How can I change the amplitude of a pwm signal ?

Yes it is possible without any external CHIPS, but then you need to use a variety of parts to emulate the function of the chip. It is absolutely IMPOSSIBLE to control even a tiny motors speed and direction with only a MCU. It will be easier and cheaper to use a purpose built chip; however, if you want to do it as an educational exercise then I highly suggest starting with a h-bridge circuit for a standard small motor (not 3-phase).