The region between two concentric spheres of radii $‘a’$ and $‘b’$,
respectively (see figure), has volume charge density $\rho = \large\frac{A}{r}$, where A is a constant and 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 :