Calculator Form
Formula Used
The basic transimpedance gain for an inverting current to voltage amplifier is:
Vout = -Iin × Zf
For a feedback resistor and capacitor in parallel:
|Zf| = Rf / sqrt(1 + (2πfRfCf)^2)
The gain in decibel ohms is:
Gain dBΩ = 20 × log10(|Zf|)
The feedback pole is:
fp = 1 / (2πRfCf)
A rough feedback capacitor estimate is:
Cf ≈ sqrt(Cin / (2πRfGBW))
How to Use This Calculator
Enter the sensor signal current first. Choose the matching current unit. Add dark current and input offset current when they matter. Enter the feedback resistor and feedback capacitor. Add the signal frequency. Enter input capacitance and gain bandwidth for a rough stability check. Set the maximum output swing. Press the calculate button. The result appears above the form.
Example Data Table
| Signal Current | Rf | Cf | Frequency | Approx Gain | Approx Output |
|---|---|---|---|---|---|
| 10 µA | 100 kΩ | 10 pF | 1 kHz | 99.998 kV/A | -1.000 V |
| 100 nA | 1 MΩ | 2 pF | 10 kHz | 992.2 kV/A | -0.099 V |
| 5 µA | 20 kΩ | 100 pF | 100 kHz | 12.47 kV/A | -0.062 V |
Transimpedance Gain Planning
A transimpedance amplifier changes input current into output voltage. It is common with photodiodes, ion chambers, current probes, and sensor front ends. The feedback resistor sets the main gain. A larger resistor gives more voltage for the same current. It also raises noise and often lowers bandwidth. The feedback capacitor controls high frequency behavior. It can reduce peaking and make the loop calmer.
Why The Calculation Matters
Small currents are easy to hide inside noise, leakage, and offset. A gain estimate shows whether the signal will be visible before the output clips. It also shows whether the chosen frequency is near the feedback pole. When the signal frequency is above that pole, gain starts to fall. The calculator uses the resistor and capacitor as a first order feedback network. This gives a practical gain value for early design checks.
Design Notes
Choose the feedback resistor from the smallest current you need to measure. Then check the largest current against the allowed output swing. If the output exceeds the limit, reduce the resistor or add range switching. Add the feedback capacitor when diode capacitance and board capacitance make the circuit ring. Too much capacitance slows the response. Too little capacitance may cause overshoot. Keep the inverting node short and clean. Guard rings help when current is very small.
Reading The Result
The transimpedance gain is shown in volts per ampere and in decibel ohms. Output voltage is based on signal current, dark current, and offset current. The sign is negative because the common circuit is inverting. The magnitude is often the main value for sizing. The estimated pole is not a full stability proof. Real stability also depends on op amp gain bandwidth, input capacitance, layout, and package parasitics. Use this result as a first design screen, then verify with simulation and measurement.
Practical Workflow
Start with realistic sensor current. Enter the intended feedback resistor. Add a small capacitor value, even if it is only board parasitic capacitance. Check the gain at your signal frequency. Export the table for design notes. Test several resistor values before choosing a final range. Compare the result with data sheet limits and expected temperature drift before committing to hardware in production boards.
FAQs
What is transimpedance gain?
Transimpedance gain is the output voltage produced for each ampere of input current. Its unit is volts per ampere, which is also equivalent to ohms.
Why is the output voltage negative?
A standard transimpedance amplifier uses the inverting input. Current entering the summing node produces an output with opposite sign. Many design checks use magnitude only.
What does the feedback resistor control?
The feedback resistor sets the main current to voltage gain. Higher resistance gives more voltage output, but it can increase noise and reduce usable bandwidth.
What does the feedback capacitor do?
The feedback capacitor limits high frequency gain. It can improve stability and reduce ringing. A larger value also slows the circuit response.
Is the bandwidth result exact?
No. The feedback pole is a useful first estimate. Real bandwidth depends on the op amp, sensor capacitance, layout, parasitic capacitance, and load conditions.
Should dark current be included?
Include dark current when using photodiodes or leakage sensitive sensors. It shifts the output voltage and can reduce available headroom before clipping occurs.
What is gain in dB ohms?
Gain in dB ohms is a logarithmic form of transimpedance magnitude. It helps compare very small and very large gain values easily.
Why does the calculator show clipping?
The clipping check compares calculated output magnitude with your allowed output swing. If output is too large, reduce gain or select a wider output range.