EFFECTS OF IMPEDANCE MISMATCH AND IMPERFECT ISOLATION ON DISTORTION CANCELLATION IN FEEDFORWARD AMPLIFIERS

Feedforward linearisation offers viable way to reduce intermodulation distortion generated in power amplification, the actual reduction depe nding on the degree of cancellation in the feedforward cancellation lo ops. Commonly, cancellation is calculated from design formulae develop ed by assuming perfect impedance matches between subcircuits and perfe ct isolation in the power splitting/combining networks, but these are difficult to achieve in practice. The effects that mismatches and impe rfect 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 circu its to compensate for these effects is also examined and simulation sh ows that mismatch and imperfect isolation can effectively be compensat ed over a very narrow frequency band by this technique and good cancel lation achieved, though it becomes less effective as bandwidth increas es. Experimental results are presented to verify the simulation result s.