When illuminated, metal-semiconductor-metal (MSM) photodetectors display a current-voltage characteristic that saturates with increasing bias voltage and which resembles the output characteristics of a field-effect transistor (FET). It is shown that operating an MSM photodetector with a GaAs FET active load can produce output voltage signal swings of over 80% of the power supply voltage from less than a 1 decade change in the MSM photocurrent, which may in turn be produced by only a 0.1 mW change in the input optical power. This substantial signal swing allows this circuit to be directly used as an extremely compact optical input to high-speed digital gate circuits without the need for any intervening amplifiers. Fully monolithic prototype optical input circuits were fabricated and tested to verify this concept. Less than -6 dBm of peak optical input power was found to be sufficient to provide noise-free switching of a standard buffered FET logic (BFL) inverter from dc up to 25 MHz, at which point the output of the BFL stage became limited by the package capacitance. The transfer function for this optical-to-electrical converter circuit is nonlinear and establishes noise margins on the optical input. In addition, the gate of the active load provides a convenient and direct control of the switching threshold, whereas a resistively loaded MSM photodetector provides only a fixed switching threshold.