b. sound wave which strike the diaphragm of the microphone are converted into electrical waves (audio voltage) and applied to the speech amplifier. The speech amplifier raises the level of tiny audio signal voltage from the microphone to a value that will provide the required modulator drive signal. The modulator is a high-power audio amplifier which provides additional power amplification for the audio signal and couples it into the RF amplifier in such a way that the amplitude or strength of the RF signal is caused to vary as the audio voltage amplitude varies. When the RF signal is radiated into space by the antenna, the intelligence is also radiated, since it has been superimposed on the carrier in the form of amplitude modulation.

e. The method of modulation show in figuren175 is known as plate modulation, since the audio voltage from the modulator causes the plate voltage of the RF amplifier to vary above and below the B+ supply voltage. This is perhaps the most widely used method of producing amplitude modulation; it is the simplest to apply, give the highest operating efficiency, and is the easiest to adjust for proper operation. The high-power requirement of the modulator stage, however, prohibits the use of plate modulation in some transmitters.