  We analyze the X-ray spectra and their timing properties of the compact X-ray
binary 4U~1820-30. We establish a number of spectral transitions in this source
seen with BeppoSAX and RXTE. During the RXTE observations (1996 -- 2009), the
source were approximately 75% of its time in the soft state making the lower
banana and upper banana transitions combined with long-term low-high state
transitions. We reveal that all of the X-ray spectra of 4U~1820-30 are fit by a
composition of a thermal (blackbody) component, a Comptonization component
(COMPTB) and a Gaussian-line component. Thus using this spectral analysis we
find that the photon power-law index Gamma of the Comptonization component is
almost unchangeable (Gamma~2) while the electron temperature kT_e changes from
2.9 to 21 keV during these spectral events. We also establish that for these
spectral events the normalization of COMPTB component (which is proportional to
mass accretion rate, \dot M increases by factor 8 when kT_e decreases from 21
keV to 2.9 keV. Before this index stability effect was also found analyzing
X-ray data for Z-source GX 340+0 and for atolls, 4U~1728-34, GX~3+1. Thus, we
can suggest that this spectral stability property is a spectral signature of an
accreting neutron star source. On the other hand in a black hole binary Gamma
monotonically increases with \dot M and ultimately its value saturates at large
\dot M.
