Optocoupler Amplifier Wiring Method

An optocoupler amplifier is wired by connecting the input LED through a current-limiting resistor, using a photodiode or phototransistor on the output side with feedback for linearity, and ensuring separate power supplies for input and output to maintain galvanic isolation.

Basic Wiring Principles

1. Input Side (LED)
   • Connect the input signal to the LED through a current-limiting resistor to set the proper forward current (I_F) for the LED. This ensures the LED emits light proportional to the input voltage or current without exceeding its ratings .
   • The input circuit is electrically isolated from the output, so it can have a separate power supply.
2. Output Side (Photodetector)
   • Use a phototransistor or photodiode to detect the LED light. The phototransistor is typically connected in a common-emitter configuration with a load resistor to convert the photocurrent into a voltage signal .
   • For linear analog applications, a photodiode in photoconductive mode is preferred, often paired with an operational amplifier to maintain linearity and gain stability .
3. Feedback for Linear Amplification
   • High-precision optocouplers like IL300 or HCNR201 include a servo photodiode that monitors LED output and feeds back to the input op-amp. This feedback loop linearizes the LED-photodiode response and compensates for temperature or LED efficiency variations .
   • The output photodiode then produces a current that accurately tracks the servo photodiode, ensuring stable transfer gain.
4. Resistor Selection
   • Input resistor (R1): Sets LED current based on input voltage. Choose to match the LED's forward current specification.
   • Output resistor (R2): Converts photodiode current to voltage. Its value depends on desired output voltage swing and photodiode current.
   • The Current Transfer Ratio (CTR) of the optocoupler affects resistor sizing and overall gain .
5. Power Supply Considerations
   • Maintain separate power supplies for input and output circuits to preserve galvanic isolation.
   • Ensure the op-amp and feedback network on the output side are powered appropriately to handle the photodiode current and maintain linearity.

Example Configurations

• Single-ended amplifier: Input LED drives a photodiode with a load resistor; output is taken across the resistor.
• Differential amplifier: Two photodiodes or a matched pair (like HCNR201) provide differential output for improved common-mode rejection.
• Servo-feedback amplifier: Uses a second photodiode to monitor LED flux and feed back to the input op-amp, achieving high linearity and stable gain (used in IL300 and HCNR201 designs), .

Summary

To wire an optocoupler amplifier effectively:

• Connect the input LED with a current-limiting resistor.
• Use a photodiode or phototransistor on the output side with a load resistor.
• Implement feedback using a servo photodiode for linearity in precision applications.
• Keep input and output circuits electrically isolated with separate power supplies.
• Choose resistor values based on LED current, photodiode current, and desired gain. This wiring method ensures accurate signal transfer, galvanic isolation, and stable amplification suitable for industrial, medical, and instrumentation applications .