Abstract: A circuit (FIG. 1) for generating a current impulse comprises an avalanche transistor 2 which responds to a stimulus to apply an electrical impulse to the bases of a plurality of further avalanche transistors 3, 6. The further transistors respond to the impulse to discharge respective equal capacitors 9-12 through a laser diode 7. Further equal capacitors 22-25 connect the emitters of the transistors to the common junction of the transistor 2 and an impedance 21 to generate the electrical impulse when the transistor 2 responds to the stimulus. The arrangement of the further capacitors ensures that once one further transistor begins to conduct, the electrical stress on the others is increased, thus ensuring the further transistors simultaneously avalanche.