A.M Modulator Circuit Diagram

Amplitude modulation is a process in which the amplitude of a carrier wave c(t) is varied about a mean value, linearly with the base-band signal m(t).
 
In amplitude modulation the amplitude of a high-frequency carrier is varied in direct proportion to the low-frequency (base-band) message signal. The carrier is usually a sinusoidal waveform that is,

C(t) = Ac . cos(wt)

Where,  is the carrier amplitude and f_c is the carrier frequency.

The base-band signal or message signal is,

m(t) = A_{m .} cos(wt) 
Where, A_m is the amplitude of message signal and f_m is the frequency of message signal.

An amplitude modulation wave may thus be described in its most general form as the function of the time as follows,

S(t) = m(t)×c(t)