# Thevenin equivalent

1. Sep 10, 2014

### nunez2005

1. The problem statement, all variables and given/known data
I'm trying to understand where the (β+1) comes from.

2. Relevant equations
vth=-βiR2
i+βi+is=0

vth=-β(-is/(β+1))R2

3. The attempt at a solution

vth=-βiR2
i+βi+is=0

vth=-β(-βi-is)R2

#### Attached Files:

• ###### problem.png
File size:
5.6 KB
Views:
97
2. Sep 10, 2014

### Staff: Mentor

Welcome to the PF.

Without seeing the original circuit that this is modeling, it's hard to be sure. But for a CE BJT amplifier, your collector current Ic = β * Ib. So your emitter current is.....

3. Sep 10, 2014

### nunez2005

i've attached the solution that i have in the book, the value of beta is 150 and R1=100k and R2=39k, the only problem I have is that how do you get to the vth while substituting beta and the resistance in both sides to be equal to -Beta (-is/beta+1), the beta + 1 has to come out from simplying and combining like terms but I just can't see it.

4. Sep 10, 2014

### Staff: Mentor

So factor out the i in that equation above:

(β + 1)i + is = 0

There's your (β + 1) term.

That doesn't look right. You want to substitute for i in your equation for Vth from above, but what you've stuck in there also contains i.