This paper presents a novel co-designed Doherty power amplifier (DPA) and output bandpass filter, in which the filter also functions as a quadrature combiner to coordinate the carrier and peaking operations, thereby eliminating the conventional DPA output network. The proposed co-design approach reduces circuit complexity and interconnection loss, enabling improved amplifier–filter integration and enhanced overall performance. A third-order quadrature-feeding coupled bandpass filter is synthesized for integration with GaN-based sub-amplifiers, leading to a filtering DPA covering 5G N78 band. Measurements validate the intended amplifier–filter frequency response and demonstrate clear Doherty efficiency enhancement, achieving 45% drain efficiency at saturation and 50% at 6-dB output back-off.

This paper introduces a new class of Schiffman phase shifters, named as the Type G Schiffman phase shifter. For the first time, it is demonstrated that the bandwidth of Schiffman phase shifters can be significantly enhanced by inserting a phase inverter within the coupled-line structure. The proposed topology maintains the electrical lengths of both the coupled line and the reference line at 90°, while the additional phase inverter is inherently compact, resulting in almost no increase in circuit area. A prototype is designed and implemented on a printed circuit board (PCB) to validate the concept. Measured results show a phase error of less than 3°, an insertion loss below 0.5 dB, and an operating bandwidth from 1 to 2.7 GHz, corresponding to a 92% fractional bandwidth.

This article presents, for the first time, a generalized theory for reduced-length coupled-line couplers. A long-standing misconception is that the electrical length of a hybrid quadrature coupled-line coupler must be exactly 90 degrees. This work demonstrates that such couplers can be designed with significantly shorter electrical lengths, at the cost of some bandwidth reduction. To validate the proposed theory, a reduced-length coupler was designed and fabricated on a 5-mil-thick Rogers Duroid 5880 PCB. The proposed reduced-length coupler is implemented on a single-layer PCB, where the coupled-line pair is realized using the top and bottom metal layers. To achieve the desired impedance, the ground plane near the coupled lines on the backside is partially removed. The prototype achieves a five times length reduction compared to a conventional 90 degrees design, while maintaining excellent performance with an insertion loss of 0.2 dB, a return loss of 20 dB, and a phase error of 0.2 degrees.

This work presents the architecture and design of a Doherty-like load-modulated balanced amplifier (DL-LMBA) operating in a quasi-load-insensitive class-E (QLIE) mode for efficiency enhancement. The QLIE mode enables efficient operation even when the impedance seen by the balanced amplifier (BA) is dynamically modulated. Furthermore, in the DL-LMBA architecture, the carrier BA is not affected by the loss of the peaking device in its off-state due to the directivity of the quadrature coupler. This characteristic leads to a further enhancement of efficiency, particularly at power back-off (PBO). In practical design, the GaN device is carefully characterized to determine the optimal output impedance for broadband QLIE operation. A high-order low-pass filter-based matching network is designed and implemented at the output, providing appropriate fundamental and harmonic terminations over the 3.2–4.0-GHz frequency range. The prototype exhibits an overall operational bandwidth from 3.0–4.0 GHz, delivering an output power of 42dBm, a back-off gain of 9–11 dB, and an efficiency of 56%–65% across the entire band at 6-dB PBO that is the highest among reported works. Under modulated signal conditions, an average efficiency of up to 53% is achieved when transmitting a 10-MHz 256-QAM 5G NR signal across the operating bandwidth.

This work presents a new coupler that unleashes an advanced load-modulation architecture, the Load-Modulated Hexagonal-Coupler-based Power Amplifier (LMHA), designed to extend the power back-off (PBO) range required by spectrally efficient $\mathbf{5 G} / \mathbf{6 G}$ modulation schemes. Unlike conventional quadrature couplers, the hexagonal coupler provides six ports and inherently non-uniform S-parameter magnitudes between port pairs, enabling multi-path power combination and higher-order load modulation. These properties make the hexagonal coupler highly favorable for achieving extended PBO. The proposed LMHA consists of one main power amplifier (PA) and four auxiliary PAs. To validate the architecture, we prototyped the hexagonal coupler using transmission lines and implemented LMHA using a Class-AB main PA and (deep) Class-C auxiliary PAs. Experimental evaluation demonstrates efficiency of $56-66 {\%}$ at $\mathbf{1 4}-\mathbf{1 6}$, dB power back-off and $52-70 {\%}$ at peak power from $1.7-2.1, \mathbf{G H z}$. In modulated measurement, up to 50% of average efficiency is achieved for transmitting $10-\text{MHz} 256$ QAM 5 GNR signal across the band.

This article presents the theory and design of a new class of load-modulated power amplifiers (PAs), named the inverse-balun load-modulated power amplifier (IBMA). This work discovers for the first time that several coupled-line-based balun architectures can be reconfigured from 180 degrees power combiners to 0 degrees power combiners by simply flipping the reflection coefficient at each termination of the coupled lines, corresponding to short-to-open transformations. Building on these newly identified 0 degrees power dividers/combiners, named as the inverse-balun power divider (IBPD), the IBMA is introduced. Similar to existing load-modulated PA architectures, IBMA employs one main amplifier and one auxiliary amplifier with Doherty-like biasing. However, unlike conventional Doherty PAs or load-modulated balanced amplifiers (LMBAs), the IBMA achieves ultrawide bandwidth using only two transistors as the core amplifying blocks. This reduces the number of required power dividers and combiners, leading to a more compact implementation. To validate the concept, a GaN-based IBMA prototype was implemented, covering 1.3-3.3GHz. Experimental results demonstrate a peak output power efficiency of 40%-61%, 6-dB output back-off (OBO) efficiency of 40%-53%, and an output power of 41-44dBm across the band.

This paper presents a novel approach for inverse design of harmonically engineered power amplifiers (PAs), aiming at minimizing human effort and intervention. The method leverages a unique synthesis technique fully within CAD tool (Keysight ADS) to automatically generate, simulate and optimize matching networks (MNs) using pixelated element-based structures. Relying on the embedded optimizers and EM-circuit co-simulation, this approach avoids the need for external scripts or costly machine-learning platforms. A corresponding inverse design flow for harmonic-tuned PA is developed with pixelated matching networks at input and output. The PA prototype designed and developed using a commercial GaN transistor device exhibits a wideband loadline response very close to the definition of ideal Class-F−1 mode. The measured performance compares favorably to state-of-the-art with efficiency ranging from 55% to 75% at peak power of around 40.1 to 40.6 dBm, validating the effectiveness of the proposed methodology. An effective reduction in the active area of the circuit is also achieved as compared to the reported counterparts.

This paper presents a novel filtering power amplifier with frequency and mode reconfigurability. The design employs a tunable filtering quadrature coupler as a post-matching power combiner, consisting of four high-Q substrate integrated cavities with contactless tuning. Balanced and quasi-Doherty modes are achieved by exchanging the bias of the sub-PAs and the termination of the output coupler. This method reduces the footprint required by the output filter and minimizes the post-amplification losses. The device was measured across different modes of operation, achieving a 1.43 tuning ratio, up to 40 dBm output power, 62% peak drain efficiency, 47% 6 dB back-off efficiency, and 16 dB gain. Modulated measurements showed good linearity with a −27 dBc adjacent channel power ratio and 54% average efficiency with a 64 QAM signal. The proposed filtering amplifier has applications in highly integrated and high-power radio front-ends.

This work presents a new self-cooling power amplifier (PA) architecture based on active second-harmonic injection for RF transmitter systems. Unlike conventional thermal solutions that rely on bulky heat sinks or liquid cooling, the proposed architecture reduces thermal stress at the device level through harmonic injection. By injecting a precisely tuned second-harmonic current, the Class-E switching transitions are reshaped to minimize power dissipation, enabling near-100% efficiency operation. Experimental results reveal temperature reductions close to 20°C at saturation and a dramatic improvement in efficiency across the operating band. The results confirm that harmonic injection can serve as an effective thermal-management mechanism, enabling compact and thermally resilient high-power RF transmitter (array) systems.

This paper presents a new class of load-modulated power amplifiers (PAs), named as the inverse-balun load-modulated power amplifier (IBMA). For the first time, it is discovered that one coupled-line-based balun topology can be reconfigured from a 180° power combiner to a 0° power combiner by performing the short-to-open transformation at each termination. Building on this newly identified 0° power divider/combiner, the IBMA is introduced. Similar to existing load-modulated PAs, IBMA employs one main amplifier and one auxiliary amplifier adopting Doherty-like biasing, with the two common-mode ports of the inverse balun combined into one main branch. However, unlike conventional Doherty PAs or load-modulated balanced amplifiers (LMBAs), the IBMA achieves ultra-wide bandwidth using only two transistors as the core amplifying blocks. To validate the proposed architecture, a GaN-based IBMA prototype is designed and fabricated on PCB, covering 1.3-3.3 GHz. Experimental results demonstrate a peak output power efficiency of 40-61, 6-dB output back-off (OBO) efficiency of 40-53%, and an output power of 41-44 dBm across the band.

This paper presents a novel filtering radio front-end based on the quadrature-balanced power amplifier (PA) architecture, which enables time-divisional duplex operation by switching off the sub-PAs. The design is based on a filtering hybrid coupler that is co-integrated with two sub-PAs. The prototype demonstrated a peak efficiency up to 63.3%, over 10.5 dB of gain, and a peak output power of 39.1 dBm. Quasi-balanced Doherty modes exhibited an improvement in output back-off efficiency of 13% compared to the balanced mode. The transmit isolation to the receiver front-end was greater than 15.5 dB, with 44.6 dB of adjacent band rejection. The receive path exhibited less than 1.66 dB of loss, with adjacent band rejection exceeding 35 dB. This paper expands the state-of-the-art time-division duplexing radios with high transmitter efficiency and low receiver loss.

The paper introduces a novel architecture called the Doherty-like load-modulated-double-balanced amplifier (DL-LMDBA), which incorporates intrinsic isolation to load mismatch. It builds upon the concept of the load-modulated balanced amplifier (LMBA) but enhances it by configuring the single-ended control amplifier (CA) into a balanced topology. This transformation enables the LMDBA to inherit the load-mismatch tolerance characteristic of balanced amplifier. To demonstrate this concept, the authors developed a wideband RF-input Doherty-like LMDBA using GaN devices and branch-line quadrature hybrid couplers operating from 1.8–2.2 GHz. Continuous-wave (CW) measurement results show promising high-efficient operation of DL-LMDBA, with efficiency around 60% at peak power of > 43 dBm and 62% to 70% at a 8-dB output back-off (OBO) under matched load conditions across the band. Modulated measurement is also conducted at 2.1 GHz, showing excellent linearizability using DPD together with 52% average efficiency with 5G-NR signal. Simulation and measurement results show that the efficiency and linearity profiles of the designed LMDBA remain robust under load mismatch, without requiring any reconfiguration, thereby validating the proposed architecture.

This article presents the first demonstration of a decade-bandwidth Doherty-like load-modulated balanced amplifier (DL-LMBA). While a frequency-agnostic, signal-flow-based broadband LMBA theory is recently developed and successfully applied to pseudo-Doherty LMBA (PD-LMBA) to validate its wideband performance both theoretically and experimentally, the broadband potential of DL-LMBA has not yet been thoroughly proven. Leveraging this newly developed theory, this work proves, for the first time, that the ideal broadband load modulation behavior in DL-LMBA can be achieved only when a constant 180 degrees phase offset is maintained between the BA and CA signal paths across the band. This condition fundamentally differs from that of PD-LMBA. The proposed theory enables us to overcome the longstanding bandwidth limitations in DL-LMBA design. Furthermore, based on the signal-flow approach, a novel reflective-type-phase-shifter-based balun (RPB) is proposed to enable the wideband operation of DL-LMBA. To validate the proposed concept, an ultrawideband RF-input DL-LMBA prototype is implemented in GaN technology, covering a frequency range from 0.2 to 2 GHz. Experimental results demonstrate peak output power efficiency ranging from 44% to 67%, and 6-dB output back-off (OBO) efficiency ranging from 43% to 75%.

The authors appreciate the comments and corrections made by Wei et al. [1] that point out errors in [2]. Based on the feedback and a thorough rederivation, we have identified and corrected inaccuracies in the theoretical derivation to ensure consistency in the theoretical framework. While the main conclusions of [2] remain valid, we regret any inconvenience caused by these errors.

This paper introduces a new approach for automatically optimizing the overall transmit performance of an antenna array during beam-steering. Based on a novel PA architecture, Hybrid Asymmetrical Load Modulated Balanced Amplifier (H-ALMBA), the antenna impedance mismatch induced by mutual coupling can be effectively handled through circuit reconfiguration. To demonstrate this concept, a dual-band 4x4 phased array is designed using slot-loaded, octagonal shaped patch antennas. Closed-loop control of all the PAs in the array is realized by monitoring their output performance and using dedicated optimization algorithm. Through circuits-antenna-array co-simulation, we demonstrate that the linearity and efficiency of all PAs can be effectively maintained close to nominal conditions at arbitrary beam-steering angles. This can eliminate the need for magnetic circulators between PAs and antennas, leading to enhanced overall performance and reduced SWaP-C of system.

This paper presents a Ka-band ellipsoidal dielectric lens antenna with the widest measured bandwidth in its type. The design is constrained to a boresight dimension of 40 mm and a monolithic fabrication. The prototype was manufactured with Roger's Radix 2.8 resin, a 3D printing dielectric material with low loss at high frequencies. The device achieved a realized gain of 22.1 dBi +/- 2 dB across the entire Ka-band. The measured bandwidth was 19.1 GHz (58%) at a center frequency of 33 GHz while maintaining a return loss below -10 dB. The presented prototype doubles the state-of-the-art bandwidth for ellipsoidal dielectric lens antennas.

This paper presents the first-ever wideband loadmodulated double balanced amplifier (LMDBA) with an intrinsic isolation to load. Building upon the foundational concept of the generic load modulated balanced amplifier (LMBA), by setting the control amplifier (CA) to balanced topology, the LMDBA can inherit the load-mismatch tolerance characteristic of balanced amplifiers, eliminating the need for a magnetic circulator. To demonstrate the principle, a wideband RF-input LMDBA is developed using GaN devices and commercial branchline quadrature hybrid couplers from 1.9-2.7 GHz. The experimental results exhibit an efficiency of 54-75 % at peak power and 45-60 % at 10-dB output back-off (OBO) under the matched condition of load across the band. An efficiency up to 70.2 % at peak power and up to 54.2 % at 10-dB OBO are measured under 2:1 VSWR of load. Additionally, the PA's linearity profiles (AMAM and AMPM) can be well maintained against load mismatch compared to matched condition without reconfiguration. In modulated measurement, the efficiency and linearity performance can well maintained under various load as well.

This letter presents the first demonstration of a decade-bandwidth Doherty-like load-modulated balanced amplifier (DL-LMBA). Leveraging the recently developed signal-flow-based broadband LMBA theory, a frequency-agnostic phase-alignment condition is identified that is critical for ensuring intrinsically broadband load modulation (LM) behavior for DL-LMBA, thereby overcoming longstanding bandwidth limitations in DL-LMBA design. In addition, a reflective-type-phase-shifter-based 180 degrees power divider is proposed for the wideband operation of DL-LMBA. To prove the proposed concept, an ultrawideband RF-input DL-LMBA is developed using GaN technology covering the frequency range from 0.2 to 2 GHz. Experimental results demonstrate an efficiency of 44%-67% for peak output power and 43%-75% for 6 dB output back-off (OBO), respectively.

This article presents the design and implementation of a novel time-division duplex (TDD) front-end architecture, namely, the Indirectly nonreciprocal load modulated balanced amplifier (INR-LMBA), targeted at emerging massive MIMO communication systems. Unlike conventional solutions that place a circulator at the power amplifier (PA) output to mitigate load mismatches, the proposed architecture relocates the circulator to the low-power control amplifier (CA) path. This significantly reduces power stress on the circulator by nearly an order of magnitude, thus paving the way for compact, nonmagnetic implementations. Combined with a transmit/receive (T/R) switch, INR-LMBA enables standard TDD functionality while offering quasi-isolation during transmission and low loss during reception. Under matched-load conditions, the architecture operates similar to a conventional pseudo-Doherty load modulated balanced amplifier (PD-LMBA). Under mismatched conditions, however, the circulator redirects reflected power to its isolation port, minimizing mismatch effects on the PA. As a proof-of-concept demonstration for this architecture, a 2-2.5-GHz INR-LMBA prototype is implemented, which achieves an efficiency of 62%-73% at peak output power and 53%-61% at 10-dB output back-off (OBO) over the in-band operation at 50- $\Omega $ load. In modulated evaluation with a 20-MHz OFDM signal, the measured average PA efficiencies are around 52% for a matched load with ACPR greater than 34 dB across the band of operation for matched load. The PA also exhibits strong mismatch resilience with higher than 43% average efficiency and >35-dB ACPR across the band for a mismatched load at 2:1 voltage standing wave ratio (VSWR).

This paper presents a novel Double-Balanced power amplifier (PA) architecture with an intrinsic load isolation. Derived from the generic load modulated balanced amplifier (LMBA), by designing the single-ended control amplifier (CA) as another balanced PA, the Pseudo-Doherty load-modulated double-balanced amplifier (PD-LMDBA) can inherit the intrinsic load-mismatch tolerance of balanced amplifier without any reconfiguration and load-impedance sensing. Theoretical analysis reveals that both the control amplifier (CA, as carrier) and primary balanced amplifier (BA, as peaking) exhibit complementary load modulation trajectories for their sub-amplifiers (CA1 and CA2, BA1 and BA2) under mismatch. This allows the PA to inherit the intrinsic load insensitivity from the generic quadrature-balanced amplifier and sustain nearly constant performance against arbitrary load variations. A prototype is implemented at 2.1 GHz, achieving 76.2% efficiency at peak power and 69.5% at 10-dB OBO with matched load. Under a 2:1 voltage standing wave ratio (VSWR) of load mismatch, an efficiency up to 72.5% at peak power and 64.1% at 10-dB OBO are measured. In modulated evaluation with a 20-MHz OFDM signal, the PA maintains linearity against 2:1 VSWR, with 2.1% of error vector magnitude (EVM) and down to -39.5 dB adjacent channel power ratio (ACPR), closely approximating the matched condition.