A Stand-Alone Distortion-Cancelling Cell for Microwave Amplifiers

This letter presents a standalone circuit (cell) to reduce the third-order inter-modulation distortion (IMD) produced in microwave amplifiers. The cell splits the output of the amplifier into two paths. In one path, the amplified signal and the IMD tones pass through toward the output. In the other path, an auxiliary FET device biased near pinchoff and a tunable phase shifter reproduce the amplifier's IMD tones but with the opposite phase as the original tones. When the IMD tones in the two paths are combined at the output, they cancel out and the input third-order intercept point (IIP
₃
) of the amplifier is improved. The cell requires only two tuning voltages: one for the auxiliary FET and another for the variable phase shifter. Experimental test results show that the cell was able to improve the IIP
₃
of a commercial, off-the-shelf, microwave amplifier from +9.5 dBm to +17.5 dBm.