Linearity of X-band Class-E Power Amplifiers in a Digital Polar Transmitter

This paper discusses the linearity of a class-E X-band PA in a digital polar transmitter. The PA is nonlinear since it is compressed by 2.2 dB when achieving a 59% PAE at 10 GHz. Load-pull is performed under a two-tone test and the resulting optimal efficiency impedance is found to be over 30% different from the single-tone load-pull. In addition standard two-tone load-pull measurements indicate that the impedance for max P_out coincide with worst nonlinearity (IMD). Surprisingly, in the two-tone polar load-pull, impedance for max P_out differs significantly from the impedance corresponding to worst IMD. Therefore, under polar modulation a different matching circuit should be designed for optimal linearity and efficiency. The PA is part of a digital polar transmitter which enables linearization including phase predistortion, with IMD levels over 11 dB lower than in the standard two-tone test. Load-pull performed on the polar class-E PA shows that the level of intermodulation products varies for different signals input into the PA.