Photodetector with wavelength @ 780nm and 1560nm: Difference between revisions

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Photodetector with wavelength @ 780nm and 1560nm

Team members: Sunke Lan

To design photodetector as power monitor with power within 10mW.

Project Outline

1. Objective

Design and evaluate photodetectors for 780 nm and 1560 nm applications.

2. Components

Photodiodes (S5971, G12180-010A)
BNC and PCB test boards
Operational amplifier (OP27G) for transimpedance amplification

---

3. 780 nm Photodetector Design

3.1 Circuit and Components Overview

The 780 nm photodetector circuit is based on a standard transimpedance amplifier (TIA) configuration, designed to convert the photocurrent generated by a silicon PIN photodiode (S5971) into a measurable voltage signal.

In this design:

The photodiode is operated under a reverse bias of 1 V, applied externally to widen the depletion region, reduce junction capacitance, and improve both linearity and response speed. Compared to higher bias voltages (e.g., 15 V), a 1 V bias offers a balance between depletion width and minimized dark current, suitable for steady-state detection at 780 nm.
The photocurrent generated by incident photons is injected into the inverting input of the OP27G operational amplifier. The amplifier maintains a virtual ground condition at the inverting input, ensuring linear current-to-voltage conversion without significant voltage swing at the photodiode terminal.
A feedback resistor $R_{f} = 10 k Ω$ connects the output to the inverting input, governing the output voltage according to:

$V_{out} = - I_{photo} \times R_{f}$ where $I_{photo}$ is the photocurrent proportional to the incident optical power.

The OP27G is selected for its excellent low input offset voltage (typically 25 µV), low noise (3 nV/√Hz at 1 kHz), and moderate gain-bandwidth product (8 MHz), making it ideal for precise low-frequency optical measurements.

To ensure stable operation:

Proper bypass capacitors (0.1 µF ceramic + 10 µF electrolytic) are placed close to the amplifier’s supply pins to suppress high-frequency noise.
Decoupling capacitors are connected across the photodiode bias supply to stabilize the reverse bias voltage.

This current-to-voltage architecture enables reliable detection of 780 nm light with sufficient sensitivity, stability, and low noise performance for laboratory testing.

3.2 Theory and Circuit Design Concept

The underlying detection principle is the direct conversion of the photogenerated current into a voltage signal using a transimpedance amplifier. By operating the photodiode under reverse bias and maintaining a virtual ground at the amplifier’s inverting input, the circuit ensures linear and efficient current-to-voltage conversion without distorting the photocurrent signal.

The output voltage follows:

$V_{out} = - I_{photo} \times R_{f}$

where $I_{photo}$ is the photocurrent and $R_{f}$ is the feedback resistance.

Careful selection of bias conditions, feedback network, and amplifier characteristics optimizes the signal-to-noise ratio and ensures stability within the low-frequency operating range appropriate for 780 nm light detection.

3.3 Feedback Resistor Selection

The feedback resistor $R_{f}$ determines the transimpedance gain of the photodetector, i.e., the voltage generated per unit photocurrent. A value of $R_{f} = 10 k Ω$ was chosen to balance signal amplitude and system stability.

Key considerations include:

**Signal strength:** Typical photocurrents under 780 nm illumination are in the nanoampere to microampere range. With a 10 kΩ feedback resistor, output voltages reach measurable levels without requiring excessive amplification.
**Noise performance:** Although increasing $R_{f}$ amplifies both signal and noise, a 10 kΩ resistor provides a favorable trade-off, maintaining a reasonable signal-to-noise ratio (SNR).
**Frequency stability:** Larger $R_{f}$ values, combined with the photodiode’s junction capacitance, can reduce bandwidth and induce phase shifts. A 10 kΩ resistor ensures sufficient stability across the targeted low-frequency range.

Thus, the 10 kΩ feedback resistor enables clean, stable, and easily detectable voltage outputs under typical laboratory conditions.

3.4 Testing and Results

The 780 nm photodetector circuit was tested under indoor lighting conditions to assess its performance. A Tektronix TDS 2024C digital oscilloscope was used to monitor the output under various amplifier supply voltages, capturing the 50 Hz AC mains modulation.

Test results are summarized as follows:

Without amplification, the photodiode output is too weak to be detected by the oscilloscope.

With a ±1 V supply, a small output fluctuation appears, indicating initial amplification.

With a ±3 V supply, a clear quasi-sinusoidal waveform corresponding to 50 Hz ambient lighting is observed, indicating effective amplification.

At ±5 V, significant distortion and oscillations occur due to instability caused by the combination of weak input signals, high open-loop gain, and parasitic effects from the breadboard assembly.

In summary:

Without amplification, the photodiode signal remains undetectable.
A ±3 V supply allows for stable and effective signal amplification.
Excessive supply voltages (> ±5 V) without proper PCB layout introduce instability.
The detector demonstrates clear sensitivity to low-frequency ambient light variations.

3.5 Discussion

While the breadboard-based photodetector successfully detects low-frequency optical signals, several limitations exist. Parasitic capacitances and imperfect grounding inherent to the breadboard setup contribute to instability at higher supply voltages. Furthermore, the lack of a fully optimized feedback compensation network restricts bandwidth and dynamic range.

Future improvements include:

Transitioning to a PCB design to minimize parasitic effects.
Implementing better shielding against environmental noise.
Optimizing feedback networks to achieve broader bandwidth and enhanced stability.

These enhancements would enable more precise and higher-speed optical measurements beyond the low-frequency regime demonstrated here.

---

4. G12180 Photodetector Design

4.1 Circuit and Components Overview

To improve stability and reduce parasitic effects, a dedicated PCB was fabricated for the G12180-010A photodiode. The layout was designed to minimize parasitic capacitance and inductance by shortening trace lengths and optimizing grounding.

The circuit adopts a TIA configuration featuring:

A 10 kΩ feedback resistor for gain control.
Bypass capacitors (0.1 µF and 10 µF) near the amplifier supply pins for noise suppression.
Decoupling capacitors across the photodiode bias supply for voltage stabilization.

The Hamamatsu G12180-010A photodiode is optimized for near-infrared and visible light detection, offering high speed and low noise characteristics.

Mechanical mounting uses a four-screw configuration for optical alignment and stability, with a BNC connector for easy signal extraction.

This setup offers significantly improved stability and sensitivity compared to the breadboard-based design.

4.2 Theory and Circuit Design Concept

The detection principle remains the direct conversion of photocurrent into voltage via a transimpedance amplifier. The PCB design significantly reduces parasitic effects, improves system bandwidth, and enhances immunity to electromagnetic interference (EMI).

The output voltage relation is:

$V_{out} = - I_{photo} \times R_{f}$

where $I_{photo}$ is the photocurrent and $R_{f} = 10 k Ω$ is the feedback resistor.

The PCB-based structure enables cleaner and more accurate optical signal measurements over a broader frequency range.

4.3 Testing and Results

The PCB-based G12180 photodetector was tested under indoor lighting conditions, with output monitored using a Tektronix TDS 2024C oscilloscope.

Output waveform of PCB-based G12180 photodetector under ambient light

As shown, the output signal is highly stable with minimal low-frequency noise. No significant oscillations or distortions were observed, marking a substantial improvement over the breadboard setup.

The enhanced performance is attributed to:

Reduced parasitic capacitance and inductance.
Effective bypassing and decoupling.
Improved grounding and signal integrity through the PCB design.

Overall, the PCB implementation enables clean, low-noise photodetection suitable for both DC and low-frequency optical measurements.

@@ Line 8: / Line 8: @@
 === 1. Objective ===
-*  Try to design photodetector for 780nm and 1560nm
+* Design and evaluate photodetectors for 780 nm and 1560 nm applications.
-*
 === 2. Components ===
-*  Photodiodes (S5917, G12180-010A), BNC test boards, PCB test boards, op-amp (OP27G) (for target 1)
+* Photodiodes (S5971, G12180-010A)
+* BNC and PCB test boards
+* Operational amplifier (OP27G) for transimpedance amplification
+---
 === 3. 780 nm Photodetector Design ===
@@ Line 22: / Line 23: @@
 [[File:testcircuit.png|center|thumb|400px|Test Circuit for 780 nm Photodetector]]
-The photodetector circuit is based on a standard transimpedance amplifier (TIA) configuration, designed to convert the photocurrent generated by a silicon PIN photodiode (S5971) into a measurable voltage output.
+The 780 nm photodetector circuit is based on a standard transimpedance amplifier (TIA) configuration, designed to convert the photocurrent generated by a silicon PIN photodiode (S5971) into a measurable voltage signal.
 In this design:
-* The photodiode is operated under a reverse bias of 1 V, supplied externally, to expand the depletion region, reduce junction capacitance, and enhance linearity and response speed. Compared to higher bias voltages (e.g., 15 V), a 1 V bias provides a trade-off between sufficient depletion width and minimized dark current, which is suitable for steady-state light detection around 780 nm.
+* The photodiode is operated under a reverse bias of 1 V, applied externally to widen the depletion region, reduce junction capacitance, and improve both linearity and response speed. Compared to higher bias voltages (e.g., 15 V), a 1 V bias offers a balance between depletion width and minimized dark current, suitable for steady-state detection at 780 nm.
-* The photocurrent generated by incident photons is injected into the inverting input of the operational amplifier (OP27G). The amplifier maintains a virtual ground condition at the inverting input, ensuring linear current-to-voltage conversion without significant voltage swing at the photodiode terminals.
+* The photocurrent generated by incident photons is injected into the inverting input of the OP27G operational amplifier. The amplifier maintains a virtual ground condition at the inverting input, ensuring linear current-to-voltage conversion without significant voltage swing at the photodiode terminal.
-* A feedback resistor <math>R_f = 10\,\mathrm{k}\Omega</math> is connected between the output and the inverting input. The output voltage is governed by:
+* A feedback resistor <math>R_f = 10\,\mathrm{k}\Omega</math> connects the output to the inverting input, governing the output voltage according to:
 <math>V_{\text{out}} = -I_{\text{photo}} \times R_f</math>
 where <math>I_{\text{photo}}</math> is the photocurrent proportional to the incident optical power.
-The OP27G was selected for its excellent low input offset voltage (typically 25 µV), low input bias current, and low noise characteristics (3 nV/√Hz at 1 kHz), making it ideal for precise low-frequency optical measurements. Its moderate gain-bandwidth product (8 MHz) provides sufficient speed while maintaining high stability under typical experimental conditions.
+The OP27G is selected for its excellent low input offset voltage (typically 25 µV), low noise (3 nV/√Hz at 1 kHz), and moderate gain-bandwidth product (8 MHz), making it ideal for precise low-frequency optical measurements.
 To ensure stable operation:
-* Proper bypass capacitors (0.1 µF ceramic + 10 µF electrolytic) are placed close to the amplifier’s power supply pins to suppress high-frequency noise and prevent self-oscillation.
+* Proper bypass capacitors (0.1 µF ceramic + 10 µF electrolytic) are placed close to the amplifier’s supply pins to suppress high-frequency noise.
-* Decoupling capacitors are connected across the 1 V photodiode bias supply to maintain a clean and stable reverse bias, minimizing potential noise coupling.
+* Decoupling capacitors are connected across the photodiode bias supply to stabilize the reverse bias voltage.
 {| style="width:100%;"
@@ Line 42: / Line 43: @@
 |}
-This current-to-voltage conversion circuit architecture enables reliable detection of light intensities at 780 nm, providing sufficient sensitivity, stability, and low noise performance for laboratory testing and calibration purposes.
+This current-to-voltage architecture enables reliable detection of 780 nm light with sufficient sensitivity, stability, and low noise performance for laboratory testing.
 ----
@@ Line 48: / Line 49: @@
 ==== 3.2 Theory and Circuit Design Concept ====
-The underlying principle of this photodetector is based on the direct conversion of the photogenerated current into a voltage signal using a transimpedance amplifier structure. By operating the photodiode under reverse bias and maintaining a virtual ground at the amplifier’s inverting input, the circuit ensures linear and efficient current-to-voltage conversion without distorting the photocurrent signal.
+The underlying detection principle is the direct conversion of the photogenerated current into a voltage signal using a transimpedance amplifier. By operating the photodiode under reverse bias and maintaining a virtual ground at the amplifier’s inverting input, the circuit ensures linear and efficient current-to-voltage conversion without distorting the photocurrent signal.
-The output voltage relationship is given by:
+The output voltage follows:
 <math>V_{\text{out}} = -I_{\text{photo}} \times R_f</math>
-where <math>I_{\text{photo}}</math> is the photocurrent proportional to the incident optical power, and <math>R_f</math> is the feedback resistor.
+where <math>I_{\text{photo}}</math> is the photocurrent and <math>R_f</math> is the feedback resistance.
-Through careful selection of bias voltage, feedback network, and amplifier characteristics, the circuit optimizes signal-to-noise ratio while maintaining stability across the low-frequency operating range suited for 780 nm light detection.
+Careful selection of bias conditions, feedback network, and amplifier characteristics optimizes the signal-to-noise ratio and ensures stability within the low-frequency operating range appropriate for 780 nm light detection.
 ----
@@ Line 62: / Line 63: @@
 ==== 3.3 Feedback Resistor Selection ====
-The feedback resistor <math>R_f</math> determines the transimpedance gain of the photodetector, i.e., how much voltage is generated per unit of photocurrent. The choice of <math>R_f = 10\,\mathrm{k}\Omega</math> in this design is a compromise between achieving a sufficient output signal amplitude and maintaining stability against high-frequency oscillations.
+The feedback resistor <math>R_f</math> determines the transimpedance gain of the photodetector, i.e., the voltage generated per unit photocurrent. A value of <math>R_f = 10\,\mathrm{k}\Omega</math> was chosen to balance signal amplitude and system stability.
 Key considerations include:
-* **Signal strength:** A typical photocurrent generated under moderate illumination at 780 nm falls in the range of nanoamperes to microamperes. With a 10 kΩ feedback resistor, the corresponding output voltage becomes easily measurable in the millivolt to volt range without requiring excessive amplification stages.
+* **Signal strength:** Typical photocurrents under 780 nm illumination are in the nanoampere to microampere range. With a 10 kΩ feedback resistor, output voltages reach measurable levels without requiring excessive amplification.
-* **Noise performance:** Increasing <math>R_f</math> enhances the signal amplitude but also amplifies thermal (Johnson) noise. A 10 kΩ resistor keeps the noise contribution low while providing an adequate signal-to-noise ratio (SNR) for steady-state measurements.
+* **Noise performance:** Although increasing <math>R_f</math> amplifies both signal and noise, a 10 kΩ resistor provides a favorable trade-off, maintaining a reasonable signal-to-noise ratio (SNR).
-* **Frequency stability:** Larger <math>R_f</math> values combined with the photodiode’s junction capacitance can reduce the system bandwidth and introduce phase shifts leading to potential oscillations. A 10 kΩ feedback resistor maintains a higher stability margin while covering the low-frequency range of interest in this application.
+* **Frequency stability:** Larger <math>R_f</math> values, combined with the photodiode’s junction capacitance, can reduce bandwidth and induce phase shifts. A 10 kΩ resistor ensures sufficient stability across the targeted low-frequency range.
-Overall, a 10 kΩ feedback resistor ensures that the photodetector system delivers clean, stable, and easily detectable voltage outputs under typical laboratory light levels at 780 nm.
+Thus, the 10 kΩ feedback resistor enables clean, stable, and easily detectable voltage outputs under typical laboratory conditions.
 ----
@@ Line 75: / Line 76: @@
 ==== 3.4 Testing and Results ====
+The 780 nm photodetector circuit was tested under indoor lighting conditions to assess its performance. A Tektronix TDS 2024C digital oscilloscope was used to monitor the output under various amplifier supply voltages, capturing the 50 Hz AC mains modulation.
-The photodetector circuit was tested under indoor illumination (standard room lighting driven by AC mains) to evaluate its sensitivity and stability. A Tektronix TDS 2024C digital oscilloscope was used to monitor the output signal under different amplifier supply voltages. The observed signals primarily reflect the 50 Hz modulation from the AC power source.
+Test results are summarized as follows:
-Test results under different conditions are as follows:
 {| style="width:100%;"
@@ Line 84: / Line 84: @@
 |}
-In the first test, the photodiode was connected directly to the oscilloscope without using the operational amplifier. As shown in the figure, no significant signal was detected. This indicates that the photocurrent generated under normal room lighting is too small (typically in the nanoampere range) to be directly observed by the oscilloscope without amplification.
+Without amplification, the photodiode output is too weak to be detected by the oscilloscope.
 {| style="width:100%;"
@@ Line 90: / Line 90: @@
 |}
-In the second test, the OP-AMP (OP27G) was powered with a ±1 V supply. A small but noticeable output fluctuation appeared. This result suggests that even with low supply voltage, the amplifier begins to operate and provides a weak but observable amplification of the photocurrent signal.
+With a ±1 V supply, a small output fluctuation appears, indicating initial amplification.
 {| style="width:100%;"
@@ Line 96: / Line 96: @@
 |}
-When the OP-AMP supply voltage was increased to ±3 V, a clear quasi-sinusoidal output waveform emerged. The output signal corresponds to the 50 Hz ambient lighting modulation from the AC mains. This indicates that the photodetector circuit is capable of effectively capturing low-frequency optical signals when properly biased and powered.
+With a ±3 V supply, a clear quasi-sinusoidal waveform corresponding to 50 Hz ambient lighting is observed, indicating effective amplification.
 {| style="width:100%;"
@@ Line 102: / Line 102: @@
 |}
-At a higher OP-AMP supply voltage of ±5 V, significant output distortion and oscillations were observed. Although the OP27G is designed to operate up to ±15 V, the combination of extremely low input signal (small photocurrent) and high supply voltage led to internal instability, likely due to excessive open-loop gain and insufficient input signal amplitude. Additionally, the use of a solderless breadboard for circuit assembly introduced parasitic capacitances and poor grounding, further exacerbating the instability and promoting self-oscillation at higher supply voltages.
+At ±5 V, significant distortion and oscillations occur due to instability caused by the combination of weak input signals, high open-loop gain, and parasitic effects from the breadboard assembly.
 ----
 In summary:
-* Without OP-AMP amplification, the photodiode signal is too weak to be detected.
+* Without amplification, the photodiode signal remains undetectable.
-* Properly powering the OP-AMP at ±3 V provides clear and stable signal amplification.
+* A ±3 V supply allows for stable and effective signal amplification.
-* Excessively high supply voltages (beyond ±5 V) without appropriate compensation or PCB layout lead to instability and self-oscillation.
+* Excessive supply voltages (> ±5 V) without proper PCB layout introduce instability.
-* The photodetector demonstrates good sensitivity to low-frequency ambient light variations when operated under appropriate conditions.
+* The detector demonstrates clear sensitivity to low-frequency ambient light variations.
+----
 ==== 3.5 Discussion ====
-While the current photodetector setup demonstrates effective detection of low-frequency optical signals under ambient lighting, several limitations remain. The use of a solderless breadboard introduces significant parasitic capacitances and imperfect grounding, which contribute to instability at higher amplifier supply voltages. Moreover, the absence of a fully optimized feedback compensation network limits the bandwidth and dynamic range of the detection system.
+While the breadboard-based photodetector successfully detects low-frequency optical signals, several limitations exist. Parasitic capacitances and imperfect grounding inherent to the breadboard setup contribute to instability at higher supply voltages. Furthermore, the lack of a fully optimized feedback compensation network restricts bandwidth and dynamic range.
+Future improvements include:
+* Transitioning to a PCB design to minimize parasitic effects.
+* Implementing better shielding against environmental noise.
+* Optimizing feedback networks to achieve broader bandwidth and enhanced stability.
+These enhancements would enable more precise and higher-speed optical measurements beyond the low-frequency regime demonstrated here.
-Future improvements could include designing a printed circuit board (PCB) for the photodetector circuit to minimize parasitic effects, implementing better shielding against environmental noise, and optimizing the feedback network to achieve higher stability and broader bandwidth. These refinements would enable more precise, high-speed optical measurements beyond the low-frequency regime demonstrated here.
+---
 === 4. G12180 Photodetector Design ===
@@ Line 125: / Line 134: @@
 [[File:back1.jpg|center|thumb|400px|Back view of G12180 PD PCB board]]
-In order to improve the stability and reduce parasitic effects observed in breadboard-based circuits, a dedicated printed circuit board (PCB) was fabricated for the G12180-010A photodiode module. The PCB layout was designed to minimize parasitic capacitance and inductance by shortening the connection traces and implementing proper grounding strategies.
+To improve stability and reduce parasitic effects, a dedicated PCB was fabricated for the G12180-010A photodiode. The layout was designed to minimize parasitic capacitance and inductance by shortening trace lengths and optimizing grounding.
-The circuit follows a basic transimpedance amplifier (TIA) configuration, using:
+The circuit adopts a TIA configuration featuring:
-* A 10 kΩ feedback resistor to set the transimpedance gain.
+* A 10 kΩ feedback resistor for gain control.
-* Bypass capacitors of 0.1 µF and 10 µF close to the operational amplifier supply pins to suppress high-frequency noise.
+* Bypass capacitors (0.1 µF and 10 µF) near the amplifier supply pins for noise suppression.
-* Additional decoupling capacitors across the photodiode bias supply to ensure a stable operating voltage.
+* Decoupling capacitors across the photodiode bias supply for voltage stabilization.
-The photodiode used is the Hamamatsu G12180-010A, which is optimized for near-infrared and visible light detection, and is particularly suitable for high-speed and low-noise applications.
+The Hamamatsu G12180-010A photodiode is optimized for near-infrared and visible light detection, offering high speed and low noise characteristics.
-Mechanical mounting was achieved using a four-screw configuration for robust optical alignment and mechanical stability. A BNC connector was used for convenient signal extraction to the oscilloscope or subsequent signal processing stages.
+Mechanical mounting uses a four-screw configuration for optical alignment and stability, with a BNC connector for easy signal extraction.
-This setup provides a more reliable platform for sensitive optical measurements compared to the previous breadboard implementation.
+This setup offers significantly improved stability and sensitivity compared to the breadboard-based design.
 ----
@@ Line 142: / Line 151: @@
 ==== 4.2 Theory and Circuit Design Concept ====
-The core detection principle remains the direct conversion of photocurrent into voltage through the use of a transimpedance amplifier. By adopting a PCB design:
+The detection principle remains the direct conversion of photocurrent into voltage via a transimpedance amplifier. The PCB design significantly reduces parasitic effects, improves system bandwidth, and enhances immunity to electromagnetic interference (EMI).
-* The parasitic capacitances and inductances were significantly reduced.
-* The stability and bandwidth of the photodetector system were improved.
-* The influence of environmental electromagnetic interference (EMI) was minimized due to better grounding and shielding options.
-The output voltage of the photodetector follows the relation:
+The output voltage relation is:
 <math>V_{\text{out}} = -I_{\text{photo}} \times R_f</math>
-where <math>I_{\text{photo}}</math> is the photogenerated current and <math>R_f = 10\,\mathrm{k}\Omega</math> is the feedback resistance.
+where <math>I_{\text{photo}}</math> is the photocurrent and <math>R_f = 10\,\mathrm{k}\Omega</math> is the feedback resistor.
+The PCB-based structure enables cleaner and more accurate optical signal measurements over a broader frequency range.
+----
+==== 4.3 Testing and Results ====
+The PCB-based G12180 photodetector was tested under indoor lighting conditions, with output monitored using a Tektronix TDS 2024C oscilloscope.
+{| style="width:100%;"
+| [[File:1560test.jpg|400px|thumb|center|Output waveform of PCB-based G12180 photodetector under ambient light]]
+|}
+As shown, the output signal is highly stable with minimal low-frequency noise. No significant oscillations or distortions were observed, marking a substantial improvement over the breadboard setup.
+The enhanced performance is attributed to:
+* Reduced parasitic capacitance and inductance.
+* Effective bypassing and decoupling.
+* Improved grounding and signal integrity through the PCB design.
-This structure enables a cleaner and more accurate measurement of incident optical signals across a wider frequency range compared to the breadboard setup.
+Overall, the PCB implementation enables clean, low-noise photodetection suitable for both DC and low-frequency optical measurements.

Photodetector with wavelength @ 780nm and 1560nm: Difference between revisions

Latest revision as of 15:18, 27 April 2025

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