Question

The current gain of a common emitter amplifier is \(50\) and its power gain is \(3000\). If the input resistance of the amplifier is \(1200\,\Omega\), then its output resistance is

• A. \(1720\,\Omega\)
• B. \(1800\,\Omega\)
• C. \(2400\,\Omega\)
• D. \(1440\,\Omega\)

Hint:

For a transistor amplifier, \[ \boxed{ A_P=A_IA_V } \] and \[ \boxed{ A_V=A_I\left(\frac{R_{\text{out}}}{R_{\text{in}}}\right). } \]

Answer 1

The Correct Option is D

Step 1: Use the relation between power gain, current gain and voltage gain. Power gain is \[ A_P=A_IA_V. \] Given, \[ A_P=3000, \qquad A_I=50. \] Hence, \[ A_V=\frac{3000}{50}=60. \]

Step 2: Relate voltage gain to resistances. For a transistor amplifier, \[ A_V=A_I\left(\frac{R_{\text{out}}}{R_{\text{in}}}\right). \] Therefore, \[ 60 = 50\left(\frac{R_{\text{out}}}{1200}\right). \] Hence, \[ R_{\text{out}} = \frac{60\times1200}{50} = 1440\,\Omega. \]

Step 3: Write the answer. Therefore, \[ \boxed{1440\,\Omega.} \] Thus, \[ \boxed{(D)} \] is the correct answer.