This paper presents the analysis and design of voltage and current amplifiers for use in protective relay testing for frequencies up to the 20th harmonic. A typical topology of DC/AC 50/60Hz converters was used to achieve the nominal and peak values produced by conventional current and voltage transformers. The stability analysis is devised considering different requirements of load and frequency response for relay inputs when compared to that of a typical single-frequency power converter. The temporal response to signals with abrupt changes and multiple decaying harmonics is also investigated. Proper test signals are stored in a PC microcomputer and played-back using the analog audio output, which is conditioned to the proposed amplifier input. The amplifier's controller was implemented using a DSP with PWM and an IGBT H-bridge. Simulations and experimental results are presented for typical faults and multi-harmonic signals.