Pen,
AM - increased depth of modulation increases sideband power and thus total transmitted power.
FM - total transmitted power always constant but increased depth of modulation requires increase in bandwith. The differnce is the bandwith required to transmit a relatively undistorted signal. Increased depth of modulation means increased deviation and therefore an increased modulation index. More distant sidebands require significant amplitudes (of modulation).
Does this help? Not from memory!!! I have a text book to hand!
[ 12 August 2001: Message edited by: Bus429 ]