This letter proposes a novel design approach of wideband 90 ^° phase shifter, which comprises a stepped impedance open stub (SIOS) and a coupled-line with weak coupling. The result of analyses demonstrates that the bandwidths of return loss (RL) and phase deviation (PD) can be expanded by increasing the impedance ratio of the SIOS and the coupling strength of the coupled-line. For RL > 10 dB, insertion loss (IL) 1.1 dB, and PD of ±5^°, the fabricated microstrip single-layer phase shifter exhibits bandwidth of 105% from 0.75 to 2.4 GHz.

This letter presents a wideband power amplifier (PA) with high efficiency integrated filtering characteristic. A novel bandpass filtering impedance transformer was proposed, which consists of a terminated coupled line structure (TCLS) and impedance tuning network (ITN), to realize the input matching network (IMN) and output impedance matching network (OMN). To enhance the efficiency of PA, the second harmonic has also been controlled perfectly. This design method can not only provide high-selectivity bandpass responses and high efficiency but also broaden the operation band. For demonstration, a wideband filtering PA operating at 2.2 GHz with a bandwidth of 400 MHz by employing a 10-W GaN HEMTs is fabricated. Measured results show the maximum drain efficiency (DE) is 68.3%-78.4% in the design band. Meanwhile, a good filtering performance was obtained. The rejection level was larger than 52.8 and 31 dB for the lower and higher suppression bands, respectively. In addition, simulation results show that the IMD2 and IMD3 were also improved simultaneously. In particular, the IMD2 value was improved about 30-40 dB.

In this letter, an adaptive basis direct learning (DL) method was proposed for the linearization of power amplifiers (PAs). The proposed method can reduce the complexity and improve the stability performance of DL digital predistortion (DPD) structure. The experimental results show that the proposed method is more stable and can achieve improvement in normalized mean square error (NMSE) and adjacent channel power ratios (ACPRs) compared with the DL. In addition, the proposed method reduces the number of coefficients by 87%.

This letter presents a cascaded neural network (NN) module behind a traditional NN digital predistortion (DPD) model to track the power amplifiers’ (PAs’) behavioral change under various operating conditions. Using the various operating conditions to control the weights of the cascaded NN’s last hidden layer, the DPD model can achieve excellent linearization performance without coefficient update in large-scale operating conditions. Since the number of controlled coefficients is relatively small, the proposed technique has higher performance and lower running complexity than existing NNs. The experiments were carried out on a commercial PA. Compared with the state-of-the-art NN models, the experimental results show that the proposed technique can improve linearization performance with lower complexity.

In this letter, a modified band-limited digital predistortion (DPD) technique is proposed to improve the performance of the conventional band-limited DPD method in wideband systems. The power amplifier's (PA's) spectral regrowth outside the passband of bandpass filter is unable to be compensated using the traditional band-limited DPD. To increase the overall linearized bandwidth and reduce the requirement for feedback signal path, this new technique introduced two modules of full-band model extraction and coefficient iteration based on the conventional structures of band-limited DPD. It provides higher linearization performance and also reduces the sampling rate of analog-to-digital converter. The experimental results for 30-W Class F PA and 40-W GaN Doherty PA with 40-MHz orthogonal frequency division multiplexing signal and 60-MHz long-term evolution signal, respectively, have shown the superiority of the modified band-limited DPD.