A buffer circuit is provided which is insensitive to a duty distortion regardless of the change of operation environment. The buffer circuit includes a current mode logic buffer and a differential-to-single-ended converter. The differential-to-single-ended converter receives first and second differential output signals to generate a single ended output signal and is configured so that an internal control node of the differential-to-single-ended converter is controlled in a negative feedback method to maintain a constant duty ratio of the single ended output signal regardless of the change of operation environment. According to some embodiments, a duty distortion of the single ended output signal due to the change of operation environment such as a process, a voltage, a temperature, etc. is reduced or minimized and thereby performance of the buffer circuit is improved and operation reliability is improved.