Q133 / 180 · AIIMS 2005 · Physics

Assertion : In a common emitter transistor amplifier the input current is much less than the output current. Reason : The common emitter transistor amplifier has very high input impedance.

  • 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
  • Correct answer
    C
    If assertion is true but reason is false
  • D
    If both assertion and reason are false.

Solution

A good amplifier stage is one which has high input resistance and low output resistance. A transistor in $C B$ configuration has a very low input resistance $(\simeq 20 \Omega)$ and a very high output resistance ( $\simeq 1 \mathrm{M} \Omega$ ). It is just the reverse of what is required. The $C E$ configuration is better, its input resistance is about $1 \mathrm{k} \Omega$ and output resistance about $10 \mathrm{k} \Omega$. Although input resistance of $C E$ is not very high but it still prefers better amplifier.

Hence, the correct answer is option: 'C'.

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