One H-bridge is used to control one coil in a traditional magnetic bearing power amplifier. In order to reduce the number of bridge arms, some of the bridge arms can be shared with vector control technology. In this paper, a power amplifier using three-leg circuit to control two coils simultaneously is studied, and the space vector pulse width modulation (SVPWM) algorithm is implemented with FPGA, which reduces the number of IGBT by 25% . Aiming at the possibly occured situation that the command signal of power amplifiers may exceed the tracking range, this paper analyzes the tracking range of the power amplifier with a three-leg circuit, and points out that the performance of the power amplifier can not be fully utilized using traditional duty cycle restriction strategies and the distortion will be increased. This paper improves the traditional duty cycle restriction strategy, which makes the utilization ratio of the power amplifier reach 100% , and can completely eliminate the distortion of one coil at most. On this basis, two duty cycle restriction strategies with different performance are proposed. Experiment results show that the power amplifier works well when the command signals are within the tracing range, and both duty cycle restriction strategies are effective and achieve the design performance when the command signals are out of the tracking range.