A "stepper motor driver" is a board with some big transistors that turns
the wimpy "step" pulses from a parallel port (or a microcontroller) into
the high-voltage, high-power, sequence of coil energy sources for the wires
of a stepper motor.
Linear drivers control the amount of power getting to the motor coil by turning a drive transistor part way on. This causes the left over energy to be dropped by the transistor, which means it gets hot. Linear drives are therefor limited to medium to low power driver systems.
Chopper drivers control the coil power by rapidly turning a drive transistor on and off. This allows for higher power drives, but comes with it's own set of costs: Because the frequence is often in the audio range, the motor coil will act as a speaker and mechanically vibrate, producing a squeal, whine or hiss which is not only annoying, but can actually cause midrange resonance issues and missed steps at specific speeds. It also introduces switching losses into the motor coil which causes the motor (instead of the driver) to heat up. As heat and magnatism don't mix well, this can lead to unexpected loss of power when the system is under load or has been operating long enough for heat to build up.
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