Calculator Inputs
Example Data Table
| Scenario | Rs | RL | Target or Existing | Signal Frequency | Estimated Result |
|---|---|---|---|---|---|
| Audio preamp stage design | 600 Ω | 10 kΩ | 20 Hz target cutoff | 1 kHz | About 1.126 µF required with 1.5 safety factor |
| Sensor interface analysis | 1 kΩ | 47 kΩ | 0.47 µF installed | 100 Hz | About 7.05 Hz cutoff and strong passband margin |
| RF control path check | 50 Ω | 1 kΩ | 100 nF installed | 10 kHz | Very low cutoff with negligible signal loss |
Formula Used
The normalized gain shows only the capacitor’s high-pass behavior. The absolute transfer magnitude also includes source-to-load resistive division, which is useful when estimating actual output amplitude.
How to Use This Calculator
- Select Design mode to size a capacitor from a target cutoff, or choose Analyze mode to test an existing part.
- Enter the source resistance and the next stage input or load resistance with correct units.
- Provide either the desired lower cutoff frequency or the installed capacitor value.
- Enter the operating signal frequency and input amplitude to estimate reactance, attenuation, phase, and output amplitude.
- Use tolerance and safety factor to create more realistic design margins before downloading CSV or PDF results.
FAQs
1. What does a coupling capacitor do?
It passes AC signals between stages while blocking DC bias. This helps one stage feed another without disturbing each stage’s operating point.
2. Why do source and load resistances matter?
They set the effective resistance seen by the capacitor. That resistance directly changes the cutoff frequency and the amount of low-frequency attenuation.
3. Why use a safety factor in design mode?
A safety factor increases the capacitor value above the mathematical minimum. It helps maintain a lower practical cutoff when tolerances, aging, or part availability shift the final value.
4. What is normalized gain?
Normalized gain shows only the high-pass effect of the coupling network. It ignores the normal midband divider loss caused by source and load resistance.
5. Why is the absolute gain lower than one?
The source resistance and load resistance form a divider. Even at high frequency, some input voltage is dropped across the source resistance, so the output may stay below the input.
6. How much higher should signal frequency be than cutoff?
A common design choice is at least three to ten times higher. Larger ratios reduce amplitude loss and phase shift, especially in audio and instrumentation stages.
7. How does capacitor tolerance affect performance?
If capacitance drops below its nominal value, the cutoff frequency rises. That can remove more low-frequency content than expected, so tolerance should be checked during design.
8. Can this tool help with amplifier stage design?
Yes. It is useful for transistor, op-amp, audio, sensor, and general analog interfaces where AC coupling and low-frequency response must be controlled.