In a common emitter amplifier, the input and output voltage are out of phase by

Options

A. 30°
B. 90°
C. 120°
D. 180°

Answer

To solve the question regarding the phase difference between the input and output voltages in a common emitter amplifier, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Common Emitter Configuration**:
- In a common emitter amplifier, the input is applied to the base-emitter junction, and the output is taken from the collector-emitter junction.

2. **Analyzing Input and Output Relationship**:
- When the input voltage (base voltage) increases, it causes an increase in the base current (IBE).
- As the base current increases, the collector current (IC) also increases due to the transistor's current amplification property.

3. **Effect on Collector Voltage**:
- The increase in collector current (IC) leads to a larger voltage drop across the collector resistor (Rc).
- The collector voltage (VC) is given by the equation VC = Vcc - IC * Rc, where Vcc is the supply voltage.

4. **Determining the Phase Shift**:
- As the collector current increases, the drop across Rc increases, which results in a decrease in the collector voltage (VC).
- Therefore, when the input voltage increases, the output voltage decreases.

5. **Conclusion on Phase Difference**:
- Since the output voltage decreases when the input voltage increases, the input and output voltages are out of phase by 180 degrees.

6. **Final Answer**:
- The input and output voltage in a common emitter amplifier are out of phase by **180 degrees**.