I'm studying Electronics and the professor gave us this homework... As you may know, the formula for dynamic resistance is: r = resistance I = current Now, the question is, where did the "26mV" come from? Explain why there is a "26mV" there. Thanks to those who will help...
...and is given by the formula where k is the Boltzmann constant k=1.38e^-23 joules/Kelvin T is the absolut temperature in Kelvin q is the electron charge q=1.602e^-19 coulomb This yields approximately a Vt of 25mV @ 20 or 26mV @ 25
It is derived from (Dr.) Shockley's Diode Equation and requires some relatively simple differentiation and algebraic manipulation. It's well known and (if interested) you'll find plenty of derivations of the dynamic resistance on the web. There's been some recent discussion recently on the forum - so try searching here. The dynamic resistance is routinely used for BJT small signal amplifier analysis.
Thanks to all!!! EDIT: So, you can derive the Thermal Voltage by differentiating the Shockley Equation right? Could someone show the detailed differentiation of Shockley's Equation to become the Thermal Voltage Equation...
Hi, sorry for double posting but, I found this... The derivation of emitter resistance is as follows: (Google "diode equation" for definitions of k, q, and T) I=Is(e^(qv/kT)-1) where kT/q=26mv at T~300 degrees Kelvin since Is<<I at normal bias currents, I/Is~e^(V/.026) V/.026~ln(I/Is) V~.026(ln(I)-ln(Is)) Recall that R=dV/dI dV/dI=.026/I QED Could somebody explain it... What does "<<" mean?