Zhijiang Dai

Bio: Zhijiang Dai is an academic researcher from Chongqing University. The author has contributed to research in topics: Amplifier & Computer science. The author has an hindex of 13, co-authored 32 publications receiving 540 citations. Previous affiliations of Zhijiang Dai include University of Electronic Science and Technology of China.

A Post-Matching Doherty Power Amplifier Employing Low-Order Impedance Inverters for Broadband Applications

Abstract: This paper presents a modified Doherty configuration with extended bandwidth. The narrow band feature of the conventional Doherty amplifier is discussed in the view of the broadband matching. To extend the bandwidth, the post-matching architecture is employed in the proposed design. Meanwhile, broadband low-order impedance inverters are adopted to replace the quarter-wavelength transmission lines. Low-pass filter topologies are used to realize both the post matching network and the impedance inverters. A modified Doherty Power amplifier was designed and fabricated based on commercial GaN HEMT devices to validate the broadband characteristics of this configuration. The 6-dB backoff efficiencies of 47%–57% are obtained from 1.7 to 2.6 GHz (41.9% fractional bandwidth) and the measured maximum output power ranges from 44.9 to 46.3 dBm in the designed band. In particular, more than 40% efficiencies are measured at 10-dB backoff throughout the operation band.

A New Distributed Parameter Broadband Matching Method for Power Amplifier via Real Frequency Technique

Abstract: A general matching method is presented in this paper for broadband power amplifier (PA) design. A novel cost function is proposed for the real frequency technique (RFT), which could straightforwardly describe PA optimal impedance along with frequency change. The new function is also developed to design a broadband transformer for the PA output matching network (MN). Based on Richard transformation, a commensurate transmission line is deployed so that the PA matching could be convenient expressed by a real positive function. More important is that the function could be directly implemented with a distributed MN through synthesis theory. Therefore, this method is practical for computer-aided design and has less calculation amount with new function for the RFT. To verify the method, a step-by-step design of a broadband PA is given. For large signals, power gain is 14.2–16.8 dB across 0.9–2.8 GHz, while output power is around 39.5 dBm. The maximum power-added efficiency is from 52.2% to 85.1%.

Design of a Post-Matching Asymmetric Doherty Power Amplifier for Broadband Applications

Abstract: In this letter, the design of a broadband asymmetric Doherty power amplifier (ADPA) with an 800 MHz (41% fractional) bandwidth is presented. The post-matching structure and low-order impedance transformation networks (ITN) are employed to achieve the broadband performance. Meanwhile, different drain biases on the main and peaking devices are used to run the asymmetric operation. The proposed ADPA shows high-efficiency performance at 8–9 dB output power back-off (OBO) throughout the whole 800 MHz band. The ADPA has been designed and implemented using commercial GaN HEMTs to validate the OBO and broadband characteristics. Maximum output power ranges from 43.7 to 45.2 dBm, 50.4%-56.2% efficiencies at 8–9 dB OBO are measured from 1.55 to 2.35 GHz.

Analysis and Design of Highly Efficient Wideband RF-Input Sequential Load Modulated Balanced Power Amplifier

Abstract: The analysis and design of an RF-input sequential load modulated balanced power amplifier (SLMBA) are presented in this article. Unlike the existing LMBAs, in this new configuration, an over-driven class-B amplifier is used as the carrier amplifier while the balanced PA pair is biased in class-C mode to serve as the peaking amplifier. It is illustrated that the sequential operation greatly extends the high-efficiency power range and enables the proposed SLMBA to achieve high back-off efficiency across a wide bandwidth. An RF-input SLMBA at 3.05–3.55-GHz band using commercial GaN transistors is designed and implemented to validate the proposed architecture. The fabricated SLMBA attains a measured 9.5–10.3-dB gain and 42.3–43.7-dBm saturated power. Drain efficiency of 50.9–64.9/46.8–60.7/43.2–51.4% is achieved at 6-/8-/10-dB output power back-off within the designed bandwidth. By changing the bias condition of the carrier device, higher than 49.1% drain efficiency can be obtained within the 12.8-dB output power range at 3.3 GHz. When driven by a 40-MHz orthogonal frequency-division multiplexing (OFDM) signal with 8-dB peak-to-average power ratio (PAPR), the proposed SLMBA achieves adjacent channel leakage ratio (ACLR) better than −25 dBc with an average efficiency of 63.2% without digital predistortion (DPD). When excited by a ten-carrier 200-MHz OFDM signal with 10-dB PAPR, the average efficiency can reach 48.2% and −43.9-dBc ACLR can be obtained after DPD.

Digital Predistortion for Power Amplifier Based on Sparse Bayesian Learning

Abstract: In this brief, a sparse-Bayesian-learning algorithm is applied to estimate the coefficients of the power amplifier (PA) behavioral models and inverse models from the view of probability. With this sparse learning method, the needed number of samplings can be reduced significantly. In addition, it also provides researchers with ideas that obtain the needed subspace of the preselected model. The performance of the algorithm is validated experimentally on a gallium nitride (GaN) PA, and the signal used to test the proposed approach is an Long Term Evolution (LTE) signal. A comparison with the state-of-the-art estimation algorithm in an open-loop digital-predistortion system is also presented, and the vast majority of tests show that the number of model coefficients is reduced by at least 50%.

IEEE Transactions on Microwave Theory and Techniques and Antennas and Propagation Announce a Joint Special Issue on Ultra-Wideband (UWB) Technology

Abstract: Before the emergence of ultra-wideband (UWB) radios, widely used wireless communications were based on sinusoidal carriers, and impulse technologies were employed only in specific applications (e.g. radar). In 2002, the Federal Communication Commission (FCC) allowed unlicensed operation between 3.1–10.6 GHz for UWB communication, using a wideband signal format with a low EIRP level (−41.3dBm/MHz). UWB communication systems then emerged as an alternative to narrowband systems and significant effort in this area has been invested at the regulatory, commercial, and research levels.

A Post-Matching Doherty Power Amplifier Employing Low-Order Impedance Inverters for Broadband Applications

Abstract: This paper presents a modified Doherty configuration with extended bandwidth. The narrow band feature of the conventional Doherty amplifier is discussed in the view of the broadband matching. To extend the bandwidth, the post-matching architecture is employed in the proposed design. Meanwhile, broadband low-order impedance inverters are adopted to replace the quarter-wavelength transmission lines. Low-pass filter topologies are used to realize both the post matching network and the impedance inverters. A modified Doherty Power amplifier was designed and fabricated based on commercial GaN HEMT devices to validate the broadband characteristics of this configuration. The 6-dB backoff efficiencies of 47%–57% are obtained from 1.7 to 2.6 GHz (41.9% fractional bandwidth) and the measured maximum output power ranges from 44.9 to 46.3 dBm in the designed band. In particular, more than 40% efficiencies are measured at 10-dB backoff throughout the operation band.

Symmetrical Doherty Power Amplifier With Extended Efficiency Range

Abstract: A symmetrical Doherty power amplifier (PA) with an extended efficiency range is proposed. This paper proves the existence of a class of symmetrical Doherty PAs having efficiency peaks for back-off levels larger than 6 dB. A design technique is developed that maintains the full voltage and current swings of both the main and auxiliary transistors. The concept is experimentally verified in a 1.95-GHz 25-W circuit demonstrator fabricated using identical GaN HEMT devices. An average power-added efficiency of 50% and adjacent power leakage ratio of $-{\hbox{49 dBc}}$ is obtained with 9-dB peak-to-average power-ratio 20-MHz long-term evolution test signals.

Highly Efficient Broadband Continuous Inverse Class-F Power Amplifier Design Using Modified Elliptic Low-Pass Filtering Matching Network

Abstract: This paper proposes a design approach for a broadband and high-efficiency continuous inverse Class-F ( ${\text {CCF}}^{-1}$ ) power amplifier (PA) based on a modified elliptic low-pass filtering (LPF) matching network (MN). From theoretical and practical perspectives, the importance of a swift impedance transition from the higher end of the fundamental frequency band to the lower end of the second harmonic band is discussed, when designing a broadband single-mode PA. After being compared with widely used Chebyshev LPF MNs, a modified elliptic LPF MN, which provides a sharp roll-off, is utilized to provide the required rapid transition. A step-by-step design procedure of the proposed modified elliptic LPF MN is presented. Experimental results show that a high-efficiency ${\text {CCF}}^{-1}$ PA is realized from 1.35 to 2.5 GHz (fractional bandwidth = 60%) with measured drain efficiency of 68%–82% and output power of 41.1–42.5 dBm. When stimulated by a 20-MHz LTE signal with an average output power of approximately 34.5 dBm, the proposed PA, combined with digital pre-distortion, achieved adjacent channel leakage ratios (ACLRs) below $-{\text {45 dBc}}$ , with average efficiency (AE) ranging from 37% to 45.8%. Similar performance is measured when the proposed PA is driven by a dual-band dual-mode modulated signal with a 100-MHz instantaneous bandwidth at a center frequency of 2.14 GHz.

A Broadband Doherty Power Amplifier Based on Continuous-Mode Technology

Abstract: In this paper, a novel broadband Doherty power amplifier based on the continuous-mode technique (C-DPA) is proposed. The amplifier is focused on manipulating harmonic components in a Doherty power amplifier (DPA) structure to achieve improved bandwidth and efficiency. In a conventional DPA, harmonic isolation is typically required between the two transistors to prevent them from modulating each other at harmonic frequencies. However, as presented in this paper, such isolation is not actually necessary. On the contrary, by allowing the two transistors to modulate each other at harmonic frequencies with the help of a properly designed postharmonic tuning network, a series of highly efficient DPA modes can be created over a continuous frequency band, leading to a broadband C-DPA. Based on the proposed method, an example of a C-DPA working from 1.65 to 2.75 GHz was designed. According to the measured results, the designed C-DPA exhibits a 52%–66% efficiency at a −6 dB power backoff and a power utilization factor higher than 1.08 over the 1.1-GHz band. In addition, when simulated by a 7.5-dB peak-to-average power ratio 20-MHz LTE signal, the example C-DPA exhibits an efficiency of 46%–62% while maintaining an adjacent channel power ratio below −45 dBc after linearization over the full 1.1-GHz band. To the best of our knowledge, this is the first proposed C-DPA and a state-of-the-art performance for broadband DPAs.