Class D audio amplifiers provide a dual benefit for battery-powered portable devices (Figure). They enhance battery life, and they produce much less power dissipation than do their linear cousins. Those features make Class D amplifiers ideal candidates for controlling speed and direction in small electric motors. The standard application circuit for a Class D audio amplifier, IC₁, requires only slight modifications. In place of the usual audio-signal input is a variable dc voltage that potentiometer R₂ generates. Resistor R₁ biases the potentiometer to match the input range of IC₁. Full-counterclockwise rotation of the potentiometer corresponds to maximum-speed reverse rotation of the motor. Midscale on the potentiometer corresponds to motor off, and full-clockwise rotation of the potentiometer produces maximum-speed forward rotation in the motor. The characteristics of a given motor may allow you to eliminate the amplifier's output filter, which comprises L₁, L₂, C₁, and C₂. But, unless the control circuitry is near the motor, you should include the filter to reduce EMI.