  We apply a method based on a BCS-type approach to the extended Hubbard model
to deal with the YBa_2Cu_3O_{7-\delta} superconductor family under pressure.
The parameters of the tight-binding band are taken from experiments, and the
coupling strength U and V are estimated from the zero pressure phase diagram Tc
X n_h, where n_h is the hole content and Tc the critical temperature. With the
assumption that the pressure P modifies the magnitude V of the attractive
potential besides n_h, we obtain a general expansion of Tc in terms of the
pressure P, as Tc(P)=Tc(n_h(P),V(P)). With this we can distinguish the charge
transfer and the intrinsic contribution to Tc. We show that the changes of Tc
with P for YBa_2Cu_3O_{7-\delta} are almost due to the intrinsic term.
