Question

In a common emitter transistor amplifier, the audio signal voltage across the collector resistance of \(2\,k\Omega\) is \(2V\). If the current amplification factor (\(\beta\)) is \(100\) and the base resistance is \(1\,k\Omega\), find the input signal voltage.

• A. \(0.1\,V\)
• B. \(0.01\,V\)
• C. \(0.02\,V\)
• D. \(0.005\,V\)

Hint:

For a common emitter amplifier: \[ A_v = \beta \left(\frac{R_C}{R_B}\right) \] Once the voltage gain is known, input voltage can be quickly found using \[ V_{in} = \frac{V_{out}}{A_v}. \]

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
We need to find the input voltage for a transistor operating in Common Emitter (CE) configuration, given the output voltage and gain parameters.
Step 2: Key Formula or Approach:
Voltage Gain (\( A_v \)) is defined as the ratio of output voltage to input voltage:
\[ A_v = \frac{V_{out}}{V_{in}} \]
For a CE amplifier, it is also expressed as:
\[ A_v = \beta \times \frac{R_c}{R_b} \]
Step 3: Detailed Explanation:
Identify given values:
Collector Resistance (\( R_c \)) = \( 2 \, k\Omega \)
Base Resistance (\( R_b \)) = \( 1 \, k\Omega \)
Current Gain (\( \beta \)) = 100
Output Voltage (\( V_{out} \)) = \( 2 \, V \)
Calculate Voltage Gain (\( A_v \)):
\[ A_v = 100 \times \left( \frac{2000}{1000} \right) = 100 \times 2 = 200 \]
Now, calculate Input Voltage (\( V_{in} \)):
\[ V_{in} = \frac{V_{out}}{A_v} \]
\[ V_{in} = \frac{2}{200} = \frac{1}{100} \]
\[ V_{in} = 0.01 \, V \]
Step 4: Final Answer:
The input signal voltage is \( 0.01 \, V \).