Advantages of Second-Order Cartesian Feedback Linearizers for Radio Amplifiers

Cartesian feedback (CF) linearization for radio amplifiers is appealingly simple, but it imposes a trade-off between signal bandwidth and feedback delay. In almost all of the CF literature, it is a first-order circuit. In this paper, we examine two second-order CF linearizers and show that they can provide deeper suppression of intermodulation distortion or greater signal bandwidth at a given loop delay. Twin-loop CF with damping factor 0.707, for example, can provide 20 dB lower intermodulation distortion and half the error vector magnitude (EVM), relative to first-order CF. For twin-loop CF and proportional-integrator CF, we provide the first analyses of their behavior, stability limits, and linearization performance. Comparison with simulations using nonlinear amplifier models shows both the value and the shortcomings of linear analysis. Our new spectral separation approach leads to insights about linearization and EVM.