A design of predistortion linearizer using 2nd order low frequency intermodulation signal injection

This paper presents a new predistortion method which injects the 2^nd order low frequency intermodulation signal of the RF signal into the input bias line of the amplifier. We show this technique can suppress the 3^rd IM and the 5^th IM apparently by adjusting 2 factors, phase and amplitude, through mathematical analysis, and then confirm it with simulation. Also, we design a digital cellular amplifier to show variation of the optimal performance without additional circuits or controls for the operating range. When the input signal CDMA IS-95 1FA was applied, measured ACPR improvements were 25 dBc, 22.5 dBc, and 6 dBc at 0.855 MHz, 1.25 MHz, and 2.25 MHz offset respectively. Also, when applying the CDMA IS-95 3FA, the measured ACPR improvement showed 16 dBc at 0.885 MHz offset.