First slide

Transistor

Question

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

Moderate

A

Both statements I and II are correct.
B

Statement I is correct but statement II is incorrect
C

Statement I is incorrect but statement II is correct
D

Both statements I and II are incorrect.

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 figure 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 input 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 $= (\frac{∆ V_{BE}}{∆ I_{B}}) V_{CE} = Constant$
Here, $∆ V_{BE}$ = voltage across base and emitter $∆ I_{B}$ = base current of the order of few micro ampere.

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