Abstract: An injector for a fuel injection system is provided with an injector housing in which a piezoelectric stack is located and with a valve housing connected with the injector housing in which a valve closing device with a jet needle is displaceably located. The valve closing device can be actuated by the piezoelectric stack. The valve closing device can be reset by a return device. A hydraulic following amplifier is located between the piezoelectric stack and the jet needle of the valve closing device. The amplifier has a displacement piston actuated by the piezoelectric stack. A control piston located downstream from the displacement piston and increases the displacement travel. A working piston that actuates the jet needle and increases the actuating force.