  The effects of pressure up to $\sim 20$ kbar, on the structural phase
transition of SrFe$_2$As$_2$ and lightly Sn-doped BaFe$_2$As$_2$, as well as on
the superconducting transition temperature and upper critical field of
(Ba$_{0.55}$K$_{0.45}$)Fe$_2$As$_2$ single crystals have been studied. All the
transition temperatures decrease with pressure in an almost linear fashion.
Under pressure, the upper critical field curve, $H_{c2}(T)$, for
(Ba$_{0.55}$K$_{0.45}$)Fe$_2$As$_2$ shifts down in temperature to follow the
zero field $T_c$ with very little change in slope. Composite $P - T$ phase
diagrams for three parent compounds, AFe$_2$As$_2$ (A = Ba, Sr, Ca), are
constructed and appear to be remarkably similar: (i) having a structural
(antiferromagnetic) phase transition line with a negative slope and (ii)
showing signs of the emerging superconducting state at intermediate pressures.
