Ring-hybrid microwave voltage-variable attenuator using HFET transistors

In this paper, a voltage-variable microwave attenuator circuit is presented. The input signal first enters a rat-race power splitter where a 0° and a 180° pair of signals is generated. The 0° signal passes through a common-gate field-effect transistor (FET) that is fully turned on, with its gate voltage set to 0 V. The 180° signal enters another common-gate transistor biased in the triode region. By changing the gate voltage of the second FET, the amplitude of the 180° signal is varied. The in-phase and out-of-phase signals are summed at the output and variable attenuation is achieved. The concept was demonstrated experimentally from 3.0 to 3.4 GHz and a variable attenuation from 6 to 30 dB was achieved. The phase response is linear over the frequency band and exhibits a group delay of 0.71 ns. The input 1-dB compression point of the attenuator is 0 dBm and the second harmonic suppression is 18.5 dB at 0-dBm input power.