difference
in small signal u asssumed that the circuit behaviour doesnt change with the input signal i.e. the o/p is linear with input , which what we did as we represent the circuit by a model that have the parameters of the dc analysis i.e. we linearized the behaviour around the q-point which can be accepted in small signal model as the i/p signal is small and we can consider the non-linear o/p c/c's as a straight line as some kind of approximation,
while in large signal the approximation is not valid anymore and we have to take the non-linear c/c's and effects into our consideration.
regards,
a.safwat