This thesis reports the results of feedback experiments on the m/n = 2/1 tearing mode in HBT-EP. The feedback algorithm is implemented in a digital-signal processing (DSP) computer with an output sampling rate of 100 kHz. The algorithm produces control signals by adjusting the gain and phase of a quadrature measurement of the 2/1 mode. Two high-power amplifiers drive a two-phase set of saddle coils with small toroidal extent (12° per phase) to produce a rotating resonant control perturbation. A normalized model of nonlinear 2/1 mode behavior is described and used to compare simulated mode dynamics to experimental data. A frequency-dependent stabilization of the mode amplitude is included in the model and observed experimentally. Two magnetic and two optical quadrature mode detection methods are compared. A novel technique using four poloidal Mirnov probes located near the plasma surface is the best performer. The fields produced by the highly-localized saddle coil set and the strength of its resonance with the 2/1 mode are determined with a 2D numerical model. The phase instability, an exponential growth of a feedback phase error away from negative feedback, is investigated and measured unstable periods show excellent agreement with theoretical predictions. The feedback system can slowly attenuate saturated 2/1 modes, but cannot prevent the growth of unsaturated modes. The phase instability decreases phase accuracy as gain increases, limiting feedback performance.