Hi mike_bihan,
when a feedback circuit is analyzed, the load effect of the feedback network must be included in the calculation of the open loop response. The feedback network affects both the input and the output of the open loop transfer function. This is readily considered suppressing the feedback at the input for looking at its load effect at the output, and conversely for the input.
This is well explained in an oooold books of the SEEC series (I guess in volume 5) and in the Gray-Searle book, but certainly there must be also in other bibliography. Look for the four topologies of feedback circuits.
Regards
Z