Abstract: A peak detector for measuring peak values of a voltage comprises a voltage comparator circuit (3), a capacitor storage element (5), and a charge and discharge circuit (8, 15) for the capacitor. A transistor (8) is connected in common base arrangement between the comparator device (3) and the capacitor with its base (9) connected to a constant voltage (E1). The reverse base-emitter diode voltage is limited by a diode (13). This reduces the value of capacitor (5) so that, with a moderate charging current from the voltage comparator device, a fast peak detector is obtained which can measure non-recurrent pulses of some volts within nanoseconds in a fairly accurate manner.