Editing Buffer amplifier (section)

====Impedance transformation using the bipolar voltage follower====
Using the small-signal circuit in Figure 4, the impedance seen looking into the circuit is
::<math> R_{\rm in} = \frac {v_x} {i_x} = r_{\pi} + (\beta + 1) ({r_{\rm O}} || {R_{\rm L}}) </math>

(The analysis uses the relation ''g<sub>m</sub>r<sub>π</sub> = (I<sub>C</sub> /V<sub>T</sub>) (V<sub>T</sub> /I<sub>B</sub>)'' = β, which follows from the evaluation of these parameters in terms of the bias currents.)  Assuming the usual case where ''r<sub>O</sub>'' >> ''R<sub>L</sub>'', the impedance looking into the buffer is larger than the load  ''R<sub>L</sub>'' 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 ''r<sub>π</sub>'', but often ''r<sub>π</sub>'' <<  (β + 1) R<sub>L</sub>, so the addition does not make much difference