Traditionally, classes A-F exist. Classes A-C are defined by the conducting phase of the input wave which they transmit. Class A conducts for 360deg, Class B for 180 deg, Class AB for >180deg, Class C for <180deg, Class D for a pwm controlled phase (0 < pwm < 360), Class E is a highly efficient RF amplifier that uses zero-point switching to minimize losses, Class F amplifiers are a version of Class D amplifiers designed to suppress specific harmonics at the loss of some efficiency.