Abstract: A gallium arsenide differential line receiver circuit has a first and second receiver means for receiving first and second differential input signal voltages. A pair of depletion transistor means for transforming the signal voltages into corresponding signal currents provides pull-up, and the depletion transistors have their gates operatively connected to the first and second receiver means. A pair of enhancement transistor means is operatively connected to the pair of depletion transistor means for outputting a gallium arsenide signal representation of the first and second differential input signal voltages. Positive feedback is provided for increasing the gain of the pair of enhancement transistors and is operatively connected to the pair of enhancement transistors. Shunting means is also provided for reducing hysteresis switching and is operatively connected to the pair of enhancement transistors.

Abstract: A photoluminescent indicator apparatus having automatic feedback means to maintain the sensitivity of the indicator apparatus with respect to variations in environmental conditions. The apparatus includes a sample of photoluminescent material having a photoluminescent decay rate which varies as a function of environmental conditions. The sample is positioned in a remote location having some unknown aspect of the environment desired to be measured. The sample is optically excited with a modulating signal to generate an excitation output signal functionally dependent on the modulating signal and indicative of the unknown environmental condition. A phase detection means is provided for comparing difference in phase between a phase reference signal and the excitation output signal to generate a phase differential signal functionally related to the photoluminescent decay rate of the sample.