Nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR)-based chemical analysis systems have been widely utilized in various areas such as medicine, security, and academic research. In these applications, the power amplifier stage plays a key role in generating the required oscillating magnetic fields within a radio frequency coil that serves as the probe. However, the bulky size and relatively low efficiency of the traditional power amplification schemes employed present a bottleneck for the realization of compact sized and portable NMR and NQR systems. To address this problem, this work presents a class D voltage-switching power amplification scheme with novel fast-start and fast-stop functions that are suitable for generating ideal NMR and NQR excitation signals. Compared to the traditional analog power amplifiers (PAs), the proposed switched-mode PA can achieve significant improvement on the power efficiency as well as the physical volume. A PA circuit for portable NQR-based explosive detection systems has been designed and built using the proposed scheme with 1 kW possible maximum output power and 10 MHz maximum operating frequency. Test results show that the presented PA achieves more than 60% measured efficiency within a highly compact volume while sustaining fast start and stop of excitation signals in the order of microseconds.