What is the purpose of a capacitor in the feedback path of a unity gain buffer?
R1 and R2 being equal (R) means that the offset error due to input bias current (but not input offset current) is eliminated. If an equal current Ib flows out of each input pin then each pin will be higher by Ib * R and if the impedance at V_in is much lower than R, the two cancel out. 







simulate this circuit &ndash; Schematic created using CircuitLab

The capacitor (if chosen to be large enough) reduces to a neglible value the phase shift in the feedback due to input capacitance of the op-amp (with R) that could reduce the phase margin and potentially cause instability. So it eliminates a problem that is caused by using R in the feedback rather than a short.