Photodiode Sensitivity / TIA Calculator
Calculate photodiode responsivity and transimpedance amplifier gain/bandwidth. Maps optical power to output voltage with shot-noise-limited sensitivity.
How to Use
- Enter photodiode responsivity (A/W, from datasheet) and optical input power.
- Enter TIA feedback resistor Rf and junction capacitance Cj.
- Tool computes output voltage, bandwidth, and shot-noise floor.
Show Work
Formulas
History of the Transimpedance Amplifier
The transimpedance amplifier (TIA) converting photodiode current to voltage became standard in the 1970s with the availability of FET-input op-amps (LF356, 1976; TL071, 1978) and low-capacitance PIN photodiodes for optical communications. Fiber-optic receivers in the 1980s pushed TIA designs to higher bandwidths (100 MHz-10 GHz) using GaAs FETs. Modern lidar, time-of-flight cameras, and FSO systems use avalanche photodiodes with internal gain plus TIA front-end to achieve single-photon sensitivity.
About This Calculator
Enter photodiode responsivity (datasheet A/W), incident optical power (typical for your wavelength source), TIA feedback resistor Rf (typical 1 kΩ to 1 GΩ depending on BW/sensitivity), photodiode junction capacitance Cj, and op-amp GBW. The tool computes photocurrent, output voltage, bandwidth, and shot-noise current floor.
For fast optical links (> 10 MHz), use low-Cj photodiode (1-2 pF) and high-GBW op-amp (100 MHz+). For high sensitivity (NEP < 10⁻¹⁴ W/√Hz), use large Rf (1-100 MΩ) and low-noise FET-input op-amp. Everything runs client-side.
Frequently Asked Questions
Responsivity?
A/W — amps of current per watt of incident optical power at a specific wavelength. Silicon: 0.4-0.6 A/W at 850 nm. InGaAs: 0.8-1.0 A/W at 1550 nm. Quantum efficiency η = R · hc/(qλ).
TIA topology?
Transimpedance amplifier: photodiode anode to op-amp inverting input, Rf in feedback. V_out = I_ph · Rf. Bandwidth limited by Cj and op-amp GBW: BW ≈ √(GBW/(2π·Rf·Cj)).
Noise sources?
Shot noise from photocurrent: √(2·q·I_ph) A/√Hz. Resistor thermal noise: √(4kT/Rf) A/√Hz. Op-amp input noise multiplied by (1 + Rf/Rs). Shot noise dominates at high light levels; op-amp noise at low.
Common Use Cases
Optical Fiber Receiver
InGaAs PD + TIA at 1550 nm for fiber-optic data links. 10 Gbit/s achievable with fast PDs.
Laser Pulse Detector
Si PD + TIA for rangefinder: detect 10 ns optical pulses with shot-noise-limited sensitivity.
Spectrophotometer
Large-area Si PD for lab instrument; low-BW TIA for high-sensitivity absorbance measurement.
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