Diode equation.Needs understanding

diodespower electronics

Kindly explain this equation for me.I do not understand what it means and from where this equation comes out?it's a diode equation
r^{\prime}_d = \dfrac{26\mathrm{mV}}{I_D} + r_B

Best Answer

A diode is a strongly non-linear electronic part. Hence, it is absolutely necessary to discriminate between static and dynamic (differential) resistances. Therefore, it is common practice to use capital letters (R) for static resistances and small letters (r) for diff. resistances. (By the way: Unfortunately, this general rule is violated by the contribution as contained in "learningaboutelectronics.com" as referenced in the first comment).

The first part of the equation under consideration is derived from the voltage-current relation of the pn junction, which is an exponential one and can be found in each textbook. It is derived from the differential quotient (slope) of the function Id=f(Vd) and contains with the "temperature voltage" Vt=26mV the temperature dependence of the unit (26mV for normal ambient temperatures). The second part (rb) is nothing else than an ohmic part (rather small) which contains the ohmic influence of the material between the external pins and the pn junction. This part has a linearizing effect for the Id=f(Vd) characteristics and comes into play for relatively large currents Id.

The static resistance of a diode is simply the ratio R=Vd/Id.