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My photomultiplier tube is connected to a Transimpedance amplifier for routine experiments. However for an experiment I need to connect the output of the PMT to lock in amplifier (SR 865A). The manual says that the lock in has inbuilt amplifier. Shall the transimpedance amplfier be removed before the connection between PMT and lock in is made or via the external amplifier is fine.

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  • \$\begingroup\$ Welcome to Stack Exchange. A schematic might help people answer your question. And a link to the SR 865A data sheet too. \$\endgroup\$ Sep 5 '18 at 22:18
  • \$\begingroup\$ Are you planning on detecting a current or individual photons? \$\endgroup\$
    – Voltage Spike
    Sep 5 '18 at 22:44
  • \$\begingroup\$ It is an analog PMT. I am reading voltages using the transimpedance amplifier. The PMT itself is current out put PMT. \$\endgroup\$
    – user188062
    Sep 6 '18 at 4:02
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You will be able to set up the Stanford lock in to use it's own amplifier (in which case you should select current input) or use the separate amplifier you have (in which case select voltage input). Compare the specifications of the built-in and separate amplifiers, and choose the one with the enough gain and the least noise in the frequency band you're interested in. We can't recommend one way or the other unless you give us the specification of both amplifiers.

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  • \$\begingroup\$ For the lock in The current input preamplifier is built with transimpedance gain of 10^6 V/A or 10^8 V/A . The spec sheet for external amplifier is thorlabs.com/drawings/… which says iis in range of 10^4. The PMT has a max output of 2 microamps hamamatsu.com/resources/pdf/etd/m-h7422e.pdf. model number H7422-50 (ANALOG NOT PHOTON COUNTING) \$\endgroup\$
    – user188062
    Sep 6 '18 at 4:13
  • \$\begingroup\$ +1 for vendor support. Thank you for taking the time. \$\endgroup\$
    – KalleMP
    Sep 6 '18 at 8:44
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The output of the PMT is current. The amount of current, or Iser(single electron response) depends on the gain of the PMT, a constant (e=1.6E-19), and the width of the signal scaled by the amount of light that is hitting the photocathode.

Iser = G*e/FWHM

That current, driven through a load resistor/input impedance of the SR 865A will generate a voltage. The current input of the SR 865A has two selectable input impedances, 100Ω and 1kΩ for a current input, and programmable current gain.

Typically the voltage generated for a single photon event into ~50Ω is well above the noise floor so you shouldn't need the extra amplification when terminating into 100Ω but you should calculate based on the amount of light you want to detect, the Iser of your photodiode, and the noise specs over the bandwidth you need for the SR 865A.

The expense of higher current gain is reduced bandwidth so depending on your detection requirements you should be able to figure out if the SR 865A is the only thing you need or if you should be tacking on anything in front of it.

In general: Read through your PMT spec sheet, read through the SR865A manual, and call SRS/Hamamatsu with questions, they will be helpful.

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  • \$\begingroup\$ I believe the native PMT output is charge, generally converted to a voltage pulse with the last divider resistor usually and then to current with the amplifier input impedance. Sometimes there is no resistor in the PMT base and this is all done in the amplifier circuitry. If the illumination is greater than single photon counting can cope with then the pulses integrate into a steady state and charge pulses per second appear as a constant current. \$\endgroup\$
    – KalleMP
    Sep 6 '18 at 8:47

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