Phenobarbital is derivative of Barbituric Acid and Barbituric Acid is derivative of Urea. (structures shown in Fig below)
Urea has H₂N- group attached to Carbonyl Group (C=O), and such class of comounds conataining H₂N-C=O bond are called as Amides.
Result:
So, Phenobarbital belongs to Amides.