this circuit 36 has two series mounted chokes 37 38 the junction point 39 of which is decoupled with respect to the ground by a capacitor 40 and it includes a connecting capacitor 41 the output microwave signal being taken at the above mentioned terminal 5 the above mentioned switching over means capable of applying either the v volts bias to the drain 2 and the 0 volts bias to the source 3 or conversely this same v volts bias to the source 3 and the 0 volts bias to the drain 2 comprise four identical relatively low frequency switching over npn transistors 42 43 44 45 which bias the electrodes 2 and 3 each by means of a choke and a microwave decoupling capacitor respectively 46 47 and 48 49 the two transistors 43 42 each have their collector connected to the dc supply source v which is for example of the order of 5 to 6 volts and is consequently as capable of supplying the switching over transistors 42 to 45 as the fet 1 their emitters are respectively connected to the electrodes 2 and 3 by means of the above mentioned microwave filters 46 47 and 48 49 to their respective base electrodes 50 51 there is applied a 0 volts or v volts control dc voltage which is provided by standard logic switching over means that are not shown consisting for example of the two outputs of an astable circuit working as a divider by.