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Voltage Amplifiers for Photodetectors
Gain: 10 V/V
Gain: 1000 V/V
Click to Enlarge
The voltage offset adjuster and polarity switch are located on the output BNC connector end.
Thorlabs' AMP200 Series of Voltage Amplifiers are designed to amplify the output signal from low-output-voltage devices such as thermopile power detectors, photoconductive detectors, or pyroelectric energy detectors. The offset caused by the dark current of a connected photodetector can be compensated for using the Zero Adjust screw on the output end of the amplifier (see image to the right). A switch allows the output signal's sign to be set based on the connected photodetector's polarity (Invert: 1/On or 0/Off).
Each voltage amplifier has an in-line box design with two female BNC connectors, and is intended to be used between two BNC cables. These amplifiers are powered through a Micro-B USB port using the included 5 V, 2 A power supply or any other available USB port. An LED next to the USB connector indicates active power supply. The internal electronics of the amplifier regulate the power to the amplification circuitry, isolating the device's performance from electrical noise that may be inherent to the power source.
The graphs below represent calculated data using an input source with 50 Ω impedance and 100 pF capacitance.
Input Voltage Connector
1 MΩ Input Impedance
Output Voltage Connector
50 Ω Recommended Termination
Power Supply Connector
Theory of Operation
A junction photodiode is an intrinsic device that behaves similarly to an ordinary signal diode, but it generates a photocurrent when light is absorbed in the depleted region of the junction semiconductor. A photodiode is a fast, highly linear device that exhibits high quantum efficiency based upon the application and may be used in a variety of different applications.
It is necessary to be able to correctly determine the level of the output current to expect and the responsivity based upon the incident light. Depicted in Figure 1 is a junction photodiode model with basic discrete components to help visualize the main characteristics and gain a better understanding of the operation of Thorlabs' photodiodes.
Modes of Operation (Photoconductive vs. Photovoltaic)
The dark current present is also affected by the photodiode material and the size of the active area. Silicon devices generally produce low dark current compared to germanium devices which have high dark currents. The table below lists several photodiode materials and their relative dark currents, speeds, sensitivity, and costs.
Bandwidth and Response
Noise Equivalent Power
Here, S/N is the Signal to Noise Ratio, Δf is the Noise Bandwidth, and Incident Energy has units of W/cm2. For more information on NEP, please see Thorlabs' Noise Equivalent Power White Paper.
Depending on the type of the photodiode, load resistance can affect the response speed. For maximum bandwidth, we recommend using a 50 Ω coaxial cable with a 50 Ω terminating resistor at the opposite end of the cable. This will minimize ringing by matching the cable with its characteristic impedance. If bandwidth is not important, you may increase the amount of voltage for a given light level by increasing RLOAD. In an unmatched termination, the length of the coaxial cable can have a profound impact on the response, so it is recommended to keep the cable as short as possible.
Common Operating Circuits
The DET series detectors are modeled with the circuit depicted above. The detector is reverse biased to produce a linear response to the applied input light. The amount of photocurrent generated is based upon the incident light and wavelength and can be viewed on an oscilloscope by attaching a load resistance on the output. The function of the RC filter is to filter any high-frequency noise from the input supply that may contribute to a noisy output.
One can also use a photodetector with an amplifier for the purpose of achieving high gain. The user can choose whether to operate in Photovoltaic of Photoconductive modes. There are a few benefits of choosing this active circuit:
where GBP is the amplifier gain bandwidth product and CD is the sum of the junction capacitance and amplifier capacitance.
Effects of Chopping Frequency
The photoconductor signal will remain constant up to the time constant response limit. Many detectors, including PbS, PbSe, HgCdTe (MCT), and InAsSb, have a typical 1/f noise spectrum (i.e., the noise decreases as chopping frequency increases), which has a profound impact on the time constant at lower frequencies.
The detector will exhibit lower responsivity at lower chopping frequencies. Frequency response and detectivity are maximized for
Aluminum Clamps, Post Mountable
Each clamp has a #8 (M4) counterbore on the bottom, allowing it to be mounted on a Ø1/2" post or any surface with an 8-32 (M4) tap. The clamp must be mounted via the counterbore before the device is attached, as the counterbore will not be accessible once the housing is secured in the clamp.
Plastic Clamp, Double Sided