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Transimpedance Amplifiers for Photodiodes![]()
AMP100 Switchable Gain: 1, 10, or 100 MV/A AMP140 Fixed Gain: 10 kV/A AMP130 Fixed Gain: 1 kV/A ![]() Please Wait ![]() Click to Enlarge The bias adjuster and polarity switch are located on the output BNC connector end. (Item # AMP140 Only) ![]() Click to Enlarge The voltage offset adjuster and polarity switch are located on the output BNC connector end. (All Amplifiers Except AMP140) Applications
Features
Thorlabs' AMP Series of Transimpedance Amplifiers are designed to amplify the output signal from unmounted or mounted photodiodes. Refer to the table below or the Specs tab for amplifier specifications. A switch on the output end of the amplifier (see photos to the right) allows the output signal's sign to be set based on the connected photodiode's polarity (AG or CG). For all models except the AMP140 amplifier, the offset caused by the dark current of a connected photodiode can be compensated for using the Zero Adjust screw (see first photo to the right). For the AMP140 amplifier (second photo to the right), this screw instead adjusts the DC bias voltage applied to the photodiode from 1.5 V up to 15 V. Each transimpedance amplifier has an in-line box design with two female BNC connectors, and is intended to be used between two BNC cables. SMA-mounted photodiodes can be connected directly using a CA2824 cable. The devices are powered through a Micro-B USB port using the included 5 V, 2 A power supply or any other available USB port. 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.
![]() Click to Enlarge Click Here for Raw Data The graph above represents calculated data using an input source with capacitance up to 10 nF. ![]() Click to Enlarge Click Here for Raw Data The graph above represents measured data using an input source with capacitance of 100 pF. Please always keep output cables as short as possible for they will induce a high capacitance. The blue curve shows the frequency response when connected to an SM05PD5A photodiode using only an SMA-to-BNC adapter; the red curve shows the results when connected via a 300 mm SMA-to-BNC cable that introduces an additional capacitance of approximately 100 pF. ![]() Click to Enlarge Click Here for Raw Data The graph above represents calculated data using an input source with capacitance up to 10 nF. ![]() Click to Enlarge Click Here for Raw Data The graph above represents calculated data using an input source with capacitance up to 10 nF. ![]() Click to Enlarge Click Here for Raw Data The graph above represents calculated data using an input source with capacitance up to 10 nF. Input Current Connector
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Material | Dark Current | Speed | Spectral Range | Cost |
---|---|---|---|---|
Silicon (Si) | Low | High Speed | Visible to NIR | Low |
Germanium (Ge) | High | Low Speed | NIR | Low |
Gallium Phosphide (GaP) | Low | High Speed | UV to Visible | Moderate |
Indium Gallium Arsenide (InGaAs) | Low | High Speed | NIR | Moderate |
Indium Arsenide Antimonide (InAsSb) | High | Low Speed | NIR to MIR | High |
Extended Range Indium Gallium Arsenide (InGaAs) | High | High Speed | NIR | High |
Mercury Cadmium Telluride (MCT, HgCdTe) | High | Low Speed | NIR to MIR | High |
Junction Capacitance
Junction capacitance (Cj) is an important property of a photodiode as this can have a profound impact on the photodiode's bandwidth and response. It should be noted that larger diode areas encompass a greater junction volume with increased charge capacity. In a reverse bias application, the depletion width of the junction is increased, thus effectively reducing the junction capacitance and increasing the response speed.
Bandwidth and Response
A load resistor will react with the photodetector junction capacitance to limit the bandwidth. For best frequency response, a 50 Ω terminator should be used in conjunction with a 50 Ω coaxial cable. The bandwidth (fBW) and the rise time response (tr) can be approximated using the junction capacitance (Cj) and the load resistance (RLOAD):
Noise Equivalent Power
The noise equivalent power (NEP) is the generated RMS signal voltage generated when the signal to noise ratio is equal to one. This is useful, as the NEP determines the ability of the detector to detect low level light. In general, the NEP increases with the active area of the detector and is given by the following equation:
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.
Terminating Resistance
A load resistance is used to convert the generated photocurrent into a voltage (VOUT) for viewing on an oscilloscope:
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.
Shunt Resistance
Shunt resistance represents the resistance of the zero-biased photodiode junction. An ideal photodiode will have an infinite shunt resistance, but actual values may range from the order of ten Ω to thousands of MΩ and is dependent on the photodiode material. For example, and InGaAs detector has a shunt resistance on the order of 10 MΩ while a Ge detector is in the kΩ range. This can significantly impact the noise current on the photodiode. For most applications, however, the high resistance produces little effect and can be ignored.
Series Resistance
Series resistance is the resistance of the semiconductor material, and this low resistance can generally be ignored. The series resistance arises from the contacts and the wire bonds of the photodiode and is used to mainly determine the linearity of the photodiode under zero bias conditions.
Figure 2: Reverse-Biased Circuit (DET Series Detectors)
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.
Figure 3: Amplified Detector Circuit
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.
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
Scroll down to read about a consideration when setting up lab equipment.
Click here for more insights into lab practices and equipment.
When an instrument offers a choice between AC and DC coupled electrical inputs, it is not unusual for the DC coupling to be the better option for a modulated input signal.
AC and DC Couplings
AC and DC couplings are interfaces between the input signal and the rest of the instrument's circuitry.
A DC coupling, which is called a direct coupling, is essentially a wire connected to the signal input. This conductive coupling transmits all of the signal's frequency components, the DC as well as the AC. The red curve in Figure 1 has a non-zero DC component.
In an AC coupling, the key feature is a capacitor placed in series with the signal input. The capacitor functions as a high-pass filter and is sometimes called a blocking capacitor. AC couplings strongly attenuate the DC and low-frequency signal components. This capacitive coupling is used to remove the DC offset from the input signal, so that only AC components are passed. The blue curve in Figure 1 has only AC frequency components.
Use the DC Coupled Input When Possible
There are many reasons to prefer the DC coupled input. Its low-frequency response is very good, it allows the DC component of the signal to be monitored along with the AC, and it does not cause signal distortion since it does not affect the frequency content of the signal.
Use of the DC coupled input is recommended unless the DC offset is large or the filtering provided by the AC coupled input is required. One problem with a large DC offset is that it can reduce the resolution of the instrument to unacceptably low levels. In extreme cases, DC offsets can cause clipping and saturation effects.
Note that using the DC coupled input does not guarantee a signal free of distortion. Distortion can occur due to other reasons, such as insufficient device bandwidth or impedance mismatch at the termination.
Reasons to Use the AC Coupled Input
By rejecting the signal's DC component, AC coupling can reduce the total amplitude of the signal. This can increase the measurement resolution provided by the instrument, as well as overcome saturation and clipping problems. AC coupling provides good results when information is carried by high frequency signal components and low frequency components are not of interest. AC coupling can also be preferred when the application does not tolerate DC frequency signal components, as is the case for some telecommunications applications.
When Using the AC Coupled Input
If AC coupling is used, it is important to keep in mind that this coupling acts as a high pass filter and affects the frequency content of the signal.
As illustrated by Figure 2, this coupling does not just remove the DC offset, it can also attenuate low frequency components that may be of interest. Due to this, AC coupling can result in signal distortion. To illustrate the effects of high-pass filtering, Figure 3 plots a binary signal, with 200 Hz repetition rate, before and after it is filtered by the high-pass filter with 35 Hz cutoff frequency (Fc).
AC-coupled, digital telecommunications signals mitigate this problem by ensuring the signals are DC balanced, so that they have no DC offset. If the signals were not DC balanced, a series of ones could cause a sustained high signal level. This would introduce a non-zero DC level that would cause the signal to be affected by the capacitive filtering. The result could be bit errors due to high states being incorrectly read as low states.
Date of Last Edit: Dec. 4, 2019
Posted Comments: | |
user
 (posted 2020-05-06 01:04:40.98) What is the input bias current of the amplifier? dpossin
 (posted 2020-05-08 04:28:52.0) Dear Customer,
Thank you for your feedback. Since the AMP102 is an transimpedance amplifier which produces an highly amplified voltage out of an input current, there is no bias specified. What we do specify instead is the maximum input current. The maximum current with respect to the amplifier settings are the following: min amplification: +/- 2mA, mid amplification: +/- 200µA, max amplification: +/- 20µA. The values can also be found in the datasheet of the AMP102. drivers
 (posted 2019-02-15 13:19:12.61) In the AMP120 spec sheet there is a transimpedance amplifier shown in the electrical schematic. I assume the feedback resistance for the TIA in this diagram is 100kV? Additionally, how much RMS drift would you expect for the offset at a stable temperature? I ask because offset adjusts are often susceptible to drift. nreusch
 (posted 2019-02-25 02:11:14.0) This is a response from Nicola at Thorlabs. Thank you for your inquiry. Yes, the resistance is 100 kOhm. The drift is mostly caused by temperature changes. A typical value for the AMP120 is 2.5 µV/°C. cbrideau
 (posted 2018-09-05 13:23:33.243) So the AG/CG switch flips the polarity of the output? (depending on how the photodiode is biased?) swick
 (posted 2018-09-14 03:38:24.0) This is a response from Sebastian at Thorlabs. Thank you for the inquiry.
AG and CG is referred to the Photodiode (PD) which is either Anode- or Cathode grounded. The polarity of the output (AMP) changes when switching from AG to CG and vise versa.
AMP110 (1 kHz) and AMP120 (100 kHz) have low bandwidth so they are actually designed to amplify signals from PDs without bias.
Basically it would work to amplify signals from PDs with bias but the AMP would decrease the bandwidth. |
Representative Photoa |
Item # | Zero Offset |
PD Bias Voltage |
Bandwidth (3 dB) |
Transimpedance Gain |
Max Input Current |
Max Output Voltage |
Max PD Capacitanceb |
Power Supply |
Mass | Dimensions |
---|---|---|---|---|---|---|---|---|---|---|---|
Fixed Gain, Compact | |||||||||||
![]() |
AMP110 | ![]() |
- | 1 kHz | 107 V/A | ±200 nA | ±2 V | 10 nF | USB, 5 V @ 2 A |
80 g | 97 mm x 32 mm x 25.4 mm |
AMP120 | ![]() |
- | 100 kHz | 105 V/A | ±20 µA | ±2 V | 10 nF | ||||
AMP130 | ![]() |
- | 100 kHz | 103 V/A | ±2 mA | ±2 V | 10 nF | ||||
AMP140 | - | ![]() |
10 MHz | 104 V/A | ±350 µA | ±3.5 Vc | 200 pF | 96 mm x 32 mm x 25.4 mm | |||
Switchable Gain, Compact | |||||||||||
![]() |
AMP100 | ![]() |
- | 1 kHz | 106 V/A | ±2 µA | ±2 V | 10 nF | USB, 5 V @ 2 A |
80 g | 97 mm x 32 mm x 25.4 mm |
107 V/A | ±200 nA | ||||||||||
108 V/A | ±20 nA | ||||||||||
AMP102 | ![]() |
- | 100 kHz | 103 V/A | ±2 mA | ±2 V | 10 nF | ||||
104 V/A | ±20 µA | ||||||||||
105 V/A | ±20 µA | ||||||||||
Switchable Gain, Benchtop with 5-Digit LED Display | |||||||||||
![]() |
PDA200C | ![]() |
![]() |
1 kHz | 108 V/A | +100 nA | ±10 Vd | 10 nF | 100 V, 115 V, or 230 V (50/60 Hz) |
<3 kg | 320 mm x 146 mm x 77 mm |
5 kHz | 107 V/A | +1 µA | |||||||||
20 kHz | 106 V/A | +10 µA | |||||||||
70 kHz | 105 V/A | +100 µA | |||||||||
250 kHz | 104 V/A | +1 mA | |||||||||
500 kHz | 103 V/A | +10 mA |
Aluminum Clamps, Post Mountable
These anodized aluminum clamps provide secure mounting for the amplifiers sold above. The clamp can snap onto the side of the device and the flexure lock can be tightened using the 2 mm (5/64") hex locking screw on the side. The ECM100 fits onto the 1.00" side, while the ECM125 fits onto the 1.25" side.
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
The EPS125 clamp is designed to connect two rectangular housings for compact setups; the clamp attaches to the 1.25" side of each device.
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