Photodetector with wavelength @ 780nm and 1560nm

Photodetector with wavelength @ 780nm and 1560nm

Team members: Sunke Lan

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

Project Outline

1. Objective

• Try to design photodetector for 780nm and 1560nm

2. Components

• Photodiodes (S5917, G12180-010A), BNC test boards, PCB test boards, op-amp (OP27G) (for target 1)

3. 780nm PD

3.1.1 Basic current-voltage transferring circuit for S5971

Test Circuit for 780nm Photodetector

Based on the reference circuit, we changed the biase voltage of photodiode to 1V, and used op-amp OP27E to do the current-voltage transferring.

the side with PD

the side with basic lump elements

3.1.2 Theory and Circuit Design Concept

The photodetector circuit in Figure 3-13 is based on the principle of converting the photocurrent generated by the photodiode into a readable voltage signal using a transimpedance amplifier (TIA) configuration.

In this design:

• The photodiode is operated under reverse bias conditions (1 V applied from an external voltage source) to widen the depletion region, reduce junction capacitance, and improve both linearity and response speed.
• The photocurrent generated by incident light is injected into the inverting input of the operational amplifier (OP27G), which maintains a virtual ground at this node.
• A feedback resistor $R_f=10\mathrm{k}\mathrm{\Omega }$ is connected between the output and the inverting input. The output voltage $V_{\text{out}}=-I_{\text{photo}}\times R_f$ follows the relation:

$V_{\text{out}}=-I_{\text{photo}}\times R_f$

• The OP27G is chosen for its low input offset voltage, low noise, and stable operation under moderate gain-bandwidth requirements, making it suitable for low-frequency, steady-state light detection.
• Proper bypass capacitors are added across the amplifier’s power supply terminals to suppress high-frequency noise and prevent self-oscillation.
• Decoupling capacitors are also placed across the bias voltage supply to maintain a stable reverse bias across the photodiode.

This basic current-to-voltage conversion architecture provides a reliable way to monitor light intensities at 780 nm and 1560 nm, ensuring sufficient sensitivity and stability for laboratory testing purposes.