  The disk corona evaporation model extensively developed for the
interpretation of observational features of black hole X-ray binaries (BHXRBs)
is applied to AGNs. Since the evaporation of gas in the disk can lead to its
truncation for accretion rates less than a maximal evaporation rate, the model
can naturally account for the soft spectrum in high luminosity AGNs and the
hard spectrum in low luminosity AGNs. The existence of two different luminosity
levels describing transitions from the soft to hard state and from the hard to
soft state in BHXRBs, when applied to AGNs, suggests that AGNs can be in either
spectral state within a range of luminosities. For example, at a viscosity
parameter, \alpha, equal to 0.3, the Eddington ratio from the hard to soft
transition and from the soft to hard transition occurs at 0.027 and 0.005
respectively. When the Eddington ratio of the AGN lies below the critical value
corresponding to its evolutionary state, the disk is truncated. With decreasing
Eddington ratios, the inner edge of the disk increases to greater distances
from the black hole with a concomitant increase in the inner radius of the
broad line region, $R_{BLR}$. The absence of an optically thick inner disk at
low luminosities gives rise to region in the size of borad line-luminosity
plane for which the relation $R_{BLR} \propto L^{1/2}$ inferred at high
luminosities is excluded. As a result, a lower limit to the accretion rate is
predicted for the observability of broad emission lines, if the broad line
region is associated with an optically thick accretion disk. Thus, true Seyfert
2 galaxies may exist at very low accretion rates/luminosities. The differences
between BHXRBs and AGNs in the framework of the disk corona model are discussed
and possible modifications to the model are briefly suggested.
