*Question E6B06:
Which of the following is a common use of a Schottky diode?*  

*Correct Answer is  D: As a VHF/UHF mixer or detector*  

### AI Response: (may contain errors)  

The correct answer to the question about the common use of a Schottky diode is:

**D: As a VHF/UHF mixer or detector.**

### Explanation of the Correct Answer:
- **Schottky Diodes** are known for their low forward voltage drop and fast switching speeds. These characteristics make them particularly useful in high-frequency applications, such as VHF (Very High Frequency) and UHF (Ultra High Frequency) mixers and detectors. 
- In mixing applications, Schottky diodes can efficiently combine signals from different frequencies, which is essential in radio communications.

### Why the Other Choices are Incorrect:
- **A: In oscillator circuits as the negative resistance element**  
  - This is incorrect because Schottky diodes do not exhibit negative resistance. Negative resistance elements are typically found in devices like tunnel diodes or certain types of transistors, not Schottky diodes.

- **B: As a variable capacitance in an automatic frequency control circuit**  
  - This is incorrect because Schottky diodes do not function as variable capacitors. Variable capacitance is typically achieved using varactor diodes, which are specifically designed for that purpose.

- **C: In power supplies as a constant voltage reference**  
  - This is incorrect because Schottky diodes are not used as voltage references. They are primarily used for rectification due to their low forward voltage drop, but they do not provide a stable voltage reference like a Zener diode would.

### Summary:
- Schottky diodes are best utilized in high-frequency applications, particularly as mixers or detectors in VHF/UHF systems due to their fast response and low voltage drop.
- Remember the specific applications of different types of diodes to help distinguish their uses in the exam.