In general analog signals are processed with opamps - but you haven't
said what the load is for the variable 12V signal - if any significant
power is needed a power amplifier may be needed.
Not sure if this helps but I was using a TIP120 transistor to power a 12v 4amp linear actuator from an arduino output pin. I just hooked the actuators positive wire directly to 12v 4amps and the negative wire thru the transistor (with a diode since it is a motor). They typically cost $1-$2 each.