Question E6F03:
What is the most common configuration of an optoisolator or optocoupler?  

Choices:  
A: A lens and a photomultiplier  
B: A frequency-modulated helium-neon laser  
C: An amplitude-modulated helium-neon laser  
D: An LED and a phototransistor    

Correct Answer is  D: An LED and a phototransistor  

Correct answer: D — An LED and a phototransistor

Why D is correct:
- An optoisolator (optocoupler) passes a signal optically while providing galvanic (electrical) isolation between input and output.
- The most common, low-cost, integrated form uses an infrared LED as the light source and a phototransistor as the detector in the same package.
- This combo offers adequate speed, reasonable linearity for logic-level coupling, high isolation voltage, and simple drive requirements.

Why the other choices are incorrect:
- A: A lens and a photomultiplier  
  - Photomultipliers require high voltage, are bulky and expensive, and are not used in standard integrated optocouplers.
- B: A frequency-modulated helium-neon laser  
  - He-Ne lasers are gas lasers needing high voltage and precision optics—far too large and complex for an optocoupler.
- C: An amplitude-modulated helium-neon laser  
  - Same issues as B; lasers are not used inside typical optoisolators.

Useful formulas and rules of thumb:
- Current Transfer Ratio (CTR):
  $$CTR(\%) = \frac{I_C}{I_F} \times 100\%$$
  - $I_F$: LED forward current (input side)
  - $I_C$: phototransistor collector current (output side)
  - Typical CTRs: 20%–200% (varies by part, temperature, and aging). Design with margin.
- LED input resistor (to set $I_F$):
  $$R_{in} = \frac{V_{in} - V_F}{I_F}$$
  - $V_F$ for IR LEDs is often about 1.2–1.4 V.
- Speed/bandwidth note:
  - Phototransistor optocouplers are relatively slow; approximate bandwidth:
    $$f_{-3\,\text{dB}} \approx \frac{1}{2\pi \tau}$$
    For higher speed and linearity, a photodiode-based optocoupler (not in the options) is used.
- Isolation ratings are commonly 2.5–5 kVrms.