AM stands for Amplitude Modulation where the amplitude of the carrier wave is varied according to the message information. There are many circuits which can create amplitude modulated signal. To explain the working principle of AM, one such simple AM circuit is shown below.
In the circuit shown, the two signals, one 500Hz(within the frequency range of speech) and a 500KHz of the desired carrier signal is to be mixed and sent to the antenna.
The audio signal is first amplified by the lower transistor. Then amplified audio signal is coupled with the carrier signal using the lower transformer and the LC tank. It is in the LC tank where the mixing takes place . This process of audio signal flow in the circuit is shown by the picture below-
The upper part is a Tuned RF Amplifier is indicated in the picture below.
The flow of carrier signal into the LC tank is shown below.
The process of mixing between the audio signal and the carrier signal takes place in the LC tank. This results in AM signal which is radiated into space via the antenna.