*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, has specific characteristics that make it useful in certain applications. Let's break down the correct answer and why the other choices are incorrect.

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

1. **Why the correct answer is correct:**
   - In an emitter follower, the output is taken from the emitter of the transistor. This configuration does not invert the phase of the signal. Therefore, the input and output signals are in-phase. This means that if the input signal goes up, the output signal also goes up, maintaining the same phase relationship.

### Why the other choices are incorrect:

2. **Choice A: Low input impedance and phase inversion from input to output**
   - An emitter follower actually has **high input impedance** and **low output impedance**, which makes it suitable for impedance matching. Additionally, there is no phase inversion; the input and output signals are in-phase.

3. **Choice B: Differential inputs and single output**
   - This describes a differential amplifier, not an emitter follower. An emitter follower has a single input and a single output, not differential inputs.

4. **Choice C: Acts as an OR circuit if one input is grounded**
   - This describes a logic gate function, not an amplifier characteristic. An emitter follower does not perform logical operations like an OR gate.

### Useful Formulas or Rules of Thumb:

- **Voltage Gain:** The voltage gain of an emitter follower is approximately 1, meaning the output voltage is nearly equal to the input voltage, but with a high input impedance and low output impedance.
  
- **Impedance Matching:** The high input impedance and low output impedance make the emitter follower ideal for buffering applications, where it can connect a high-impedance source to a low-impedance load without significant signal loss.

Understanding these characteristics helps in identifying the correct application and behavior of an emitter follower in electronic circuits.