A transponder (1) comprising an antenna (4), a demodulator (5) and a signal processing circuit (6) which converts a modulated signal received via the antenna (4) into a signal suitable for processing in the demodulator (5). The signal processing circuit (6) comprises an amplifier (7) with a predefined amplification factor and a closed loop control circuit (8) serving to maintain the voltage swing of the processed signal applied at the input of the demodulator (5) substantially constant. In a preferred embodiment the closed loop control circuit comprises a capacitor which is continually discharged and differingly charged as a function of the strength of the output signal of the amplifier of the signal processing circuit, the voltage resulting across the capacitor controlling a controllable resistor connected in parallel to the input of the signal processing circuit and forming with a coupling capacitor a voltage divider.