a tad
A small amount, portion, or degree; a bit. Can you turn the heat down a tad? It's getting pretty warm in here. I'm feeling a tad sick, so I think I'll stay home from work today.
a tad bit
A small portion, degree, or amount. It got a tad bit warm when the sun started shining so I'm not going to need to wear this coat today. I forgot my lunch, can I have a tad bit of yours? I'm afraid I've caught a tad bit of a cold.