Checking the Servo Loop in a Linear Optocoupler Amplifier

A linear optocoupler amplifier can use one photodiode in a local feedback loop and another to transfer the signal across the isolation barrier. The feedback reduces sensitivity to LED output changes, but the complete gain still depends on the transfer relationship and surrounding circuit. Review both sides of the optical path.

Identify the two photodiode roles

The Vishay application note on IL300 linear amplifiers defines the servo and output gains and their ratio. It explains how the local optical feedback controls LED current while the second photodiode produces the isolated output signal.

Mark the feedback photodiode and output photodiode on the schematic. Confirm their polarity and operating mode. Interchanging them or reversing a connection can alter loop behavior even though the package still appears correctly oriented.

Trace the closed loop operating point

For the input range, calculate the required servo photocurrent and corresponding LED drive range using the selected circuit equations. Compare amplifier output swing and current capability with that requirement.

Check both ends of the range. Near zero input, offsets and leakage may become significant; near full scale, LED drive or amplifier headroom may limit the feedback loop. A gain measurement at midscale alone cannot establish linearity over the complete input span.

Budget the isolated output gain

Include the specified optical transfer ratio, feedback and output resistors, amplifier errors, and relevant temperature behavior. Distinguish correction of LED efficiency from correction of every gain error in the system.

If calibration is used, document whether it removes offset, gain, or a nonlinear residual. A two-point correction cannot automatically eliminate curvature or frequency dependent error.

Verify stability and transient behavior

Apply controlled input steps and inspect the local servo output as well as the isolated output. Check for ringing, saturation, or slow recovery. Photodiode capacitance and the amplifier network influence the loop’s dynamic behavior.

Measure gain and error across the intended input range, frequency band, and operating temperatures. Keep the receiving load consistent with the final application and use a measurement arrangement that preserves the intended isolation boundary.

Save photodiode mapping, component values, LED drive range, transfer assumptions, calibration, and measured residuals. A successful implementation demonstrates that the local loop remains in control and the isolated output meets the required accuracy. The presence of optical feedback does not make the complete amplifier inherently unity gain, perfectly linear, or independent of component selection and operating conditions. Preserve the fitted transfer ratio group with the calibration record.