Power amplifier

A power amplifier comprising:

• amplifier circuits of multiple stages, wherein
• each of the amplifier circuits of multiple stages includes at least one bipolar transistor and a base electrode,
• each bipolar transistor included in each of the amplifier circuits of multiple stages includes a collector layer placed on a substrate, a base layer placed on the collector layer, and at least one emitter mesa placed on part of a region of the base layer, and the at least one emitter mesa has a shape elongated in one direction in plan view,
• the base electrode includes at least one base main portion arranged in such a manner as to be separated from the emitter mesa with a gap in a direction orthogonal to a lengthwise direction of the emitter mesa in plan view, and the base main portion of the base electrode has a shape elongated in a direction parallel to the lengthwise direction of the emitter mesa in plan view and is electrically connected to the base layer, and
• of the amplifier circuits of multiple stages, a width, which is a dimension in the direction orthogonal to the lengthwise direction, of each base main portion of the bipolar transistor of a final stage amplifier circuit is wider than a width, which is a dimension in the direction orthogonal to the lengthwise direction, of each base main portion of the bipolar transistor of another stage amplifier circuit.