A transistor is used in common emitter mode as an amplifier. Then
(1) the base-emitter junction is forward biased
(2) the base emitter junction is reverse biased
(3) the input signal is connected in series with the voltage applied to the base-emitter junction.
(4) the input signal is connected in series with the voltage applied to the base collector junction.

Options

A. A,B
B. A,D
C. A,C
D. only C

Answer

To solve the question regarding the operation of a transistor in common emitter mode as an amplifier, we will analyze each statement provided in the question.

### Step-by-Step Solution:

1. **Understanding Common Emitter Configuration**:
- In a common emitter configuration, the transistor has three terminals: the emitter, the base, and the collector. The emitter is common to both the input and output circuits.

2. **Analyzing Statement 1**:
- **Statement**: The base-emitter junction is forward biased.
- **Analysis**: For a transistor to operate as an amplifier in common emitter mode, the base-emitter junction must indeed be forward biased. This allows current to flow from the base to the emitter, which is essential for amplification.
- **Conclusion**: This statement is **True**.

3. **Analyzing Statement 2**:
- **Statement**: The base-emitter junction is reverse biased.
- **Analysis**: If the base-emitter junction were reverse biased, the transistor would not conduct, and thus it would not amplify the input signal. Therefore, this statement contradicts the operation of the transistor in common emitter mode.
- **Conclusion**: This statement is **False**.

4. **Analyzing Statement 3**:
- **Statement**: The input signal is connected in series with the voltage applied to the base-emitter junction.
- **Analysis**: In common emitter configuration, the input signal is indeed applied in series with the voltage that forward biases the base-emitter junction. This allows the input signal to modulate the base current, which in turn controls the collector current.
- **Conclusion**: This statement is **True**.

5. **Analyzing Statement 4**:
- **Statement**: The input signal is connected in series with the voltage applied to the base collector junction.
- **Analysis**: The input signal is not applied to the base-collector junction in common emitter mode. Instead, the base-collector junction is typically reverse biased. Therefore, this statement is incorrect.
- **Conclusion**: This statement is **False**.

### Final Conclusion:
Based on the analysis:
- **True Statements**: Statement 1 and Statement 3.
- **Correct Option**: The correct answer is **Option C**.