Fundamental differences in design influence the way that linear and digital optocouplers are degraded by radiation. Linear optocouplers are more affected by current drive conditions because the detector operates in a high-injection region when the LED produces normal light output, and do not have the extra operating margin that is inherent in digital optocouplers. Although LED degradation is often the dominant degradation mechanism in space environments, degradation of optocouplers with improved LEDs is limited by photoresponse degradation. Phototransistor gain has a relatively minor effect except at very high radiation levels.
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