High-speed optical receivers use a photodetector to convert a received optical signal to an output current signal. The output current signal is typically coupled to the input of a transimpedance amplifier (TIA) that converts the current signal to a voltage signal.
Optical receivers and monitors using high speed photodetectors and TIAs typically integrate an external capacitor between the power supply and ground. The external capacitor substantially reduces the reception of power supply transients by the TIA that might otherwise be received and amplified into a noise contribution to the output signal. However, the output of the photodetector is typically wire bonded to the input of the external TIA. The wire bonds introduce a parasitic inductance that tends to degrade the signal generated by the photodetector.
In addition the return path for this signal is from the TIA ground through wire bonds to the circuit board, then through traces on the circuit board, the external capacitor and additional wire bonds, to the photodetector. This path also introduces parasitic inductance, which may also degrade the signal out of the transimpedance amplifier.