A phase modulator for communication applications is constructed in so as to be easily implemented by way of MMIC technology. A phase modulator section is followed by a saturated amplifier section so as to provide substantially constant amplitude for all bits or states. The phase modulator section is a bi-phase shift keyed or quadrature-phase shift keyed modulator constructed in accordance with the present invention for discretely varying the phase of an input carrier signal between two or four possible phase states, respectively. The bi-phase shift keyed modulator includes at least a first pair of first and second transmission line segments and a pair of switches which each serve as a single-pole double-throw switch for operatively connecting one of the pair of transmission line segments between an input associated with the first switch, and an output associated with the second switch. The length of the transmission line segment of each of the first and second line segments differs by a first selected difference value determined at a desired operating carrier signal. A control means is provided to control the gate voltages of each of the field effect transistors of each of the switches so as to provide Bi-phase switching of the carrier signal. The quadrature-phase shift keyed modulator is accomplished by adding, in tandem, a second switch selectable pair of transmission line segments to the first pair constituting the bi-phase shift keyed modulator. The post amplifier section employs an amplifier driven into saturation so as to ensure minimal amplitude variation in the output signal whole while maintaining a substantially constant selected phase state.
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