an oscillator comprising a delay circuit having an input terminal and an output terminal , and a tristate inverter having an input connected to the output terminal of the delay circuit and an output connected to the input terminal of the delay circuit . this tristate inverter is adapted to be switched by a control signal between an operable condition and an inoperable condition . a wave shaping inverter is connected to the output of the tristate inverter to generate a shaped pulse signal . further , there is provided at least one tristate inverter having an input directly connected to an output of the tristate inverter itself so that it can generate a predetermined potential corresponding to the threshold of the tristate inverter itself . this voltage generating tristate inverter is connected at its output terminal to a selected one of the input and output terminals of the delay circuit . this voltage generating tristate inverter is adapted to be put in response to the control signal , when the first tristate inverter is put in the operable condition , in such a biasing mode as to generate the above predetermined voltage at the output terminal , and when the first tristate inverter is put in the operable condition , in such a floating mode as to bring its output terminal into a floating condition , so that the first tristate inverter oscillates because of a feedback given by the delay circuit .