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A Hybrid Photodiode from DEP was used as a photo-intensity reference for most light tests in the lab. It particular it served to calibrate the detection efficiencies of the candidate Silicon Photomultipliers (SiPMs) for the Tagger Microscope.
[[Image:S20photocathode_QE.jpg|frame|HPD Photo-cathode efficiency as a function of wavelength.]]
The quantum efficiency of this device is shown in the adjacent figure. The DEP-supplied device provides a gain factor of 2700 at the HV of 12 kV and additionally requires 40 - 80 V bias to collect the electrons across the diode junction. Built into the package is a transimpedance amplifier with 50 kΩ || 1.5 pF feedback, requiring ±5 V supplies. The amplifier circuit also contains a 51.1 Ω resistor in series with the output. Reading out the HPD signal from the amplifier with 50 Ω termination creates a about a factor of two (50 Ω/(50 Ω + 51.1 Ω)) on top of the 50 kΩ current to voltage conversion.
== Frequency Response ==
Based on the circuit described above, a two-pole response model was developed of the form:
<math>\tilde{V}_{out}(\omega)=\frac{\alpha}{a\omega^2+ib\omega+c}\tilde{I}_{in}(\omega)</math>
where the parameters in the gain pre-factor are:
* <math>\alpha = -G/C_F</math>
* <math>a = -(1+C_{HPD}/C_F)</math>
* <math>b = G + 1/(R_F C_F)</math>
* <math>c = G/(R_F C_F)</math>
The subscript 'F' in the above expressions stands for op-amp feedback components. G is the gain-bandwidth product - nominally 1.6 GHz but was adjusted to 21 GHz to produce physical results. (Applying this response to the measured responses of LED pulses with the narrow nominal bandwidth produced negative LED intensities!)
The time domain
Depending on the constants provided, this model produces two poles situated on the positive imaginary axis or along the -ib/2a line. For the physical constant of our circuit, the former set of poles are relevant.
[[Image:S20photocathode_QE.jpg|frame|HPD Photo-cathode efficiency as a function of wavelength.]]
The quantum efficiency of this device is shown in the adjacent figure. The DEP-supplied device provides a gain factor of 2700 at the HV of 12 kV and additionally requires 40 - 80 V bias to collect the electrons across the diode junction. Built into the package is a transimpedance amplifier with 50 kΩ || 1.5 pF feedback, requiring ±5 V supplies. The amplifier circuit also contains a 51.1 Ω resistor in series with the output. Reading out the HPD signal from the amplifier with 50 Ω termination creates a about a factor of two (50 Ω/(50 Ω + 51.1 Ω)) on top of the 50 kΩ current to voltage conversion.
== Frequency Response ==
Based on the circuit described above, a two-pole response model was developed of the form:
<math>\tilde{V}_{out}(\omega)=\frac{\alpha}{a\omega^2+ib\omega+c}\tilde{I}_{in}(\omega)</math>
where the parameters in the gain pre-factor are:
* <math>\alpha = -G/C_F</math>
* <math>a = -(1+C_{HPD}/C_F)</math>
* <math>b = G + 1/(R_F C_F)</math>
* <math>c = G/(R_F C_F)</math>
The subscript 'F' in the above expressions stands for op-amp feedback components. G is the gain-bandwidth product - nominally 1.6 GHz but was adjusted to 21 GHz to produce physical results. (Applying this response to the measured responses of LED pulses with the narrow nominal bandwidth produced negative LED intensities!)
The time domain
Depending on the constants provided, this model produces two poles situated on the positive imaginary axis or along the -ib/2a line. For the physical constant of our circuit, the former set of poles are relevant.