A connector compatible fiber optic replacement module for use in a local area network coaxial hub (192) for direct communication with TTL level signals to provide a fiber optic communication link. The system uses a first amplification stage (20) having a circuit design which provides a precisely controlled input impedance to allow unit to unit repeatability of response of the circuit in a simple and easy manner. High-frequency response of the circuit is provided by a cascode transistor (50, 70) configuration which limits the effects of virtual capacities. Direct coupled biasing (52, 56, 58) is employed throughout the receiver circuit to limit the number of components utilized. The transmitter circuit employs a standard TTL open collector driver circuit (126) used in peripheral units to drive an infra red (140) LED for transmission over fiber optic communication link.