I have duplicated this circuit using a 40106 hex inverter to drive a motor using pwm. The only difference in my design was to switch out the transistor stage for an Irfz34n MOSFET. The motor is a 12v 2amp drill motor. When I tested it, I received decent pwm, adjusting the motor speed a little, however the MOSFET almost burned up immediately. I have bought a heatsink for test #2, however I'm concerned I am missing a crucial variable regarding thermal conductivity. Although MOSFET gate impedence is high, should I have a small resistor to check current along with a pulldown resistor? The invertor going low seems to me would take care of floating charge but I am still learning. I'm also concerned a resistor to the gate would slow the Gate charge time and mess with the pulsed switching speed. Any thoughts would be appreciated.
Edit: I should add, the motor stage is powered independently with a 12v lead acid battery.