I use lm324 op-amp and feed signal to non-inverting input.
Inverting input gets feedback to source of IRF640 power mosfet and source connected to GND with 10 Ohm resistor.
Please keep in your mind gate connected to output of LM324 and drain connected to transformer and other input of transformer connected to VCC=12Volts.
This is my schematic.
" "" I WANT TO CONVERT VOLTAGE TO CURRENT AND FEED IT TO TRANSFORMER TO CHANGE LEVEL"""
My circuit on PULSE INPUT works good but I can't use it for sine wave(Full Rectified with 100Hz )
The non-inverting input pin 3 of the U1A cannot be driven below ground without blowing it up! R1 is likely to0 high in value (the max current is limited to less than 1A). The slew-rate of the LM324 is too whimpy to generate a 10V 100KHz pulse at either pin 1 or pin 7.