High-power T/R switch with GaN HEMT technology is successfully developed, and the design theory is formulated. The proposed switch employs an asymmetric series-shunt/shunt configuration. Because the power handling capability of the proposed switch is mainly dependent of the breakdown voltage of FETs, the proposed circuit can make full use of the characteristics of the GaN HEMT technology. The switch has a high degree of freedom for the FET gate widths, so the low insertion loss can be obtained while keeping high-power performances. To verify this methodology, T/R switch has been fabricated in X-band. The fabricated switch has demonstrated an insertion loss of 1.8 dB in Rx-mode, 1.2 dB in Tx-mode and power handling capability of 20 W in 53% bandwidth.

A novel nonlinear analysis method of high power amplifier instability has been developed. This analysis method deals with a loop oscillation in a closed loop circuit and presents the conditions for oscillation under large-signal operation by taking account of mixing effect of FETs. Applying this analysis to the high power amplifier instability that an output power for the fundamental wave (f0-wave) decreases at some compression point where a half of the fundamental wave (f0/2-wave) is observed, it has been found that this instability is caused by an f0/2 loop oscillation. In addition, it has been verified by analysis and experiment that the oscillation can be removed by employing an isolation resistor in a closed loop circuit.

A C-Ku band 5-bit MMIC phase shifter using optimized reflective series/parallel LC circuits is presented. The proposed circuit has frequency independent characteristics in the case of 180 phase shift, ideally. Also, an ultra-broad-band circuit design theory for the 180 optimized reflective circuit has derived, which gives optimum characteristics compromising between loss and phase shift error. The fabricated 5-bit MMIC phase shifter with SPDT switch has successfully demonstrated a typical insertion loss of 9.4 dB 1.4 dB, and a maximum RMS phase shift error of 7 over the 6 to 18 GHz band. The measured results validate the proposed design theory of the phase shifter.

A novel design method for wideband low-noise multi-stage amplifiers is presented. It utilizes a RL-SFC (esistive oaded eries eedback ircuit) comprised of a series feedback circuit with additional lossy match stubs to achieve low noise figure, low VSWRs, flat gain, and high stability simultaneously. In addition, each stage amplifier employs a different approach for designing the RL-SFC to achieve the best compromise between noise figure, VSWR, gain, and stability as a multi-stage amplifier. With the use of this novel design method, the 3-stage amplifier has demonstrated the state-of-the-art wideband low-noise performance of 2.3 dB over 6 to 18 GHz.