sentences:  Combining the two difference stages output currents yields four-quadrant operation.However, in the cells invented by Gilbert, shown in these figures, there are two additional diodes.This is a crucial difference, because they generate the logarithm of the associated differential (X) input current in such a way that the exponential characteristics of the following transistors result in an ideally perfect multiplication of these input currents with the remaining pair of (Y) currents.