Modulation is the process of changing some characteristics (e.g. amplitude, frequency or phase) of a carrier wave in accordance with the intensity of the signal is known as modulation.
The figure shows the electronics circuit of a simple am modulator. It is essentially a CE amplifier having a voltage gain of A. The carrier signal is the input to the amplifier. The modulating signal is applied in the emitter resistance circuit.
The carrier ec is applied at the input of the amplifier and the modulating signal es is applied in the emitter resistance circuit. The amplifier circuit amplifies the carrier by a factor A, so that the output is Aes. Since the modulating signal is a part of the biasing circuit, it products low frequency variations in the emitter circuit. This in turn causes variations in “A”.
The result is that amplitude of the carrier varies in accordance with the strength of the signal. Consequently, amplitude modulated output is obtained across RL. It may be noted that carrier should not influence the voltage gain A; only the modulating signal should do this. To achieve this objective, carries should have a small magnitude and signal should have a large magnitude.