# Explain the equation [closed]

Explain the following equation...

$$r^{\prime}_d = \dfrac{26\mathrm{mV}}{I_D} + r_B$$

Explain from where and how this equation comes out . Also elaborate the terms used in above mentioned equation.

## closed as unclear what you're asking by Matt Young, Leon Heller, Scott Seidman, Alfred Centauri, user17592 Sep 18 '14 at 19:30

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• Explain the question – ACD Sep 18 '14 at 17:33
• Sounds like homework. – Ignacio Vazquez-Abrams Sep 18 '14 at 17:34
• Can you give more context? The equation is like of the dinamic resistance for a polarized P-N junction... right? – Martin Petrei Sep 18 '14 at 17:35
• Homework questions with no attempt at a solution are closed. – Leon Heller Sep 18 '14 at 17:36
• Sir,it;s not homework question.I just read it and don not understand that's why I asked. – Ali Awan Sep 18 '14 at 17:39

If it is the dynamic resistance of the diode, $r_d$ is the value of this resistance, when the diode is forward biased with a current $I_D$.

This starts from the analysis of the diode equation

$$I = I_s\cdot \left(e^{\dfrac{q_e\,V}{\eta\,K\,T}} -1\right)$$

and defining the dynamic resistance as

$$r_d = \dfrac{dV}{dI}$$

replacing $K$ for the Boltzmann's constant, $T$ the temperature, $\eta$ a coefficient between 1 y 2, $q_e$ the electron charge and $I_s$ the saturation current (minoritary carriers), an aproximate value is

$$r_d \approx \dfrac{26\,\mathrm{mV}}{I}$$

You can find a more detailed explanation here