A GaAs Monolithic Bipolar Variable Attenuator with Improved Control Linearity

A novel circuit topology is presented to improve the control linearity of wide range variable attenuators. Five pairs of cold MESFETs are employed as variable devices. They have different gate widths and their drain and source are connected in parallel to a quadrature hybrid. The gate of each MESFET is driven by external dc control voltage. The gate width and voltage ratio of each MESFET are optimized in circuit design to achieve high linearity. A prototype bipolar variable attenuator is fabricated using the GaAs MMIC process. Its control performance is measured in the S band. Sensitivity deviation within ±5% is observed for over 50 % of the full varistor control range. This bipolar variable attenuator is extended to an orthogonal vector generator by integrating a pair of bipolar variable attenuators coupled to each other, a quadrature 3 dB hybrid, and a Wilkinson 3 dB combiner.