Dynamic Resistance

Thread Starter

xKael

Joined Jun 30, 2010
24
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... :D
 

Georacer

Joined Nov 25, 2009
5,182
...and is given by the formula
\(V_{\small T}=\frac{k \cdot T}{q}\) 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\(^{\tiny{o_C}}\) or
26mV @ 25\(^{\tiny{o_C}}\)​
 

t_n_k

Joined Mar 6, 2009
5,455
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.
 

Thread Starter

xKael

Joined Jun 30, 2010
24
Thanks to all!!! :D


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...
 
Last edited:

Thread Starter

xKael

Joined Jun 30, 2010
24
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?
 
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... :D
whats f mean in If?
it mean forward bias?!!??
 
Top