {{V}_{BE}}\] = voltage across base and emitter, \[\Delta {{I}_{B}}\] = base current of the order of few micro ampere.