This paper describes the results of several experiments conducted on a Cartesian feedback amplifier with soft landing. This amplifier uses a new method for adaptively controlling the local signal phase in a Cartesian demodulator. We tested the soft-landing Cartesian feedback (SL-CFB) method in the 900 MHz band using d4-shifted QPSK modulation to determine its applicability for pocket terminals in Japanese digital cellular systems.We developed a smooth loopgain control mechanism using a variable gain amplifier with an FET and a continuous phase-shifter control circuit for digital processing in the IF band. We showed that the SL-CFB method can maintain the optimum local signal phase in the demodulator. This method therefore solves the problem at applying CFB amplifiers. We adopted circuit configurations that are easy to incorporate into LSI chips so that the amplifier can be used in compact terminals.The results show that the SL-CFB amplifier is a promising candidate for developing highly efficient linear amplifiers, which are very important in digital cellular telephone terminals.
This paper describes the use of a nonlinear distortion canceller (NLDC) with a Cartesian feedback loop to improve the output backoff of an 800-MHz GaAs FET power amplifier when a QPSK signal with a rolloff factor of 0.5 is applied. This amplifier module (Fujitsu FMC080802-04) uses three GaAs FETs. The amplifier's saturated power is 31 dBm, and its maximum power-added efficiency is 50 X .Excellent operation of the NLDC was confirmed within a backoff range of more than 4 dB. Within this range, the out-ofband emission was kept 55 dB below the center frequency level. Since the peak output power coincides with the saturation power at a 4 dB backoff, the NLDC improves the backoff to the theoretical limit.
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