What is the formula for Diode Dynamic Resistance?

The Diode Dynamic Resistance formula is rd = VT / Id (≈ 26 mV / Id), where rd—Small-signal dynamic resistance (Ω); VT—Thermal voltage (≈26 mV); Id—DC bias current (A).

What does rd mean in the Diode Dynamic Resistance formula?

rd is Small-signal dynamic resistance (Ω).

Diode Dynamic Resistance

Q: How do you calculate Diode Dynamic Resistance?

Use the formula rd = VT / Id (≈ 26 mV / Id). The dynamic (AC) resistance of a forward-biased diode is the slope of its V–I curve at the operating point, equal to the thermal voltage divided by the bias current. Enter the values you know in the calculator above and it solves for the remaining variable.

rd = VT / Id

Calculator

Dynamic Resistance

Thermal Voltage

Diode Current

Result

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Formula

rd = VT / Id (≈ 26 mV / Id)

Description

The dynamic (AC) resistance of a forward-biased diode is the slope of its V–I curve at the operating point, equal to the thermal voltage divided by the bias current. It is what an AC signal "sees" through the diode and governs the behaviour of diode mixers, log amplifiers, temperature sensors, and the emitter junctions of transistors.

Variables

rd — Small-signal dynamic resistance (Ω)
VT — Thermal voltage (≈26 mV)
Id — DC bias current (A)

Practical Notes

At 1 mA, rd ≈ 26 Ω. This is much smaller than the diode’s DC (static) resistance V/I, because the exponential I–V curve is steep at the operating point.

Related Concepts

All Discrete Amplifiers formulas →

Thermal Voltage Small-Signal Emitter Resistance Transistor Transconductance Diode Dynamic Resistance Common-Emitter Voltage Gain BJT Alpha–Beta Relationship Darlington Pair Current Gain Emitter-Follower Input Impedance

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