A voltage detector operates to detect a system power supply voltage and generate a trigger signal. A control signal generator responds to the trigger signal and generates a control signal. A DC bias generator responds to the control signal by generating a DC bias. The control signal controls the DC bias to have a first value when the power supply voltage is a first voltage and have a second value when the power supply voltage is a second voltage different from the first voltage, wherein the first value is different from the second value. A dynamic DC bias is generated which can not only support a larger voltage scope, but also significantly improves signal to noise ratio. The system power supply detection may concern stop/start operation of an automobile engine.