Voltage-variable attenuator MMIC using phase cancellation

A new microwave voltage-variable attenuator integrated circuit operating from 1.0 GHz to 3.5 GHz with a large attenuation range is demonstrated in this work. The input signal enters an active balun circuit, which generates an in-phase (0°) signal and an out-of-phase (180°) signal of equal amplitudes. The signals then pass through a pair of source-follower buffer circuits and then through two common-gate (CG) NMOS devices. The drains of the two CG circuits are connected together at the load. While one CG transistor is kept fully on, the gate bias of the second CG device is varied and the amplitude of the signal passing through it changes. Therefore, when the two signals emerging from the CG transistors are added at the output, variable attenuation occurs. The circuit exhibits a measured attenuation range of approximately 30 dB. The 1 dB compression point occurs at an input power of −13 dBm when the attenuation is set to 12 dB. The second harmonic is suppressed by at least 20 dB up to the 1 dB compression point and the measured IIP3 is 3 dBm. The IC was implemented using 0.18 μm CMOS technology. The circuit measures 575 μm by 275 μm including bonding pads and it consumes 18 mW of DC power.