(The analysis uses the relation "gmrπ = (IC /VT) (VT /IB)" = β, which follows from the evaluation of these parameters in terms of the bias currents.) Assuming the usual case where "rO" Â» "RL", the impedance looking into the buffer is larger than the load "RL" without the buffer by a factor of (β + 1), which is substantial because β is large. The impedance is increased even more by the added "rπ", but often "rπ" Â« (β + 1) RL, so the addition does not make much difference