In the design of an AM transmitter there are two ways to go: 

In the case of the low level modulation approach, one could use either a 2 quadrant or 4 quadrant multiplier as the modulator. 

	The difference being: with the 2 quadrant multiplier, negative modulation of greater than 100% causes severe distortion as well as interference on adjacent bands. This is due to the carrier being cut off when the 2 quadrant multiplier cannot furnish any output for negative values of the modulating signals, hence the RF output becomes a pulsed spectrum.
	If, however, a 4 quadrant multiplier is used, negative modulation starts to appear as a double sideband suppressed carrier--or in this case, reduced carrier.

The High Level modulation cannot handle negative modulation of greater than 100%. As with the 2 quadrant multiplier in the first approach, the carrier is cut off during negative peaks that exceed 100% negative modulation. 

Most commercial AM and FM transmitter output stages--called "Finals"--use Class "C" amplifiers. 

Other transmitters, like Television (visual), SSB, etc., use "Linear Amplifiers," Class AB₁ or AB₂, which are a combination of Class A and Class B (both being much less efficient than the Class C amplifier).