Enhanced Mage Armor

You touch a willing creature who isn't wearing armor, and a protective magical force surrounds it until the spell ends. The target's base AC becomes 13 + its Dexterity modifier. The spell ends if the target dons armor or if you dismiss the spell as an action.

As the caster grows in strength, the spell can be upgraded using a material component appropriate to the protection desired.   Every 5 caster levels the base AC can be increased by 1 (AC 14 at 5th level, 15 at 10th level, etc.)