Linear Amplifier Motor Driver
A simple driver for a motor is a "push-pull" current amplifier. This circuit uses two transistors to "push" or "pull" current through the motor. The transistors are npn-type and are activated by an applied voltage. The op-amp linearizes the circuit and provides a current gain. The input is an analog voltage.
This is a very simple linear amplifier. It provides a voltage across the motor equal to the voltage at the noninverting input of the op amp. A better linear amplifier, and the associated board available for stuffing in the NU mechatronics lab, is described here. With that circuit, you can decide whether it is the voltage across the motor, or the current through the motor, that is proportional to the analog input voltage.
While linear amplifiers can work very well, they are not very power efficient. Lots of power can be dumped into heat when the transistors are not saturated, as the power dissipated as heat by a transistor is equal to the voltage from the collector to the emitter multiplied by the current flowing through it. As a result, large heatsinks may be required for the transistors even when using small motors. For a more power-efficient method of driving motors, which also has the benefit that only on-off voltages are required, consult Pulse Width Modulation. In this control mode, the driving transistors are nearly always saturated, meaning little power dissipated as heat by the transistors.