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
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.
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.
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.
however, a 4 quadrant multiplier is used, negative modulation starts to
appear as a double sideband suppressed carrier--or in this case, reduced
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.
AM and FM transmitter output stages--called "Finals"--use Class "C" amplifiers.
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).