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This study presents a switchable Doherty power amplifier (S‐DPA) based on dual compensating reactance and adjusted drain voltage with 124% fractional bandwidth (FBW) using single‐input architecture. The proposed S‐DPA can be operated in wideband mode (WB‐MOD) and dual‐band mode (DB‐MOD) with a larger power back‐off region through one‐click switching directly. The needed back‐off impedance in both operation modes is achieved by using the dual compensating reactance for bandwidth extension. Meanwhile, a load modulation enhancement technique through adjusting the drain voltage of both the carrier and peaking amplifiers is proposed for proper load modulation in DB‐MOD. To verify the proposed techniques, an S‐DPA was designed and fabricated. Measurement results show that the proposed S‐DPA has an efficiency of 51.3%–70.3% at saturated power of higher than 43.4 dBm over the frequency band of 0.8–3.4 GHz with 124% FBW. The 6‐dB back‐off efficiency in WB‐MOD (1.3–2.9 GHz) is 45.3%–56.1% and the 8‐dB back‐off efficiency in DB‐MOD (0.8–1.3 GHz & 2.9–3.4 GHz) is 41.6%–57.9%. 80‐MHz modulated signals with 6.2‐dB and 8.2‐dB peak to average power ratios are used to evaluate the modulated signal performance in two modes. The DPA can achieve an average efficiency of up to 56% with high linearity after digital pre‐distortion linearisation.

Recording reliably extracellular neural activities is an essential prerequisite for the development of bioelectronics and neuroprosthetic applications. Recently, a fully differential, two-stage, integrating pre-amplifier was proposed for amplifying and then digitising neural signals. The amplifier featured a finely tuneable offset that was used as a variable threshold detector. Given that the amplifier is integrating, the DC operating point keeps changing during integration, rendering traditional analysis (AC/DC) unsuitable. In this work, we analyse the operation of this circuit and propose alternative definitions for validating the necessary key performance metrics, including gain, bandwidth, offset tuning range and offset sensitivity with respect to the memory states of the employed memristors. The amplification process is analysed largely through investigating the transient behaviour during the integration phase. This benchmarking approach is finally leveraged for providing useful insights and design trade-offs of the memristor-based integrating amplifier.

The noise is an inherent part of a transceiver system, which becomes more severe on low‐cost systems. The power amplifier (PA) further enhances this noise and the signal to be propagated to the receiver. The conventional approach of digital predistortion (DPD) assumes an ideal transceiver system while extracting data for the predistortion function generation. As a result, performance limitation arises due to residual signal–noise interaction. This study presents the accuracy and implementation issues of DPD on a low‐cost transceiver having lower bit resolution in the presence of transceiver noise. Different model architectures, as well as processing algorithms, are compared in terms of numerical stability of the solution (condition number of observation matrix), efficient field‐programmable gate array (FPGA) implementation (dispersion of coefficients, in‐band model performance (normalised mean square error), and out‐of‐band model performance (adjacent channel power ratio). The simulation results are tested on FPGA and direct conversion transceiver‐based platform using PA. A long‐term evolution signal with 64 quadrature amplitude modulation is used for performance evaluation. The suitability of the various polynomial models for fixed‐point implementation and the required memory size for implementing the DPD model is further established.

This work outlines the design, implementation, and experimental results of a Class‐F power amplifier (PA) with a novel output matching topology by utilizing substrate integrated waveguide (SIW). The design methodology is first proposed with one highly integrated SIW matching structure. The impedances at fundamental, second harmonic and third harmonic frequencies are analysed to explain good impedance matching for the PA as well as the isolation mechanism of the direct current (DC) and radio frequency (RF). For demonstration, an S‐band 10‐W Class‐F PA is fabricated and tested. The simulation and measurement results agree very well with each other, which verifies the proposed technology of SIW output matching. To the authors’ knowledge, this is the first time to use a highly integrated SIW matching in Class‐F PA, which simplifies the output matching for third‐order harmonics and DC/RF isolation structures.

The second‐order distortion, often using second‐order intercept point (IP2) as an indicator, has been considered easy to be filtered in narrowband radio frequency (RF) communications because it is located far away from the passband of transmitted signals. However, with rapidly increased applications in wideband RF communications, the second‐order intermodulation (IM) might become important, partially evidenced by the increased appearances of IP2 in the amplifier datasheets in recent years. In this article, a power spectrum model of the second‐order IM is derived to quantify the second‐order distortion using the IP2. This model could be used to determine under what conditions, in terms of the relation between the carrier frequency and bandwidth, the second‐order IM will affect the passband or the adjacent bands or other applications, using three types of frequency scenarios. These discussions are beneficial for RF engineers and spectrum planners to predict interferences from second‐order IM. The experimental measurement at the end of the article validates the spectrum model.

Purpose In simultaneous transmission and reception (STAR) MRI, along with the coupling of the excitation pulse to the received signal, noise, and undesired distortions (spurs) coming from the transmit chain also leak into the acquired signal and degrade image quality. Here, properties of this coupled noise and its relationship with the transmit amplifier gain, transmit chain noise density, isolation performance, and imaging bandwidth are analyzed. It is demonstrated that by utilizing a recently proposed STAR technique, the transmit noise can be reduced. The importance of achieving high isolation and careful selection of the corresponding parameters are demonstrated. Theory and Methods A cancellation algorithm, together with a vector modulator, is used for transmit-receive isolation. The scanner is modeled as a pipeline of blocks to demonstrate the noise contribution from each block. With higher isolation, coupled transmit noise can be reduced to the point that the dominant noise source becomes acquisition noise, as in the case for pulsed MRI. Amplifiers with different gain and noise properties are used in the experiments to verify the derived noise-transmit parameter relation. Results With the proposed technique, more than 80 dB isolation in the analog domain is achieved. The leakage noise and the spurs coupled from the transmit chain, are reduced. It is shown that the transmit gain plays the most critical role in determining sufficient isolation, whereas the amplifier noise figure does not contribute as much. Conclusion The transmit noise and the spurs in STAR imaging are analyzed and mitigated by using a vector modulator.

This letter proposes a unipolar thin‐film transistor (TFT)‐based amplifier design that can provide enhanced DC offset suppression. The amplifier consists of an input AC coupling network and a capacitor bootstrap amplifier. By making the first pole of the input AC‐coupling network coincide with that of the capacitor bootstrap amplifier, a DC offset suppression capability of 40 dB/dec, which is 2× higher than that of the prior arts, can be obtained. An implementation is also proposed and a Rensselaer Polytechnic Institute (RPI) TFT model whose parameters are turned to fit the measured data of our metal‐oxide TFT is used to simulate the circuit. Simulation results show that the implementation has 31 dB midband gain, 0.5 Hz–5 kHz bandwidth, 300 μW power consumptions, and the expected 40 dB/dec DC offset suppression. This design is suitable for applications that need to block large DC interference, such as sensor readout amplifiers.

PURPOSE B1+ shimming is an important method for mitigating B1 inhomogeneity in high-field MRI. Using independent power amplifiers for each transmit (Tx) element is the preferred method for B1 shimming but comes with a high cost. Conversely, the simplest approach to control a Tx array is by using coaxial cables of varying length in the Tx chain, but this approach is cumbersome and impractical for dynamic shimming. In this article, a system is described that enables dynamic, phase-only, eight-channel B1+ steering on a 7T MR scanner with only two power amplifiers. METHODS Power dividers were utilized to first split the existing two-channel Tx signal into eight channels. Digitally controlled phase shifters on each channel were designed to provide independent phase shifts with a resolution of 22.5° (from 0°, 22.5° … 337.5°). To validate the system, an eight-channel body dipole array was simulated and constructed for bench and 7T imaging and evaluation. RESULTS The phase conjugate B1+ steering method was employed at three different spatial positions in simulation, bench measurements, and scanner measurements-all with matching results. At the desired points, regions with homogenous B1+ were generated, indicating good Tx steering to the selected region. CONCLUSION The described system can be used as a simple retrofit to existing hardware to provide phase control while avoiding the need to manually switch cables and without requiring independent power amplifiers for each channel, thus demonstrating the ability to perform dynamic B1+ shimming with increased degrees of freedom but without significantly increased hardware cost.

A modified load‐modulation network to design a fully integrated wideband Doherty power amplifier (DPA) from 4.6 to 5.5 GHz is presented in this letter. The modified load‐modulation network uses only two π‐type high‐pass equivalent quarter wavelength transmission lines (QWTLs). Unlike the existing methods, the bandwidth of the load‐modulation network composed of two merged π‐type equivalent QWTLs is broadened. Furthermore, a drain bypass capacitor compensation method is proposed to reduce the leakage of RF power after the drain supply inductors are merged. For verification, a wideband monolithic microwave integrated circuit DPA was designed and fabricated using a 0.25‐μm gallium nitride based high‐electron‐mobility transistor process. Measurement results show that a saturated power of 41.6–42.4 dBm, a saturated drain efficiency (DE) of 65.3–72%, and a 6‐dB back‐off DE of 45–55.4% are achieved from 4.6 to 5.5 GHz. Using a 160‐MHz orthogonal frequency‐division multiplexing signal with a peak‐to‐average power ratio of 6.2 dB, the high average efficiency of 49% with an adjacent channel leakage ratio of –47.2 dBc after linearisation was obtained at an average output power of 35 dBm at 4.9 GHz.

This letter presents an analytical framework to study the distortion effect of power amplifiers (PA) in a single‐stream millimetre wave (mmWave) system with a radio‐frequency beamforming architecture. A third‐order nonlinearity PA model is used. The impact of PA nonlinearity on the transmitter performance is evaluated by means of its normalised mean square error. Also, the effect of PA compression on the system capacity in a line‐of‐sight channel is analysed. All analytical results are supported with Monte‐Carlo simulations.

Stimuli-responsive photodynamic therapy (PDT) is a hot topic in precise medicine, but the low abundance of responsive trigger molecules in early-stage disease limits application. Here we designed an amplifier with multiple upconversion luminances to achieve a near-infrared photo-switched cascade reaction triggered by specific microRNA and precise PDT of early-stage cancers. This amplifier was composed of photo-caged DNA nanocombs and an upconversion nanoparticle (UCNP) sensitized with IRDye 800CW. The nanocomb was prepared by assembling a photozipper-protected hairpin and two kinds of hybridizable hairpin probes on a DNA skeleton. Upon 808-nm light irradiation, the produced UV light cleaved off the photozipper to induce microRNA-responsive cascade hybridization reaction, activating the photosensitizers linked to different hairpins to generate reactive oxygen species (ROS) under the simultaneously emitted blue light for efficient PDT.

Purpose To implement a modular, flexible, open-source hardware configuration for parallel transmission (pTx) experiments on medical implant safety and to demonstrate real-time mitigation strategies for radio frequency (RF) induced implant heating based on sensor measurements. Methods The hardware comprises a home-built 8-channel pTx system (scalable to 32-channels), wideband power amplifiers and a positioning system with submillimeter precision. The orthogonal projection (OP) method is used to mitigate RF induced tip heating and to maintain sufficient B 1 + for imaging. Experiments are performed at 297MHz and inside a clinical 3T MRI using 8-channel pTx RF coils, a guidewire substitute inside a phantom with attached thermistor and time-domain E-field probes. Results Repeatability and precision are ~3% for E-field measurements including guidewire repositioning, ~3% for temperature slopes and an ~6% root-mean-square deviation between B 1 + measurements and simulations. Real-time pTx mitigation with the OP mode reduces the E-fields everywhere within the investigated area with a maximum reduction factor of 26 compared to the circularly polarized mode. Tip heating was measured with ~100 μK resolution and ~14 Hz sampling frequency and showed substantial reduction for the OP vs CP mode. Conclusion The pTx medical implant safety testbed presents a much-needed flexible and modular hardware configuration for the in-vitro assessment of implant safety, covering all field strengths from 0.5-7 T. Sensor based real-time mitigation strategies utilizing pTx and the OP method allow to substantially reduce RF induced implant heating while maintaining sufficient image quality without the need for a priori knowledge based on simulations or in-vitro testing.

Purpose In the spirit of overcoming the signal-to-noise limitations of active matrix, flat-panel imagers (AMFPIs) which employ array circuits based on a-Si:H thin-film transistors (TFTs), an empirical investigation of the noise properties of prototype active pixel arrays based on polycrystalline silicon (poly-Si) TFTs is reported. Like a-Si:H, poly-Si supports fabrication of large area, monolithic x-ray imaging arrays and offers good radiation damage resistance, while providing electron and hole mobility orders of magnitude higher. Compared to pixel circuits typically consisting of a single addressing switch in an AMFPI array, the pixel circuit of an active pixel array includes an amplifier that magnifies the imaging signal prior to readout by external acquisition electronics. Also, while readout erases signal stored in the pixels for AMFPI arrays, active pixel arrays allow multiple nondestructive readout, which can be used to reduce noise. The prototype arrays investigated in this paper were developed to explore the effect of variation in amplifier design on noise. Methods A pair of prototype arrays incorporating single-stage and two-stage poly-Si pixel amplifiers were examined. The arrays incorporate various amplifier designs in which dimensions of some of the three (or four) poly-Si TFTs per pixel circuit for the single-stage array, and some of the five poly-Si TFTs for the two-stage array, were varied. The arrays were operated using a recently developed electronic data acquisition system that allows variation of operational conditions such as voltages and timing of control signals. The arrays were operated in the absence of radiation in various correlated multiple sampling modes, with and without the injection of charge directly into the pixel circuits for measurements of in-pixel gain and pixel noise. Pixel noise, referred back to the input of the pixel amplifier, was compared to predictions generated by a sophisticated circuit simulation model. Results Across the various pixel circuit designs, the median in-pixel gain for the single-stage and two-stage arrays was measured to be ×9.3 and ×25, respectively. These gain levels were sufficient to reduce the contribution of external noise, defined as the electronic additive noise in the absence of noise contributions from circuitry in the pixel and referred back to the input of the pixel amplifier, to less than 340 e. As a result, median pixel noise results as low as ~695 e and 866 e, acquired using eight samples, were observed from the best-performing single-stage and two-stage designs, respectively. While the magnitude of pixel noise predicted by simulation was lower than the measured results, there was generally good agreement between simulation and measurement for the functional dependence of noise on operating voltages, timing, and sampling mode. Conclusions The single-stage and two-stage arrays examined in this study demonstrated pixel noise well below that typically demonstrated by AMFPIs. Through proper design, it should be possible to maintain the noise levels observed in this study irrespective of the size and pitch of an active pixel array. Further reduction in pixel noise may be possible through more optimized pixel circuit design, faster readout, or improvements in fabrication.

Traditionally, common radio frequency (RF) structures involved single‐input and single‐output devices, with a common ground reference. But with the advent of advanced microwave monolithic integrated circuits and the move to using CMOS technology to higher frequencies, more and more RF circuits are being designed using differential devices. The mixed‐mode parameters may also be defined for a 3‐port network, comprising one single‐ended port and one mixed‐mode port, which is usually defined as the differential or balanced port. But, it must be recognized that for a device referenced to ground, a balanced port always has the possibility to support a common mode signal as well. The most common use of a single‐ended to differential device is as a balun, used to drive a differential device from a single‐ended measurement instrument. With that advent of 4‐port vector network analyzers, the need for baluns in characterization of linear, passive measurements was eliminated.

One of the challenges in noise‐figure measurements is in determining the noise figure of systems that integrate the low‐noise amplifier (LNA) stage into a single‐chip down‐converter, where the output is not at the same frequency as the input and the LNA cannot be separated from the rest of the system. This chapter illuminates the details of noise figure and noise‐power measurements, including their uncertainty, verification, and methods to improve them. It discusses active antenna noise‐figure measurements and describes noise figure in mixers and frequency converters. Mixers and frequency converters are often the first component after the antenna in a receiver system, and as such the noise figure of the first converter predominantly sets the noise figure of the system. The chapter also discusses noise power measurements with a vector network analyzer spectrum analyzer and noise figure measurements using spectrum analysis as well as carrier‐to‐noise measurements.

A novel architecture for an AC (i.e. high-pass) amplifier is proposed allowing a drastic reduction of the cutoff frequency to the sub-Hertz range. It builds upon the classic AC configuration with a high gain amplifier and a parallel RC circuit in the feedback loop, by increasing the feedback resistance through bootstrapping. Resistance multiplying factors higher than four orders of magnitude are easily achievable. The basic principle can be applied to several practical implementations, though in this letter it is demonstrate with measurement results of an op-amp based discrete implementation. This work was financially supported by the following grants from the Spanish Research Agency: TEC2016-80396-C2-1-R and PID2019-107258RB-C32 (AEI/FEDER). M.Martincorena Arraiza was funded by the Ministry of Universities under grant BES-2017-080418.