To amplify the output of a linear Hall Effect device for the purpose of driving a voice coil
For a straight forward circuit to convert your 0V to 2.5V sensor voltage to a 20mA to 100mA drive current for the voice coil you would want to use a circuit more like the following:



This servos the voice coil current against the hall sensor output voltage. The 1K and 100pF capacitor provide some feedback frequency compensation to make the circuit more stable. You would likely need to adjust these values based upon the actual op-amp part number that you use and the final physical circuit layout properties. 

Note that for a 100mA output current you may actually want to select an NPN transistor or N-Channel MOSFET that has a metal tab package better equipped to dissipate a bit more power than your typical TO-92 package of a 2N3904. 

Performance of the circuit was evaluated using the free LTSpice circuit simulator. The picture below shows the input voltage ramping from 0v to 2.5V and the corresponding current through the R7 load resistor. The op-amp used is a rail to rail type for both its input common mode range and its output swing range. The circuit will not work with an op-amp that does not support an input range down near GND or an output capable of swinging within about a volt of its negative (GND) rail.