Active transponder with very low electric power consumption in standby mode

An active transponder including a logic circuit and an RF input-output circuit, wherein said input-output circuit is formed by an input amplifier arranged between an envelope detector for the signal received by an antenna of the transponder and a demodulation circuit for said signal that also forms said input-output circuit, wherein said transponder includes an activation unit for said input amplifier, which is formed of a frequency changer or mixer circuit, which receives as input an envelope signal of the antenna signal and a clock signal generated by an internal clock at a clock frequency that is different from, but close to a modulation frequency of an interrogation signal of an RF system for which the transponder is intended, and which supplies as output, when it receives said interrogation signal as input, a low frequency signal, whose value is approximately equal to the difference between said modulation frequency and said clock frequency, said activation unit also including a low pass filter and low frequency amplifying means for amplifying said low frequency signal, and said activation unit supplying a wake up signal to said input amplifier when it receives an envelope signal having said modulation frequency and a sufficiently large amplitude.