Common Emitter amplifier Bias Circuit Simplification
The equivalent circuit shows only the components relevant to the signal.

The capacitors are assumed to have an impedance low enough to be considered a short for the signal so they're replaced by a wire.

R1 supplies bias current to the NPN in the equivalent circuit we already know what the bias current is (it is derived in the original circuit) so R1 in the equivalent circuit is there only to show the influence of R1 on the signal (a voltage divider consisting of Rs, R1 and R2).