Question E6B09:
What is a common use for point-contact diodes?  

Choices:  
A: As a constant current source  
B: As a constant voltage source  
C: As an RF detector  
D: As a high-voltage rectifier    

Correct Answer is  C: As an RF detector  

Correct answer: C — As an RF detector

Why C is correct:
- Point-contact diodes (early crystal detectors; modern equivalents are small-signal Schottky or germanium) have very low junction capacitance and low forward voltage, making them fast and sensitive for demodulating AM and for RF sensing.
- They rectify the RF, and an RC network recovers the audio envelope.

Useful detector rule-of-thumb:
- Choose RC so it filters out the RF but follows the audio envelope:
  $$\frac{1}{f_{RF}} \ll RC \ll \frac{1}{f_{audio(max)}}$$
  Equivalently, for the RC low-pass corner:
  $$f_{audio(max)} \ll \frac{1}{2\pi RC} \ll f_{RF}$$
- Typical forward voltage: germanium/Schottky $V_f \approx 0.2$–$0.3$ V (good for weak signals).

Why the other choices are incorrect:
- A: As a constant current source
  - A diode alone is not a current source; current sources require active biasing or specialized circuits. Point-contact diodes are used for detection/mixing, not for setting constant current.

- B: As a constant voltage source
  - That’s the role of Zener diodes (voltage reference). Point-contact diodes do not maintain a fixed voltage; their I–V curve is not suited for regulation.

- D: As a high-voltage rectifier
  - Point-contact diodes have low reverse breakdown voltage and low current capability. High-voltage rectification uses robust PN silicon rectifiers designed for large reverse voltages and currents.