1. Apparatus for calculating a sum of products, each product having two factors, comprising: a junction point; a smoothing stage having an input coupled to the junction point and having an output at which the sum of the products is taken off; an inverter having an output coupled to the junction point and having as inputs a plurality of input voltages, each input voltage corresponding to a first factor of each of the products; a plurality of switches, equal in number to the number of products being summed, each switch having an output and each switch having one of the plurality of input voltages as an input; means for generating switching pulses proportional to voltages corresponding to the respective second factors of the products, the pulses being coupled as control inputs to the switches; and a proportional stage coupling the output of each switch and of the inverter to the junction point.