Calculator
Formula Used
The calculator uses the diode thermal small signal model.
- Thermal voltage: VT = kT / q
- Small signal resistance: rd = nVT / ID
- Diode bank resistance: rbank = rd × series diodes / parallel paths
- Voltage attenuation: dB = 20 log10(Vout / Vin)
- Insertion loss: loss = −attenuation dB
Here, k is Boltzmann constant, q is electron charge, T is absolute temperature, n is ideality factor, and ID is forward diode current.
How to Use This Calculator
- Select the attenuator topology.
- Enter the forward DC bias current through the diode bank.
- Choose the current unit.
- Enter junction temperature and diode ideality factor.
- Add diode count, fixed resistors, source impedance, and load impedance.
- Enter the input signal level in Vpp.
- Press Calculate to show the result above the form.
- Use CSV or PDF export for records.
Example Data Table
| Example | Bias current | Temperature | Ideality factor | Diode bank | Approximate rbank |
|---|---|---|---|---|---|
| RF pi attenuator | 1 mA | 25 °C | 1.8 | 1 series, 1 parallel | 46.25 Ω |
| Strong bias path | 10 mA | 25 °C | 1.8 | 1 series, 1 parallel | 4.62 Ω |
| Light bias path | 100 µA | 25 °C | 2.0 | 1 series, 1 parallel | 513.85 Ω |
| Parallel diode bank | 2 mA | 30 °C | 1.7 | 1 series, 2 parallel | 11.10 Ω |
Diode Attenuator Small Signal Resistance Guide
Why Small Signal Resistance Matters
Diode attenuator small signal analysis helps designers turn a bias current into an RF or audio control resistance. The diode is not treated as a simple switch. It is treated as a biased junction. Around that operating point, a small AC signal sees a dynamic resistance. That value changes when bias current, temperature, or ideality factor changes.
How the Model Works
This calculator starts with the thermal voltage. Thermal voltage rises with absolute temperature. It then multiplies that value by the diode ideality factor. Finally, it divides by the forward bias current. A higher current gives a lower small signal resistance. A lower current gives a higher resistance. This is the main control action in many diode attenuators.
Attenuator Circuit Context
Circuit context also matters. A single diode may sit in series with the signal path. It may shunt part of the signal to ground. Several diodes may also be arranged as a pi attenuator. The tool includes these common approximations. It combines series diode count and parallel diode count before calculating the network response.
Design Limits
Use the result as a design estimate. Real circuits include package capacitance, reverse recovery, carrier lifetime, layout parasitics, and transformer effects. High frequency work may need a measured S parameter model. Still, the small signal resistance gives a fast starting point. It is useful for bias planning and quick attenuation checks.
Impedance Effects
The source and load impedance fields estimate voltage ratio. A matched RF system may use fifty ohms. Audio circuits may use higher values. When the diode resistance approaches those impedances, attenuation changes strongly. The output voltage and insertion loss fields show that effect in a direct way.
Getting Better Results
Good results need realistic entries. Use the DC current through each active diode bank. Use the expected operating temperature. Use an ideality factor near one for ideal behavior, or near two for many practical junction conditions. Add fixed resistors when your attenuator contains padding or protection resistance.
Exporting Results
This page also exports CSV and PDF summaries. Those files help document bias sweeps, design reviews, and lab notes. Compare several currents. Then check the predicted attenuation against bench measurements. Keep each exported record with schematic revision, supply voltage, and diode part number. This makes later troubleshooting easier and keeps assumptions visible during prototype testing sessions too.
FAQs
What is diode small signal resistance?
It is the dynamic resistance seen by a small AC signal around a DC bias point. It is different from static DC resistance.
Which current should I enter?
Enter the forward DC current through the active diode or diode bank. Use the current at the expected operating point.
What ideality factor should I use?
Use one for an ideal diode estimate. Use values near two when recombination effects or practical junction behavior are expected.
Does this replace an RF diode model?
No. It gives a fast approximation. RF designs may also need capacitance, package parasitics, and measured S parameter data.
Why does resistance fall when current rises?
The formula divides thermal voltage by forward current. More current lowers the dynamic resistance around the bias point.
Can I use this for audio attenuators?
Yes. The same small signal idea applies. Use realistic source and load impedances for the audio circuit.
What does the pi attenuator option assume?
It assumes equal diode banks in both shunt arms and one diode bank in the series arm, with optional fixed resistors.
Why are CSV and PDF exports useful?
They make it easier to record bias conditions, compare design cases, and share calculation results during review.