a control circuit controls the operation of a brushless dc sensorless motor having a first terminal connected to a first winding , a second terminal connected to a second winding and a third terminal connected to a third winding . a driver circuit applies drive signals to the first and second terminals and places the third terminal in a high - impedance state . the drive signals include first drive signals at a first current amplitude and second drive signals at a second current amplitude different from the first current amplitude . a differencing circuit senses a first mutual inductance voltage at the third terminal in response to the first drive signals and senses a second mutual inductance voltage at the third terminal in response to the second drive signals . the differencing circuit further determines a difference between the first and second mutual inductance voltages and produces a difference signal that is used for zero - crossing detection and rotor position sensing .