1. A 0.degree.-180.degree. active phase shifter for microwave frequencies, characterized in that it comprises a differential input amplifier receiving an input signal and having an inverting output and a non-inverting output, and a first and a second output stage of the push-pull type realized with MESFET transistors, each stage comprising an inverting input and a non-inverting input, said inverting output controlling the inverting input of the first output stage and the non-inverting input of the second output stage and said non-inverting output controlling the non-inverting input of the first output stage and the inverting input of the second output stage, the outputs of the two output stages constituting the outputs of the phase shifter.