The light is detected by the D91 photo detector and fed to
SA5211 chip[9] which is a transimpedance, wide-band,
low noise amplifier with differential outputs, particularly
suitable for signal recovery in fiber optic receivers. A
differential amplifier followed will amplify the output of
SA5211 as necessary for comparator circuit. High
Impedance Comparator Circuit will receive the signal from
trans-impedance circuit and generate two wave forms which
can be used to extract three positive levels it receive prior to
converting back to HDB3 wave form. The signal it
generates directed to a switching circuit.