Harmonic backscattering is an attractive technique in very-low power communication scheme. The hardware implementation is extremely simple due to the absence of selfjamming issue (as compared to fundamental backscattering communication) and it can even allow full-duplex communication. Unfortunately, the conversion power losses from fundamental to harmonic frequency is hampering the communication distance. This paper reports a tunnel diode-based reflection amplifier used in its nonlinear negative resistance region. Measured under an input power of -60dBm, it has a 33dB gain, which can enhance the communication distance by a factor of 5.6. The amplifier operate under 200mV voltage supply, which can easily be provided by ambient RF energy harvesting.