Dragon 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 16 + its Dexterity modifier. The spell ends if the target dons armor or if you dismiss the spell as an action. Component cost of 10 GP worth of Gemstones.

The target's base AC becomes 17 + its Dexterity modifier, when cast at 3rd level. Component cost of 50 GP worth of Gemstones.

The target's base AC becomes 18 + its Dexterity modifier, when cast at 4th level. Component cost of 100 GP worth of Gemstones.

The target's base AC becomes 21 + its Dexterity modifier, when cast at 5th level. Component cost of 500 GP worth of Gemstones.