In a practical AM transmitter it's easier than it seems. In an AM CB radio, when switched to transmit, the audio amplifer output transformer secondary is disconnected from the speaker and transferred to the supply rail that powers the transmitter output stage. When there is silence, the supply current passes through the secondary as though it was a peice of wire, the transmitter receives 12 V.
This is called "high level modulation". It's the most economic way when there's already an amplifier and transformer. When speaking, voice-shaped currents from the transformer secondary vary the supply voltage of the transmitter output stage. If the voltage swing on the transmitter supply rail reaches +24V and 0V, there would be 100% modulation.
OK, you can low-level modulate, synthesise the AM signal with a multiplier IC, but if the modulator is followed by RF amplifier stages, they have to be designed to behave linearly, otherwise the modulation is distorted.
It all depends on what the application is, to decide what's best for your circuit, can you post some more info?