(1) The low level generation of AM (DSB + Carrier) and the
progressive amplification of that RF signal with the final stage being a Linear RF amplifier--Class AB.
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.
(2) The second method is the progressive amplification of only the Carrier Wave with the output stage being, a
more efficient, Class C (non-linear) RF amplifier; the modulation is introduced as a very high level audio signal at the final stage --more precisely, the positive plate supply of the RF "Final" Amplifier
is made to vary as the modulation audio input signal.
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).