The main function of the preamplifier is to amplify the very smallcharge or current signal likean order of 1fC or 1pC and it usually has a output dynamic range of 0-1 V or so.Most semi-conductor detector needs preamplifier.
On the other hand, the function of the amplifier is simple.Just amplify the voltage signal and sometime perform the waveform shaping.Its output dynamic range is 0-10V or so.
Usual diagram of the circuit is below.
detector => preamplifier => amplifier
by Shinsuke Ota