For a Power Opamp, you use resistors to set the gain, and then the load is connected between the output pin and the tap between the split power supplies. Since you want a non-inverting gain of 20, then you would use R2/R1 = 19, as shown below:
Note that I answered the question about the "back EMF", too. There is none, as long as the magnet is not suddenly open circuited...
If the input is driven by a 1V square wave, the 1 Henry magnet is driven by a 20V square wave, but the current increases/decreases with a classic exponential waveform, with no voltage peaking.