An inverter-based operational transconductance amplifier (OTA) robust to process, voltage and temperature (PVT) variations is proposed in this Letter to enable the design of low-voltage power-efficient continuous-time sigma-delta modulators (CT-SDMs). The proposed OTA significantly reduces the unity gain-bandwidth variation across PVT corners while presenting a DC gain of 41.9 dB at nominal conditions when operated at 0.6 V. The effectiveness of the proposed circuit was verified in a 180 nm CMOS process by transistor-level simulations including PVT variations and mismatch. Also, a 0.6 V third-order CT-SDM is designed to verify the robustness against voltage and temperature variations.