Abstract: A transmitter optical subassembly may include an optical modulator for modulating output light from the light source. The optical modulator has a characteristic that a current depending on amount of optical absorption has a positive correlation with an applied voltage thereto. The transistor at the second terminal is connected in series to the optical modulator. A drive voltage applied to the optical modulator and the transistor is divided into a first voltage applied to the optical modulator and a second voltage applied to the transistor. A drive current flowing through the optical modulator and the transistor depends on the control signal input to the first terminal. The first voltage is based on the drive current and is subject to the characteristic of the optical modulator. The second voltage fluctuates in response to the first voltage.