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

*Correct Answer is  A: A Zener diode*  

### AI Response: (may contain errors)  

The correct answer to the question about a stable voltage reference is:

**A: A Zener diode**

### Explanation of the Correct Answer:
- **Zener Diode**: A Zener diode is specifically designed to operate in the reverse breakdown region, where it maintains a constant voltage across its terminals despite variations in current. This property makes it an excellent choice for providing a stable reference voltage in circuits.

### Why the Other Choices are Incorrect:
- **B: A digital-to-analog converter (DAC)**: 
  - A DAC converts digital signals into analog voltages but does not inherently provide a stable voltage reference. Its output can vary based on the input digital signal and power supply variations.

- **C: An SCR (Silicon Controlled Rectifier)**: 
  - An SCR is a type of semiconductor device used for switching and controlling power. It does not provide a stable voltage reference; instead, it is used for controlling high power applications.

- **D: An analog-to-digital converter (ADC)**: 
  - An ADC converts analog signals into digital data. Like the DAC, it does not serve as a stable voltage reference; its output depends on the input analog signal.

### Useful Formulas or Rules of Thumb:
- The Zener voltage ($V_Z$) can be defined as the voltage across the Zener diode when it is in reverse breakdown:
  
  $$V_Z = V_{in} - I_Z \times R$$

  where:
  - $V_{in}$ = input voltage
  - $I_Z$ = Zener current
  - $R$ = series resistance

This formula illustrates how the Zener diode can maintain a stable output voltage ($V_Z$) as long as the Zener current ($I_Z$) remains within specified limits.