Amplitude modulation is defined as a system of modulation in which the amplitude of carrier is made proportional to the instantaneous amplitude of modulating voltage.

Hence, in AM the instantaneous modulating voltage variations are superimposed on the carrier amplitude. as shown in fig b, when there is temporarily no modulation, the amplitude of carrier is equal to its un-modulated value.

(a)

(b)

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the amplitude of carrier is varied by the instantaneous value when the modulation is present. the amplitude of “ the top envelope “ of the AM wave is given by the relation

A = V_c + V_m SIN W_m*T.

Similarly the maximum negative amplitude of the bottom envelope is given by

–A = –(V_c + V_m SIN W_m*T).

This is shown in fig c where one cycle of the modulating sine-wave is shown, it extends between two limiting envelopes and has a repetition rate equal to the un-modulated carrier frequency. It is mention that the amplitude of both positive and negative half cycles of carrier wave are changed in accordance with signal. for the instant when the signal is increasing in positive sense, the amplitude of carrier wave also increases. On the other hand,during negative half cycle of the signal, amplitude of carrier wave decreases. amplitude modulation is done by an electronic equipment called modulator. In short we can say that;

1. According to the intensity of the signal, the amplitude of carrier wave changes.
2. Modulating wave or signal frequency f_m governs the amplitude variations of the carrier wave.
3. the frequency of amplitude modulated wave remains same to the carrier frequency.