Basic AM modulator circuit?
One common way of doing AM modulation is to multiplying a baseband signal with a square wave that is at the carrier frequency, and then run the chopped baseband signal through a band-pass-filter tuned to the carrier frequency.

The reason this is done is that multiplying by a square wave is pretty easy to do with as little as one high frequency switching transistor.

If the signal plus DC bias is connected through a resistor to the source of a FET, and a square wave at the carrier frequency is connected to the gate, then you will get multiplication by a square wave.  When the FET is off the output signal is equal to the input (multiplied by 1), when the FET is on the output is 0 (multiplied by 0).

A square wave at the carrier frequency can be decomposed into an infinite series of sinewaves that are multiples of the carrier frequency.  So if we multiply the input by a square wave at the carrier then the output is equal to the input multiplied by a sine-wave at the carrier (which is exactly what we want for AM), plus the input multiplied by sine-waves at the harmonics of the carrier (which is something we don't want).

If the output of the chopper is run through a band pass filter which passes only the signals near the carrier, then the signals at the harmonics are rejected and we are left with only what we want (the AM modulated signal).

See...
http://homepages.udayton.edu/~hardierc/ECE401/ECE401L/ECE_401L_Lab2.pdf

http://www.analog.com/library/analogDialogue/archives/43-09/EDCh%204%20rf%20if.pdf