I'm studying the emitter-follower configuration for bjt on the book "Practical Electronics for Inventors". On pag 439 there is an example related to the design of this system. In this problem it knows the parameters Rload, Vcc, hfe and f3db, and the target is to find the other parameters:
- choose the value of Ic = 1mA;
- Find the resistance Re = (0.5*Vcc)/Ic;
- Find the ratio of R1/R2 such that Ve = Vb-0.6V;
Than it says that R1//R2 <= 0.1 Rin(base),dc because this prevents the voltage divider's output from lowering under loading conditions. I don't understand this statement.
My first question is: why R1 and R2 are in parallel? If you apply the simplified model (I show after) to find the quiescent operating point R2//Rin not R1//R2. Am I wrong? Con you explain the meaning of this statement?