RF transistor amplifiers include a Group III nitride-based RF transistor amplifier die that includes a semiconductor layer structure, a conductive source via that is connected to a source region of the Group III nitride-based RF transistor amplifier die, the conductive source via extending through the semiconductor layer structure, and an additional conductive via that extends through the semiconductor layer structure. A first end of the additional conductive via is connected to a first external circuit and a second end of the additional conductive via that is opposite the first end is connected to a first matching circuit.