What is the Thevenin equivalent circuit?

[Defennder]

Homework Statement

I really don't see how the Thevenin equivalent circuit is obtained from the one on the left. The way learned to find Thevenin equivalent circuit considers the circuit network in between 2 nodes. This one appears to have 3 nodes. And why is the thevenin resistance given as though the resistance were connected in parallel? I don't see why they are in parallel.


2. Homework Equations /pictures
Thevenin equivalence.
http://img380.imageshack.us/img380/5082/theveninequivalentvm4.jpg

 

[CEL]

To obtain the open circuit voltage you use a voltage divider with the 10V source and the two resistors, getting 3.33V.

For the equivalent resistance you short circuit the source, so the two resistors are in parallel (the upper terminal of R1 is connected to ground).

 

[Defennder]

Changed image

Yeah, thanks, I think I can see it for the Thevenin resistance. I assume that the above gray box is also equivalent to this:

http://img508.imageshack.us/img508/8425/thevenincircuitwi3.jpg

But the open circuit voltage is between V1 and Vb or V2 and Vb?

 

[ahmedibrahim7]

for this circuit there is an expression to calculate v(th)& R(th) which is:
R(th)=R1//R2
v(th)=R(th)[(v1/R1)+(v2/R2)]
where v1=10v,v2=0v
// means parallel

 

[Defennder]

Where did you get that expression for v(th) from? And more importantly is the open-circuit voltage between V1 and Vb or V2 and Vb? That's all I need to be able to figure out the thevenin voltage.

 

[zeitghost]

v(th) comes from the Voltage divider rule.

v(th) is measured between vb and v2.

 

[Defennder]

But why isn't it measured between V1 and Vb?

 

[CEL]

But why isn't it measured between V1 and Vb?

Because V2 is ground and you want to polarize the transistor, so you are really interested in the voltage between base and emitter: Vbe = Vb - Ve.

 

[Defennder]

Oh yeah you're right. It's the voltage across the Vbe that's relevant here. Thanks a lot!