transistors biasing

[kliker]

hi i cant understand something in voltage regulators

http://www.youtube.com/watch?v=9FJJre-HG_0&feature=channel

at 11:35 he starts explaning why Vb's equation is like that but i dont get it

at 12:00

he says that Vcc/(R1+R2) gives the current through the two resistors

but how is that possible? I mean in the circuit these two resistors dont have the same current because they are connected in parallel, so why does he say that they have the same current?

i cant get it, i would appreciate any explanation, this is the only thing that i cant understand from his lecture and i really need to understand it because i cant solve problems because of it

thanks in advance

[vela]

R1 and R2 aren't connected in parallel. Why do you think they are?

He's using the approximation that the base current IB is very small, so you can neglect it. This means all the current flowing through R1 also goes through R2. In other words, R1 and R2 are effectively in series so you have a simple voltage divider.

[kliker]

R1 and R2 aren't connected in parallel. Why do you think they are?

He's using the approximation that the base current IB is very small, so you can neglect it. This means all the current flowing through R1 also goes through R2. In other words, R1 and R2 are effectively in series so you have a simple voltage divider.

thanks a lot ;)

one more question, can we always assume that Ib is too small?