Unless I missunderstood what you wrote, You can control the H bridge with an analog/variable voltage but you lose part of the advantage of the H-bridge. By driving the bridge with a pulse, the bridge is very efficient and it can control a large motor( high current) without the need for a large heat sink. Pulse Width Modulation (PWM) Switches on and off quickly so the MOSFET doesn't have to dissipate as much power. example: Using an analog voltage in a 12 volt system running the motor at half speed, you will have 6 volts across your MOSFET. Power is I*E. Using PWM, the MOSFET is turned on 1/2 of the time. WHen it's on only a small amount of voltage is across your MOSFET. Another advantage of the PWM is torque. You will have better torque at half speed, because you're slamming the motor with the full voltage and current. Also, if you use an analog voltage to control the bridge, you need to have a feedback loop to control the voltage out of the bridge. You add a lot of complexity, lose a lot of power, increase the size (and cost of you MOSFETs, heatsinks, cooling, etc.). You need a Hi side Lo side driver to drive the N channel MOSFETS. P channels are more expensive and they don't have as low of a RDSon. A MOSFET with and RDSon of .005 ohms ( 5 milliohms) will have .5v across it at 100 amps. ( E=I*R) 100amps * .005= .5 volts Power = I*E = 100*.5 = 5 watts. If this is a 12 volt system, you're controlling 100*12 =1200 watts of power. Your efficiency is 99%. Yes, you are going to lose more power than that due to switching losses. Rload = 100 ohm /12 volts= .12 ohms So, in your analog system, at half voltage out I =12/.12 = but looking at your 12 volt system, 1/2v out would give you the following 6v/.12ohms= 50 amps power wasted across the MOSFET is I*E= 50amps*6volts= 300Watts. That's a lot of heat and if you're running on batteries, you are really wasting a lot of run time.