*Question E7B09:
What is characteristic of an emitter follower (or common collector) amplifier?*  

*Correct Answer is  D: Input and output signals in-phase*  

### AI Response: (may contain errors)  

An emitter follower, also known as a common collector amplifier, is a type of transistor amplifier that has specific characteristics. Let's break down the question and the choices.

### Correct Answer: D: Input and output signals in-phase

- **Explanation**: In an emitter follower configuration, the output is taken from the emitter of the transistor. This configuration provides a voltage gain of slightly less than 1, meaning the output voltage closely follows the input voltage. Therefore, the input and output signals are in-phase, which is a key characteristic of this type of amplifier.

### Why the Other Choices are Incorrect:

- **A: Low input impedance and phase inversion from input to output**  
  - **Incorrect**: An emitter follower actually has a high input impedance, which makes it suitable for interfacing with high-impedance sources. Additionally, it does not invert the phase; the output is in-phase with the input.

- **B: Differential inputs and single output**  
  - **Incorrect**: This description is more characteristic of differential amplifiers, not emitter followers. An emitter follower typically has a single input and a single output.

- **C: Acts as an OR circuit if one input is grounded**  
  - **Incorrect**: This statement does not apply to an emitter follower. An emitter follower does not function as a logic gate like an OR circuit. It simply amplifies the input signal without any logical operation.

### Useful Rules of Thumb:

- **Input Impedance**: Emitter followers have high input impedance, making them ideal for buffering applications.
- **Voltage Gain**: The voltage gain is approximately 1, meaning the output voltage is nearly equal to the input voltage.
- **Phase Relationship**: The output signal is in-phase with the input signal, which is crucial for applications requiring signal integrity.

This understanding of the emitter follower's characteristics will help you in the Extra Class exam and in practical applications of amplifiers.