A communications system includes an optical receiver for receiving optical signals and for converting the optical signals into electrical signals, a transimpedance amplifier (“TIA”) for filtering the electrical signals, a limiting amplifier coupled with the TIA, an automatic threshold control (“ATC”) coupled with the TIA for providing a reference voltage for the limiting amplifier. The ATC further includes a common emitter circuit and an emitter follower circuit, wherein logic high signals and logical low signals in the electrical signals are determined based on the reference voltage output from the ATC.