Calculator
Formula Used
The small signal resistance of a forward biased diode is:
rd = nVT / ID
VT = T / 11604.51812
Req = series count × (rd + Rs) / parallel branches
Here, rd is junction dynamic resistance. n is ideality factor. VT is thermal voltage. ID is diode current in amperes. T is junction temperature in kelvin. Rs is added series resistance.
How to Use This Calculator
Enter the diode forward bias current and select its unit. Add the ideality factor from your data sheet or circuit estimate. Enter junction temperature, not only air temperature, when heat rise matters. Add series resistance if the diode package or external path has measurable ohmic resistance. Use series and parallel fields for diode strings or branches. Press Calculate to show the result above the form. Use CSV or PDF buttons to save the same result.
Example Data Table
| Current | n | Temperature | Series resistance | Estimated rd |
|---|---|---|---|---|
| 1 mA | 2 | 25 C | 0 ohms | 51.385 ohms |
| 10 mA | 1 | 25 C | 0 ohms | 2.569 ohms |
| 100 uA | 2 | 75 C | 1 ohm | 600.003 ohms |
Why Small Signal Resistance Matters
A diode is nonlinear. Its current changes sharply when the bias point moves. Small signal resistance turns that curved behavior into a useful local resistance. Designers use it when a tiny alternating signal rides on a steady forward current. The value is not a fixed part rating. It depends on operating current, temperature, and the ideality factor. A higher current gives a lower resistance. A higher temperature raises thermal voltage, so resistance increases for the same current.
Practical Design Use
The calculator models the diode around one bias point. It first converts current and temperature into base units. Then it finds thermal voltage from absolute temperature. The ideality factor adjusts the slope for real diode behavior. Silicon signal diodes often use values near two at low current and near one at stronger forward current. Schottky and LED parts can differ. Always compare the result with a data sheet when accuracy matters.
Including Series and Parallel Effects
Many circuits contain more than one diode. A clipper may use opposing diodes. A bias string may use several devices in series. Rectifier banks may place branches in parallel. This tool lets you include series count, parallel branches, and added ohmic resistance. Those settings estimate the equivalent resistance seen by the small signal. The result is useful for gain estimates, ripple checks, detector loading, and bias stability work.
Limits of the Model
The formula assumes a forward biased exponential region. It is not valid for zero current, reverse breakdown, or large signal swings. If the signal voltage is too large, the diode curve cannot be treated as a straight line. Use the incremental current result only for small signals. For switching, recovery, capacitance, and noise studies, add separate diode parameters or circuit simulation.
Getting Better Results
Use measured bias current when possible. Enter the real junction temperature, not only room temperature. Include bond, contact, and external series resistance for power devices. For precision work, repeat the calculation at minimum and maximum expected conditions. That shows how the diode resistance may shift during normal operation. Store each run with the export buttons. The saved values help compare prototype measurements against calculated expectations during reviews and later troubleshooting sessions in lab.
FAQs
What is small signal resistance of a diode?
It is the local resistance of a biased diode for tiny voltage changes. It depends on the diode current, temperature, and ideality factor.
Is this the same as DC resistance?
No. DC resistance is voltage divided by current. Small signal resistance is the slope resistance at one operating point.
Why does higher current lower rd?
The diode current curve becomes steeper at higher forward current. A steeper curve means a smaller incremental resistance.
What ideality factor should I use?
Use the value from the diode data sheet when available. For rough estimates, silicon diodes often use values between one and two.
Should I enter junction temperature?
Yes. Junction temperature gives the best result. Ambient temperature can be used only when self heating is small.
What does series resistance do?
Series resistance adds directly to the junction small signal resistance. It can matter in power diodes, wiring, and high current circuits.
Can I use this for reverse bias?
No. This formula is for a forward biased diode in its exponential region. Reverse bias and breakdown need other models.
When is the small signal model inaccurate?
It becomes inaccurate when the AC signal is large. It also misses capacitance, recovery time, noise, and high frequency package effects.