Unity gain buffer for AC signals. The input impedance is dominated by the voltage divider ( 2M2 & 3M3 ) as the impedance looking into the base is around 30M. In reality capacitance in real transistors would tend to lower the input impedance at higher frequencies. As with any emitter follower the gain is slightly less than 1, but only by a very small amount as hFE is around 10000 minimum for a small signal Darlington such as a BC517
