The region between two concentric spheres of radii 'a' and 'b',

The region between two concentric spheres of radii 'a' and 'b', respectively (see figure), has volume charge density $\rho=\frac{\mathrm{A}}{\mathrm{r}}$, where $\mathrm{A}$ is a constant and $\mathrm{r}$ is the distance from the centre. At the centre of the spheres is a point charge $Q$. The value of A such that the electric field in the region between the spheres will be constant, is :-