Most investigations of feedback control of magnetically confined plasma have used the approach in which the control variable is the perturbed plasma density. In this paper, the use of another basic type of feedback control, namely the (φ-φ) type, in which the control variable is the local potential perturbation is examined. The basic equations for the study of electrostatic oscillations in inhomogeneous magnetized plasma are deduced and the results are applied to some known mirror instabilities. It is found that for present-day amplification systems the threshold densities set by microinstabilities can be increased by factors of the order of 100 or more and that, in some cases, geometric stabilization can also be achieved by feedback control.