A transponder device having an LC oscillator circuit, an energy storage capacitor and an integrated transponder circuit powered by energy from the storage capacitor. The device operates either in a charge mode in which RF energy is received through the LC oscillator circuit and stored in the energy storage capacitor, or in a transmit mode in which data from the transponder circuit are transmitted from the transponder device through the LC oscillator circuit by sustaining oscillation of the LC oscillation circuit and selectively modulating the oscillator frequency. The device further includes a stimulating circuit to feed energy from the storage capacitor into the LC oscillator circuit during the transmit mode and to sustain oscillation thereof, a peak detector for detecting a peak voltage level of an RF oscillator signal in the LC oscillator circuit and a pulse generator for providing trigger pulses to the stimulating circuit in response to an output from the peak detector. The stimulating circuit is controlled in a manner to prevent the amplitude of the oscillator signal in the LC oscillator circuit from exceeding a predetermined peak value, by suppressing a stimulating pulse when the oscillator signal gets close to the predetermined peak value.