Transistor lasers (TLs) can be current-modulated as a diode laser or voltage-modulated as a Franz-Keldysh electro-absorption modulator. These two modulation schemes result in opposite frequency shifts that can cancel each other. In this work, we present the principle of generating optical signals with minimal frequency chirps using TLs. With the proper setting of pulse shapes for the current and voltage inputs, optical signals that are robust against distortions can be directly output from TLs for the medium-distance or long-distance optical fiber communication.