*Question E7B02:
What is a Class D amplifier?*  

*Correct Answer is  A: An amplifier that uses switching technology to achieve high efficiency*  

### AI Response: (may contain errors)  

A Class D amplifier is a type of amplifier that uses switching technology to achieve high efficiency. Let's break down the correct answer and why the other options are incorrect.

### Correct Answer: A
- **Class D Amplifier**: This amplifier operates by rapidly switching the output devices (transistors) on and off, which allows it to achieve efficiencies often exceeding 90%. The output signal is then filtered to remove the high-frequency switching components, resulting in a clean audio or RF signal.
- **Efficiency**: The high efficiency is primarily due to the fact that the output transistors are either fully on or fully off, minimizing the power lost as heat.

### Why the Other Choices are Incorrect:
- **B: A low power amplifier that uses a differential amplifier for improved linearity**  
  - This describes a different type of amplifier, often used in audio applications for better linearity, but it does not define a Class D amplifier. Class D amplifiers are not limited to low power and do not specifically use differential amplifiers.

- **C: An amplifier that uses drift-mode FETs for high efficiency**  
  - While FETs (Field Effect Transistors) can be used in Class D amplifiers, the term "drift-mode" is not standard in describing Class D technology. Class D amplifiers are characterized by their switching operation rather than the specific type of transistor used.

- **D: An amplifier biased to be relatively free from distortion**  
  - This describes Class A or Class AB amplifiers, which are designed to operate in a linear region to minimize distortion. Class D amplifiers, while they can be designed to reduce distortion, primarily focus on efficiency through switching.

### Summary
- **Class D amplifiers** are known for their high efficiency due to their switching operation.
- Other classes of amplifiers (like A and AB) focus on linearity and distortion, which is not the primary characteristic of Class D.

### Useful Rule of Thumb
- **Efficiency Comparison**: Class D amplifiers can achieve efficiencies of over 90%, while Class A amplifiers typically have efficiencies around 20-30%. This makes Class D amplifiers ideal for battery-powered and high-power applications.