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This paper reports a sensorless control technique for a Brushless DC (BLDC) motor based on an improved sliding mode observer, where a signum function is replaced by sigmoid function. Conventional sliding mode observer use signum function as the switching function which leads to chattering in the estimated back EMF. A low pass filter (LPF) is used to eliminate the chattering problem which indeed produces a phase delay with respect to the actual rotor position. In order to avoid the phase delay, a phase compensator is used to match with the actual rotor position. To overcome the chattering problem without usage of LPF is developed by replacing sigmoid function with signum function. The validity of the improved control strategy has been demonstrated with simulation and real experiments.