There migh be a module, where you just add some bonuses to AC based on level, and actually I am not against such a bonus, just as I am not against a bonus to "to-hit" by level. But this bonus needs to be individual for each class, not a strict function of level.
So a Fighter migt get +1 to AC once in a while, a monk too.
A wizard however should not, but some wizards may gain spells that augment AC, and that spell migt be "at-will" So a war wizard or an abjurer might get an at will spell, that increases AC by +1 per 5 levels. So if a player asks: why do you have a higher AC, you have an in game explanation: Magic!
The fighter, who gets +1 to AC per 3 levels also has: Skill at arms!
The rogue might get a dodge ability, where he can increase his AC by 4 1/5 levels per day, and can explain it: "I am just lucky"
You could also start differentiating hp by this: the fighter just has very high hp, the wizard a lot lower, but magic spells to protect him... (oh ok, that is old)
There is a second problem which will arise, if HP and AC are linear functions of level: the defensive power rises quadratic. So the game does not scale very well. (See early 4e) So one of those stats should be fixed. And in D&D it is traditionall AC which is fixed and HP rises. There are different games, where you increase an AC like stat (parry values etc) and hp remain more or less the same!
edit: sorry for the formatting... enworld seems quite buggy for me since the downtime