Stability analysis of Cartesian feedback linearisation for amplifiers with weak nonlinearities

RF amplifier linearisation by Cartesian feedback is analysed using a multiple input multiple output (MIMO) model. The effects of RF amplifier distortion, namely AM/AM and AM/PM distortion, are shown to degrade the stability of the Cartesian feedback loop. A versatile technique is presented that allows the amount of stability degradation to be determined for RF amplifiers provided distortion is not too large (i.e. class A to class AB amplifiers). Loops with low stability margins are shown to exhibit out-of-band noise peaking. It is concluded from the noise analysis undertaken, that the highest proportion of gain possible should be placed ahead of the loop compensation to minimise out-of-band noise