What is RB in this CE circuit?

Question

I'm working through this handout I3.PDF to help me understand feedback in common emitter amplifiers. (The author does not reveal his/her name in the PDF and so I can't ask them directly.) They show a common emitter circuit at the top and a small-signal model just below it. What expression defines the resistance \$R_B\$ in the model?

I'm imagining two possibilities. First, it might be be some combination of \$R_1||R_2\$. This doesn't seem correct. In the upper schematic \$R_1\$ appears between base and ground whereas \$R_2\$ appears between \$V_{CC}\$ and the base. But in the small-signal equivalent, \$R_B\$ appears only between base and ground. The second possibility is that it is simply \$R_1\$ alone. With us only considering small-signal (AC) voltages, \$V_{CC}\$ would appear as ground and so we ignore it and the resistor directly connected to it? Both of my guesses seem flawed.

Answer 1

RB in the model is the effective parallel resistance of components external to the transistor.

In this case it will be the the parallel resistance of R1 and R2.

From the small signal point of view VCC and ground are the same so R1 and R2 are in parallel. It is assumed that the power supply has a low impedance for AC.

Note that RC, at the output, is also shown connected to ground where in the upper schematic it connects to VCC.