This paper presents two interface circuit designs that improve the piezoelectric energy harvesting performance at offresonance. Complex impedance matching at different frequencies is achieved by introducing delay into the conventional switching methods. The proposed techniques can also be used in other applications, such as electromagnetic energy harvesting. The system is analyzed using equivalent impedance at fundamental of operating frequency. Design considerations such as two possible modes of operation, sensitivity to voltage and timing errors, loss due to nonlinear operations, and conduction loss of switches are detailed. As compared with the conventional methods, analytical results show that, with the electromechanical coupling coefficient k2e of 0.076 and the mechanical damping ratio ζm of 0.02, the proposed techniques can improve the 3-dB bandwidth by 76% with a flipping quality factor of 10. Experimental results with a commercially available piezoelectric device show a 23% improvement in 3-dB bandwidth and a 66% improvement in extracted power with 5% of frequency mismatch.