I'm trying to understand the operation of a class A common-emitter amplifier, but I'm struggling to understand how the equivalent resistance for the cutoff frequency of C1 is determined. I also can't see how the equivalent circuit at the bottom left is correct.
Since the circuit splits in 3 after C1, it makes sense that there would be three parallel resistances. My first instinct would be to follow each of these to ground, and sum resistance along the way. Which would result in R1 | R2+RC+RE | RE, which is clearly wrong. I also thought to sum only the resistors before reaching the NPN, which left me with R1 | R2+RC.
So my question is, is there a rule for which resistors to include in the equivalent resistance of this circuit?