Designs of Branch-Line Couplers by Considering the Parasitic Effects of P-I-N Diodes

Branch-line couplers (BLCs) are commonly used in the wireless systems. To achieve reconfigurable applications, switchable BLCs with p-i-n diodes can be used. Several studies have used diode parasitic reverse-biased capacitor and forward-biased inductor to approach off and on states. Although the capacitance and inductance are usually low, the parasitic effect may degrade predicted switching responses. This study proposes five reconfigurable switching microstrip BLCs. Each of the first two presented BLCs uses shunt to ground diodes for realizing two switching modes. The first mode with reverse-biased capacitors for perfect matching design is equivalent to a conventional branch-line coupler (BLC). The second mode uses low forward-biased inductances to approach shunt to ground, which transfers most signal power from Port 1 to Port 2/4; however, parasitic inductors produce some mismatches. To improve this problem, the proposed third or fourth BLC achieves two perfect matching modes by using shunt stub-loaded diodes. Specifically, by using four stub-loaded diodes, the proposed final BLC exhibits three perfect matching modes and one perfect isolation mode under a lossless ideal circuit condition.