A bottom level detection device is disclosed for a burst mode optical
receiver. The receiver includes a converter for converting a burst mode
signal into a voltage signal and an automatic gain controller for
automatically controlling a gain of the converter. The bottom level
detection device detects a bottom level of an output signal from the
converter and controls the automatic gain controller on the basis of the
detected bottom level. The bottom level detection device includes an
amplification circuit for receiving the output signal from the converter
at its first input terminal and a fed-back output signal from the device
at its second input terminal, respectively, and amplifying a difference
between the received signals, a detector for detecting a bottom level of
an output signal from the amplification circuit, and an output buffer.
The detector includes an RC rectifier for receiving a power supply
voltage, and a transistor having its emitter connected to an output
terminal of the amplification circuit and its base and collector
connected in common to the RC rectifier.