The circuit described in this paper can accurately define the time at leading-edge of pulses from radiation detectors. The basic circuit consists of two input shaping networks and an amplitude comparator generating output pulse having a fixed time almost independent of input pulse height. The output pulse shifts only ±0.5 ns when the input having risetime range of 0.1 to 0.5 μs is varied from 0.25 to 10 V in amplitude. © 1969.