A motor driving circuit is described for three-phase brushless DC motors, which have a three-phase-coil and first and second Hall sensors to detect the magnetic field of a rotor. The motor driving circuit includes first and second comparators, comparing a first and second pair of Hall signals from the Hall sensors, and outputting a first and second Hall signals. An adder unit receives the first and second pair of Hall signals to output a third pair of Hall signals to a third comparator, which outputs a third Hall signal. A motor driver is controlled by the first, second, and third Hall signals of the first, second and third comparators to change directions of currents flowing through phases of the three-phase coil accordingly to rotate the rotor of the motor. The first and second Hall signals can be amplified to match the level of the third Hall signal, or vice versa.