Diode Small-Signal Resistance Calculator

Enter the DC bias current to get the diode's small-signal (dynamic) resistance and the thermal voltage at that temperature.

Calculator

K
Small-signal resistance rd
25.85 Ω
Thermal voltage VT
25.85 mV

Type numbers like 4.7, 0.002 or 2e-3. Results are rounded to 4 significant figures.

The formulas

rd=nVTIDr_d = \dfrac{n V_T}{I_D}

rd in ohms, ID the DC bias current in amperes, n the ideality factor (1 for an ideal diode, up to about 2).

VT=kTqV_T = \dfrac{kT}{q}

k = 1.380649 × 10⁻²³ J/K, q = 1.602176634 × 10⁻¹⁹ C and T in kelvin, so VT = 25.85 mV at 300 K.

  • It holds only for small signals around the bias point (a few millivolts) and in forward bias.
  • Many textbooks round VT to 26 mV or 25 mV. Use the value your question gives.

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