Removing the effects of baseband impedance on distortion in FET amplifiers

The baseband drain node impedance of FETs has previously been found to influence the distortion produced by amplifiers employing such devices. For a two tone test, variations in this impedance lead to variations in the magnitude and phase of the intermodulation distortion products. This results in distortion characteristics which vary with tone separation, and consequently limits the performance of many linearisation schemes. A scheme capable of removing the dependency of intermodulation distortion on baseband drain node impedance is presented, and operates over a bandwidth of 30 MHz. Practical tests conducted on a real amplifier subjected to a two tone input signal show that the scheme can suppress phase asymmetry variations of up to 70 degrees.