# A Derivation w.r.t AC Signal

Discussion in 'Homework Help' started by imfaizzi, Oct 30, 2008.

1. ### imfaizzi Thread Starter Member

Oct 11, 2008
20
0
hi,

can anyone gimme the Solved derivation (or its link/pdf/anything) for the following equation.

Code ( (Unknown Language)):
1.
2. [B]__
3. [SIZE=4]R[/SIZE]E = [U]25mV
4. [/U][SIZE=4]    I[SIZE=2]E[/SIZE][/SIZE][/B][SIZE=4][SIZE=2]
5. [/SIZE][/SIZE]

Also, what is RE and IE anyway?

2. ### mik3 Senior Member

Feb 4, 2008
4,846
63
Re is the impedance looking into the emitter of the transistor and Ie is the DC bias current flowing through the emitter.

3. ### imfaizzi Thread Starter Member

Oct 11, 2008
20
0
so how's it can be derived?

4. ### mik3 Senior Member

Feb 4, 2008
4,846
63
Use the diode equation

I=Io*((expV/VT)-1)

and find dV/dI to get Re

dI/dV=(Io*exp(V/VT))/VT

but Io*exp(V/VT)=I+Io

thus dI/dV=(Io+I)/VT, because Io is very small we ignore it and we get

dI/dV=I/VT

but Re=dV/dI=VT/I=25mV/I, because VT equals about 25mV at room temperature.