BJT Emitter Degeneration Rπ
There are TWO high-pass filters in that circuit.

The input cap is one.

The emitter-resistor-bypass is the other.

Both of these capacitors cause a rising output signal level.

Thus 12 db/octave or 40dB/decade.

Its your job to manage the timeconstants of these TWO HPF.



What is the input impedance?

Zin is Z(c1) + the parallel combination of

a) R1

b) R2

c) Miller Capacitance [stage voltage gain * Cob (C collecter to base) ]

d) the sum of beta* [1/gm + { R4 in parallel with Z(20uF) } ], 
   where beta*1/gm is R_pi.