A small signal model is a linear model which is independent of amplitude. It may or may not have time dependence (i.e. capacitors).

The small signal model for a nonlinear component such as a BJT is a linear model about some nominal operating point. The deviations from the operating point are small enough that it approximates the nonlinear component over a limited range of amplitudes

Simple Small Signal BJT Model Implementing relationships,$ i c = g_m v_i, i c = g_o v_{ce}, and v_i = r_πi_b$, into a schematic model gives,

Note that the small signal model is the same for either a npn or a pnp BJT.