1. An arrangement for comparing an input signal from a source with a reference signal comprising: a CTD channel, a first electrode in said CTD channel, a second electrode in said channel sequentially positioned along said channel from said first electrode and coupled to said source, a third electrode in said channel sequentially positioned along said channel from said second electrode, voltage buffering means coupling said third electrode to said first electrode, a source of clamping potential coupled to said third electrode through a switch, whereby said CTD channel and said first and second electrodes create a charge bucket with charges therein and with a barrier formed by said second electrode responding to said reference signal, said second electrode responding to an input signal to change the barrier so as to provide positive feedback if said barrier is lowered, to transfer some charges over said barrier, said charge bucket thereby emptying over said barrier.