 There is no need to take into account the forward volt drop of the diode (which is necessary in the preceding simple circuits as this adds to the reference voltage).The effect of the diode volt drop on the circuit output will be divided down by the gain of the amplifier, resulting in an insignificant error.The circuit also has a great improvement in linearity at small input signals in comparison to the simple diode circuit and is largely unaffected by changes in the load.