The quality factor (Q -factor) of a coil can be improved by using a negative impedance converter (NIC) with transistors. Wireless power transfer (WPT) system with the high Q -factor coil can have extremely enhanced power transfer efficiency (PTE) and received power at the receiver side. The NIC is composed of a bipolar junction transistor (BJT), a voltage divider, a resonance circuit, elements for impedance matching, and a tunable impedance. The negative impedance at 6.78 MHz can be obtained by using the tunable impedance in the NIC, and it can be adjusted as desired by the user. As a result, the loss of the transmitting coil is significantly reduced from 2 Omega to 0.1 Omega The Q -factor of the coil is enhanced from 370 to 7,067, which is 19.1 times higher than that of the typical coil. The measured PTE is 96% and 79% at a transmission distance of 50 and 200 mm, respectively. Hence, the performance of a WPT system, which uses an NIC with a BJT, demonstrates that it can be applied to mobile applications.