by simple you mean small, so you can carry it with you, right...?
this sort of device would either need to be soldered into your phone to take the output signals and boost them, in addition to adding a better antenna for receiving.
or it would need to have a receiver circuit that received your phone signal and re-transmitted it at higher power, while preventing your phone from communicating with the tower.
and either way its going to require working with RF signals in the several gigahertz range, which is not particularly easy.
if your home has poor reception and you would like to boost your signal while in your house, I believe verizon had some units that plug into your wall and act just like a wireless modem... but for your phone. I do not remember what they were called or if they were actually released.