*Question E7D03:
What device is used as a stable voltage reference?*  

*Correct Answer is  A: A Zener diode*  

### AI Response: (may contain errors)  

A Zener diode is used as a stable voltage reference because it is designed to maintain a constant voltage across its terminals when reverse-biased, even if the current through it changes. This makes it ideal for providing a stable reference voltage in circuits.

### Explanation:

1. **Correct Answer: A Zener diode**
   - **Zener Diode Functionality**: When reverse-biased, a Zener diode allows current to flow in the reverse direction once the voltage across it reaches a certain value known as the Zener breakdown voltage. This voltage remains nearly constant over a wide range of currents, making it an excellent voltage reference.
   - **Application**: Zener diodes are commonly used in voltage regulation circuits to provide a stable reference voltage.

2. **Why the Other Choices are Incorrect**:
   - **B: A digital-to-analog converter (DAC)**
     - **Functionality**: A DAC converts digital signals into analog voltages or currents. It is not inherently used for providing a stable voltage reference.
   - **C: An SCR (Silicon Controlled Rectifier)**
     - **Functionality**: An SCR is a type of thyristor used for controlling power. It acts as a switch and is not used for voltage reference purposes.
   - **D: An analog-to-digital converter (ADC)**
     - **Functionality**: An ADC converts analog signals into digital data. It does not provide a stable voltage reference.

### Key Points:
- **Zener Diode Voltage Regulation**: The Zener diode maintains a constant voltage, $V_Z$, across it when reverse-biased, which is used as a reference.
- **Zener Diode Symbol**: The symbol for a Zener diode is similar to a regular diode but with bent edges on the bar, indicating its special breakdown characteristics.

In summary, the Zener diode is specifically designed to provide a stable voltage reference, which is why it is the correct answer for this question.