THz oscillators operating up to 680 GHz have been developed in this work based on a 250-nm InP heterojunction bipolar transistor (HBT) technology. This work shows that the common-base cross-coupled push-push oscillator topology is improved by adopting a coupled-line structure, in which DC blocking capacitors and other transmission lines are replaced by a pair of coupled lines. The coupled lines enable efficient impedance and phase matching with a small area, resulting in improved output power and efficiency. Three types of oscillators with a slight dimensional variation were fabricated. The measured oscillation frequency of the three oscillators are 628 - 682 GHz, 556 - 610 GHz and 509 - 548 GHz, respectively, with bias-based tuning. The maximum output power and DC-to-RF efficiency of oscillators are up to -10 dBm and 0.19 %, respectively. The circuit occupies only around 0.014 mm2 excluding the probing pads.