Voltage Regulators: Understanding Transistors Biasing at 11:35

[kliker]

hi i can't 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 don't 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 don't have the same current because they are connected in parallel, so why does he say that they have the same current?

i can't get it, i would appreciate any explanation, this is the only thing that i can't understand from his lecture and i really need to understand it because i can't 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 I_B 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 I_B 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?