Effects of impedance mismatch and imperfect isolation on distortion cancellation in feedforward amplifiers

Feedforward linearisation offers a viable way to reduce intermodulation distortion generated in power amplification, the actual reduction depending on the degree of cancellation in the feedforward cancellation loops. Commonly, cancellation is calculated from design formulae developed by assuming perfect impedance matches between subcircuits and perfect isolation in the power splitting/combining networks, but these are difficult to achieve in practice. The effects that mismatches and imperfect isolation have on cancellation are studied and investigated and it is shown that the transmission characteristics of the cancellation loops can deviate significantly from those calculated using the above assumptions, resulting in poorer cancellation. The use of tuning circuits to compensate for these effects is also examined and simulation shows that mismatch and imperfect isolation can effectively be compensated over a very narrow frequency band by this technique and good cancellation achieved, though it becomes less effective as bandwidth increases. Experimental results are presented to verify the simulation results