Akku-Überwachung (Bleiakku-Spannung)

Multiply X and Y and you get XY -all very simple and straightfor-ward -at least on paper. But what if X and Y are analogue voltages, which may be of either polarity? How does one go about multiplying two such quantities? The following circuit for a `four quadrant multiplier` -a circuit which will multiply two input voltages and ensure the product is of the correct polarity -shows one way of approaching this problem.