PURPOSE: To obtain a power amplifier suitable for MIC(microwave IC) forming with a small size by constituting the input/output of the amplifier by a 180° transformer comprising a coplaner line and a slot line.

CONSTITUTION: The amplifier consist of a coplaner line A, a slot line B, an input side matching circuit 2, an output side matching circuit 3 and a transistor 4. Through the constitution above, a signal from the unbalanced coaxial line is sent once to an unbalanced line and converted at an unbalanced-balanced section of coplaner line A - slot line B. Since the phase differs by 180° at the part of the slot line B, the circuit is active in an equivalent shown in figure. Both the coplaner part A and the slot part B are formed n an alumina substrate or an equivalent insulation substrate. Thus, the size is decreased and the power amplifier suitable for MIC forming is obtained.