TRANSFORM IMPEDANCE WITH A BRANCH-LINE COUPLER.

Carefully designed branch-line couplers can eliminate interstage impedance-matching transformers in the typical hybrid-combined transistor amplifier, reclaiming several tenths of a db in circuit losses, and cutting sbustrate area by up to 50%. Branch-line couplers are traditionally designed with four, 50-ohm ports to match system impedance. However, most L and S-band power transistors present impedances of 5 to 50 ohms, in spite of internal matching circuitry. Interstage impedance transformers have been the classical solution. A more practical approach incorporates the function of impedance transformation into the hybrid coupler. This article presents guidelines for designing branch-line couplers with unequal input and output port impedances, and equal or unequal power split. The analysis is based on well-established design formulas for branch-line couplers with an arbitrary power splitting ratio and equal impedances at all four ports.