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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.

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

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