Photodiode transimpedance
Photodiode transimpedance with circuit equations and operating-limit checks.
Development preview. Check the assumptions below before relying on a result. Calculations run in your browser.
How it calculates
Single-pole amplifier transimpedance: H(s) = −Rf/[1+s(Rf Cf+1/ωt)+s² Rf(Cin+Cf)/ωt]
Q: Q = √[Rf(Cin+Cf)/ωt]/(Rf Cf+1/ωt)
Noise: Integrated en²|NGclosed|² + (in²+2q|Iphoto+Idark|+4kT/Rf)|Zt|²
Assumptions
- Photocurrent and dark current flow into the summing node; amplifier bias current flows out of the node into its input. Op-amp model uses A(s)=ωt/s, without extra poles. Capacitance design targets Q and reports the corresponding model phase margin; it does not imply the same phase margin for a real amplifier. Noise integrates from 1 Hz, with no flicker noise.
Worked example
These are the calculator's default inputs. Change any input above to recalculate.
Inputs
- Feedback selection
- design
- Photocurrent into summing node
- 0.000001 A
- Feedback resistor
- 1000000 Ω
- Total photodiode and input capacitance
- 2e-11 F
- Chosen feedback capacitor
- 1e-12 F
- Target second-order Q
- 0.707107
- Amplifier GBW
- 10000000 Hz
- Dark current into node
- 1e-9 A
- Input bias current into amplifier
- 1e-12 A
- White voltage noise
- 5e-9 V/√Hz
- White current noise
- 1e-12 A/√Hz
- Noise integration upper limit
- 1000000 Hz
- Temperature
- 25 °C
Results
- Feedback capacitor
- 7.97726e-13 F
- Photocurrent output voltage
- -1 V
- Model −3 dB bandwidth
- 276632 Hz
- Model Q
- 0.707107
- Dark/bias DC contribution
- -0.000999 V
- Integrated output noise
- 0.000644993 V RMS
- Single-pole loop phase margin
- 65.828 deg