I am working on designing a stepper motor controller. Not something I've done before.

this is the general design i'm starting from:
http://209.41.165.153/stepper/Tutorials/chopping.gif

my thought was to replace the transistor with a MOSFET, replace the comparator with an op-amp which directly drives the gate of the transistor, and an open-collector inverter also driving the gate so that a high signal will pull the gate low, turning off the MOSFET and switching off the coil.

the idea is that the voltage on the feedback resistor will control the mosfet via the op-amp so that the coil gets a fixed current. any thoughts on things i missed about this design?

I imagine there has to be some reason most other designs are using comparators/AND gates which gives a sawtooth waveform for the current, vs. a more smooth waveform i would imagine with an op-amp... but i can't see it at the moment.