This paper describes a monolithic microwave integrated circuit (MMIC) active module with small phase variation and low insertion loss for beamforming network in S-band. The MMIC active module composed of a digital phase shifter, a digital attenuator and a buffer amplifier, has characteristic to control amplitudes and phase shifts by using digital control signals. By using the digital attenuator, the MMIC active module has obtained the excellent performances. This paper also describes the exact on-state resistance of FET switch for designing the digital attenuator.

An improved equivalent circuit model of a GaAs FET switch for MMIC phase shifters is proposed that incorporates distributed lines into a lumped-constant equivalent circuit to account for distributed-line effects. The validity of the proposed model is demonstrated by applying a coupled-wave analysis to the FET switch. Comparison of the measured and the simulated phase angles of the S-parameters shows that the improved model gives much bettter accuracy than the lumped-constant model. X-band six-bit MMIC phase shifters designed using the improved model are also described.

We propose an accurate FET model for microwave nonlinear circuit simulation, which has been modified from the Statz model. We have greatly enhanced the accuracy of both dc and capacitance expressions, especially in the knee voltage region where Ids begins to saturate. In the expression of dc characteristics, our model improves the accuracy by incorporating the drain-source voltage dependence of pinch-off voltage, the gate-source voltage dependence of knee voltage, and the non-square dependence of drain current against the gate-source voltage. The non-square-root voltage dependence of gate capacitances is considered as well. All modifications are simple and the parameter extraction is kept as simple as that of the Statz model. By using this model, good agreement has been obtained between simulated and measured characteristics of a GaAs FET. For the dc characteristics and the S-parameters, each of estimated error is within 5% and 10%. The model accuracy has been verified by comparison of simulated and measured results of power amplifier performances over a wide range of operating conditions.