I need some basic advice for impedance matching: I have a photodetector (Det01CFC from Thorlabs, https://www.thorlabs.com/drawings/275cb418cdc2cf1f-A2E46103-D9C0-3581-4553E3D3CA93AD83/DET01CFC-Manual.pdf) that generates a current. For my experiment, I would like to amplify this signal with an amplifier to subsequently read it out with a digital oscilloscope.

The amplifier that I use is the LNA-1440 (https://rfbayinc.com/products_pdf/product_88.pdf) with an input impedance of 50 Ohm. Since the input impedance of the amplifier matches the source impedance of the photodetector, I connected them directly with an SMA-cable.

The problem that I am facing right now is that the signal gets attenuated for frequencies larger than a few MHz, i.e. it seems as if my circuit acts as a low pass filter. Do you have an idea why this could be the case?

Thank you very much for your advice!

  • \$\begingroup\$ How do you know that it isn't your driven light source that has the problem? \$\endgroup\$
    – Andy aka
    Jan 14, 2021 at 10:45
  • \$\begingroup\$ How are you measuring bandwidth? Are you using a 50 ohm oscilloscope? I suspect you are missing something. \$\endgroup\$ Jan 14, 2021 at 15:09
  • \$\begingroup\$ The issue is that if I add a parallel resistor of 50 Ohm between the photodetector and the amplifier, I do not observe this attenuation anymore. \$\endgroup\$ Jan 14, 2021 at 19:01
  • \$\begingroup\$ Yes, I measure with a 50 Ohm oscilloscope - I also suspect that something is missing, but I cannot spot it right now... \$\endgroup\$ Jan 14, 2021 at 19:02
  • \$\begingroup\$ I realized that when I use a 50 Ohm parallel resistor between the amplifier and the photodetector, the attenuation of the signal is no longer observable on the oscilloscope. One possibility of how I could understand this is that due to the photodetector, there is a capacitance parallel to the load resistance of the oscilloscope. Since those two act as a low-pass filter, I could see this attenuation. By adding a parallel resistance between the photodetector and the amplifier, however, this "low pass filter effect" gets reduced. What do you think? :) \$\endgroup\$ Jan 14, 2021 at 19:19


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