Improvement of TWT Nonlinearity with a Field Effect Transistor Predistortion Circuit

In this paper, a Field Effect Transistor (FET) operating in close to cut off frequency is used to replace the attenuator and the phase-shifter in most TWT predistortion circuit, which can realize good control for the amount of gain expansion with little phase expansion variation. The experiments with practically designed circuit show that, in the frequency range of 8.38~8.58 GHz, about 6 dB gain expansion and 45 degrees phase shift can be compensated for the TWTA saturation response in input power dynamic range, and the improvement of about 15.4 dB Carrier to Intermodulation (C/IM) at TWTA Output Power Back Off (OPBO) of only -3 dB can be reached with the linearizer.