AM modulation with BJT transistor .. need help please

please take a look at the following circuit :

http://www.zen22142.zen.co.uk/spice/ammod.htm

my question is what is the mechanism used to evaluate R1 , R2 , R4,R5 and C3 for the circuit to work as an AM modulator? how does the circuit work ?should the transistor work in the active region ?

if u can give me simple equations of how to calculate R's and C3 this will be very appreciated.
thank you.

Unmodulated carrier comes in at "Vcarrier". Modulated carrier goes out at the mislabeled "Vmod". Audio voltage is AC coupled in at "Vaudio", and as the audio voltage goes up and down, it changes the bias point of the amplifier transistor--thereby varying the carrier level. With no audio applied, the output should be a carrier with 50% of its voltage amplitude. That way the carrier can be varied +/- 50% from the set point.