A new concept to cancel insertion phase variation in MMIC amplitude controller

A concept which keeps a very low insertion phase variation in an analog monolithic microwave IC (MMIC) amplitude controller is presented. This new approach, applicable to dual-gate FET (DGFET) variable-gain amplifiers, is to connect an active variable load to the second gate of the DGFET. This load makes use of a cold FET (with Vds=0 V) whose impedance varies depending on the amplitude command of the circuit. Then, the cold FET compensates the insertion phase change introduced by the DGFET attenuation control over a wide dynamic range. Based on this principle, a C-band GaAs MMIC attenuator for active antenna application has been designed. The phase variation versus gain is always lower than +or-2 degrees over a 20-dB gain/attenuation range.<>