In this circuit a technique called bootstrapping is employed to greatly increase the effective input impedance allowing for easier design at the cost of a few more components, this circuit works by isolating the input signal from the voltage divider, since the AC voltage is nearly equal on each side of the base shunt resistor the current through it is small which results in a high impedance, for optimal performance the feedback and input signal need to be the same phase and amplitude.
This technique can also be applied to the JFET and MOSFET, although impedance is usually high enough as it is.
