A power amplifier module comprising: a substrate (210, 510, 810, 910) formed from a plurality of dielectric material layers (318, 319, 320, 321, 322, 323, 324), and having a mounting surface and a second surface opposite the mounting surface, wherein a plurality of non-overlapping zones (104, 201, 202, 203, 204, 205) is defined at the mounting surface; a first transmission line (172, 272) coupled to the substrate and located within a first zone of the plurality of non-overlapping zones; a first power transistor die (132, 232, 252) coupled to the mounting surface in a second zone of the plurality of non-overlapping zones, wherein the first power transistor die includes a first transistor (237, 257) integrated within the first power transistor die, and wherein the first transistor includes a first output terminal (138, 238, 258); a second power transistor die (132, 232, 252) coupled to the mounting surface in a third zone of the plurality of non-overlapping zones, wherein the second power transistor die includes a second transistor (237, 257) integrated within the second power transistor die, and wherein the second transistor includes a second output terminal (138, 238, 258), wherein a first end of the first transmission line is coupled to the first output terminal, and a second end of the first transmission line is coupled to the second output terminal; and a ground plane height variation structure (520, 720, 820, 920) extending from the second surface into the substrate within the first zone to provide a ground plane in the substrate underlying the first transmission line at an upper boundary of the ground plane height variation structure, a portion of the substrate is present between the upper boundary of the ground plane height variation structure and the first transmission line, and the ground plane height variation structure includes a conductive path between the upper boundary of the ground plane height variation structure and the second surface, and characterized in that a height of the ground plane height variation structure is at least half of a total thickness of the substrate, wherein said height is configured to set the characteristic impedance of the first transmission line for a given power level of the power amplifier module. a substrate (210, 510, 810, 910) formed from a plurality of dielectric material layers (318, 319, 320, 321, 322, 323, 324), and having a mounting surface and a second surface opposite the mounting surface, wherein a plurality of non-overlapping zones (104, 201, 202, 203, 204, 205) is defined at the mounting surface; a first transmission line (172, 272) coupled to the substrate and located within a first zone of the plurality of non-overlapping zones; a first power transistor die (132, 232, 252) coupled to the mounting surface in a second zone of the plurality of non-overlapping zones, wherein the first power transistor die includes a first transistor (237, 257) integrated within the first power transistor die, and wherein the first transistor includes a first output terminal (138, 238, 258); a second power transistor die (132, 232, 252) coupled to the mounting surface in a third zone of the plurality of non-overlapping zones, wherein the second power transistor die includes a second transistor (237, 257) integrated within the second power transistor die, and wherein the second transistor includes a second output terminal (138, 238, 258), wherein a first end of the first transmission line is coupled to the first output terminal, and a second end of the first transmission line is coupled to the second output terminal; and a ground plane height variation structure (520, 720, 820, 920) extending from the second surface into the substrate within the first zone to provide a ground plane in the substrate underlying the first transmission line at an upper boundary of the ground plane height variation structure, a portion of the substrate is present between the upper boundary of the ground plane height variation structure and the first transmission line, and the ground plane height variation structure includes a conductive path between the upper boundary of the ground plane height variation structure and the second surface, and characterized in that a height of the ground plane height variation structure is at least half of a total thickness of the substrate, wherein said height is configured to set the characteristic impedance of the first transmission line for a given power level of the power amplifier module.