An FM is a constant envelope signal, due to the fact that the envelope
of the carrier does not change with changes in the modulating signal. Hence the
transmitted power of an FM signal is constant regardless of the amplitude of the
message signal. The constant envelope of the transmitted signal allows efficient
Class C power amplifiers to be used for NP' power amplification of FM. In AM,
however, it is critical to maintain linearity between the applied message and the
amplitude of the transmitted signal, thus linear Class A or AB amplifiers, which
are not as power efficient, must be used.
The issue of amplifier efficiency is extremely important when designing
portable subscriber terminals since the battery life of the portable is tied to the
power amplifier efficiency. Typical efficiencies for Class C amplifiers are 70%,
meaning that 70% of the applied DC power to the final amplifier circuit is converted
into radiated RF power. Class A or AB amplifiers have efficiencies on the
order of 30-40%. This implies that for the same battery; constant envelope FM
modulation may provide twice as much talk time as AM.
of the carrier does not change with changes in the modulating signal. Hence the
transmitted power of an FM signal is constant regardless of the amplitude of the
message signal. The constant envelope of the transmitted signal allows efficient
Class C power amplifiers to be used for NP' power amplification of FM. In AM,
however, it is critical to maintain linearity between the applied message and the
amplitude of the transmitted signal, thus linear Class A or AB amplifiers, which
are not as power efficient, must be used.
The issue of amplifier efficiency is extremely important when designing
portable subscriber terminals since the battery life of the portable is tied to the
power amplifier efficiency. Typical efficiencies for Class C amplifiers are 70%,
meaning that 70% of the applied DC power to the final amplifier circuit is converted
into radiated RF power. Class A or AB amplifiers have efficiencies on the
order of 30-40%. This implies that for the same battery; constant envelope FM
modulation may provide twice as much talk time as AM.
