Input Resistance and Mid-band Gain of a CE-CE Stage Cascade Amplifier
What I've learned, the input resistance in emitter circuit \$ r_e \approx r_{BE} = \beta U_T / I_C \$, the \$  U_T \$ is the temperature voltage of about 26 mV at room temperature. More precisely: $$ U_T = kT/e_0 = \frac{1.38\cdot10^{-23} \text{ J/K} \cdot 296 \text{ K}}{1.60\cdot 10^{-19}\text{ C}} = 25.5 \text{ mV}$$
The output doesn't influence the input resistance so much and can be neglected in that emitter circuit.