Tunable branch-line hybrid circuit using electrical length variable quasi-distributed TFMS line

In three-dimensional (3-D) MMICs, a me and er-like line is realized easily for in creasing the integration level and many devices such as metal-insulator-metal (MIM) capacitors and FETs can be located on transmission lines. The circuits which use FETs have variable frequency response because the FETs have variable capacitance. In this paper, a transmission line composed of high impedance thin film microstrip (TFMS) lines and high electron mobility transistors (HEMTs) which are realized easily by 3-D MMIC topology is described. This type of TFMS transmission line is called “electrical length variable quasi-distributed thin film microstrip (QD-TFMS) line”. Moreover, a tunable branch-line hybrid circuit in X band is realized by using the QD-TFMS lines. The relative tuning range of the branch-line hybrid circuit is up to about 20%. The intrinsic circuit area of the branch-line hybrid circuit is reduced to more than 70% smaller than that of conventional one.