Inverse class-F Research Articles

High-Efficiency Low Voltage Inverse Class-F Power Amplifier Design Based on Harmonic Control Network Analysis

A high-efficiency inverse Class-F (Class-F−1) power amplifier (PA) with a low voltage pHEMT is presented in this paper. The design process is based on a novel harmonic control network (HCN), consisting of a bias circuit (BC) and a low pass filter (LPF). The goal is reducing the impact of fundamental frequency reflection on DC supply, elimination of harmonics, improvement of power added efficiency (PAE), shaping the voltage and current waveforms along with enhancing the performance of the PA. The simulation is performed based on the harmonic balance analysis in order of 9. The implemented Class-F−1 PA is simulated and measured at 2.4 GHz. The maximum measured PAE of 76% and the drain efficiency of 82% are achieved, with a low drain voltage of 3 V and 11.2 dB gain under a 16 dBm input power. The proposed PA is fabricated and measurement results validated the simulations.

A High-Efficiency Inverse Class-F Microwave Rectifier for Wireless Power Transmission

A high-efficiency inverse class-F microwave rectifier for wireless power transmission is described in this letter. The proposed inverse class-F harmonic termination network is realized by connecting a $\lambda /12$ open-ended transmission line in parallel with the diode cathode, and a $\lambda /8$ short-ended transmission line in series with the diode anode. This structure yields a compact layout, and measurement results have demonstrated a peak conversion efficiency of 80.4% for an input power of 13 dBm at 2.35 GHz, and an efficiency larger than 60% within the bandwidth 2.25–3 GHz and for a wide range of output loads.

A Novel High-Pass Delta–Sigma Modulator-Based Digital-IF Transmitter With Enhanced Performance for SDR Applications

This brief presents a new transmitter architecture based on digital-intermediate frequency (IF) high-pass delta–sigma modulator (HPDSM). The proposed digital-IF transmitter topology utilizes a novel complex HPDSM topology to address the high quantization-noise power problem in Cartesian band-pass BPDSM and HPDSM-based digital-IF transmitters, without increasing the oversampling ratio of the signal or the clock rate of the system. To evaluate the performance of the new digital-IF transmitter system, a comparison with its Cartesian band-pass and high-pass counterparts, in terms of signal to noise and distortion radio (SNDR) and coding efficiency, was established. A simulation using 8 dB peak to average power ratio long term evolution signals with 1.24-MHz bandwidth showed that by integrating the proposed second-order complex HPDSM digital-IF in the transmitter, the power of the quantization noise is significantly reduced. The transmitter digital blocks were implemented on the BEEcube software-defined radio prototyping platform. The input signal is encoded by the new HPDSM topology and is up-converted and then fed to the inverse class-F switch-mode power amplifier. An overall efficiency of 12% was achieved by the proposed topology. Moreover, the output signal SNDR reached 37.8 dB and the adjacent channel leakage ratio at the lower and upper 1.25-MHz offset frequencies measured is equal to −35 dBc.

Simplified First-Pass Design of High-Efficiency Class-F−1 Power Amplifiers Based on Second-Harmonic Minima

This paper investigates the source and implication of efficiency minima that is typically observed during the second-harmonic load–pull of transistor amplifiers. The study starts with the theoretical derivation of time-domain voltage and current waveforms as a function of conduction angle ( $\alpha$ ) at the second-harmonic efficiency minima, where the output power and drain efficiency (DE) are at minimum. Thereafter, this paper unfolds a systematic re-engineering approach that is developed to recover the performance degradation and to exploit the region of efficiency minima in favor of design and implementation of high-efficiency inverse class-F power amplifiers (PAs). Interestingly, the inferences drawn from the in-depth analysis are shown to provide a simplified first-pass design approach that guarantees inverse class-F PA operation without an a priori knowledge of device parasitic elements. Theoretical postulations and simulation results are experimentally validated using an on-wafer active harmonic load–pull and a prototype design using 1.95-mm NXP gallium nitride die at a frequency of 2.6 GHz. The designed PA delivers an output power of 40 dBm with DE of 76% and gain of 12 dB at 3-dB gain compression. The measurement results confirm the theoretical framework reported in this paper.

Analysis and design optimisation for inverse Class‐F GaN Doherty amplifier

An analytical model of an inverse Class-F (Class-F-1) Doherty power amplifier (DPA) with a proposed compensation method is presented. The derived model was shown to be effective in predicting the behaviour of the fundamental current and optimum load impedance in both a carrier and peaking cell for a Class F-1 DPA. Based on the analysis results, the degradation of the RF performance, including the peak output power, efficiency, and linearity, caused by an inherent lower current driving of a peaking cell is also proven. To compensate this issue, a dynamic gate bias for a peaking cell using a proposed voltage shaping is introduced. For verification, a 2.4 GHz GaN Class-F-1 DPA was designed and fabricated. Using the proposed gate shaping method, improvements of the maximum output power of 2.6 dB and peak power drain efficiency of 5.6% are achieved compared with a conventional case. With a 10 MHz 6.5 dB peak-to-average power ratio signal, the Class-F-1 DPA achieves an average output power of 36 dBm with a DE of 54% at a 28 V supply voltage. The measured adjacent channel rejection ration under such conditions is below -30 dBc.

A Highly Efficient X-Band Inverse Class-F SiGe HBT Cascode Power Amplifier With Harmonic-Tuned Wilkinson Power Combiner

A highly efficient sub-Watt X-band inverse class-F SiGe heterojunction bipolar transistor (HBT) cascode power amplifier (PA) is presented. With a multi-harmonic resonance filter and a low loss lumped-element Wilkinson power combiner as an output matching network, the inverse class-F operation is realized successfully and high output power and power added efficiency (PAE) are obtained simultaneously. A cascode configuration with a low base impedance termination and reduced voltage–current waveform overlap extends ${V} _{\rm{CE}}$ swing of the upper SiGe HBT in the cascode beyond BVCBO, leading to improvement in output power and PAE. As proof of concept, the proposed PA was implemented in a 0.13- ${{\mu }}\text{m}$ SiGe BiCMOS technology platform. Measured results shows 53.4% peak PAE with 26.1 dBm output power at 10 GHz, when operated on a 3.0-V supply. No performance degradation is observed after 24-h continuous mode operation. To the author’s best knowledge, this brief has the highest PAE among any Si-based X-band PAs with comparable output power, which demonstrates that the proposed harmonic-tuned Wilkinson power combiner approach is an appropriate solution for efficient PA design at X-band.

History of Class-F and Inverse Class-F Techniques: Developments in High-Efficiency Power Amplification from the 1910s to the 1980s

Today, class-F amplification is a well-established technique for increasing the efficiency of RF power amplifiers (PAs). The increase in efficiency afforded by class F reduces power consumption and increases talk time, both of which are driving forces in the design of RF PAs. The operational efficiency of radio transmitters was of great interest right at the dawn of the development of high-power radiotransmission systems. Many if not most readers will be surprised to learn that the basic concepts of class-F amplification were discovered at the end of the 1910s, just a few years after the first vacuum tube transmitters were developed.

Investigation of load modulated inverse Class-F power amplifier with extended conduction angle

A novel design space of load modulated (LM) inverse Class-F power amplifiers (PAs) with extended conduction angle is proposed. The effects of the driven level factor β and the biasing operation factor ρ on the third-harmonic generation are discussed. The harmonic generation mechanism shows that the knee voltage effects are the source of third-harmonics for LM inverse Class-F PAs. The definition of the inverse Class-F mode is consistently valid under the circumstance of load modulation in a limited output power back-off (OPBO) range. Meanwhile, the conduction angle θ0 can be extended from π/2 to a limited value (<110°) to keep the standard waveforms of inverse Class-F mode and maintain high efficiency. After introducing the “continuous concept” and the second harmonic manipulation method, the mathematical design space of inverse Class-F PAs with extended conduction angle is derived. Calculation shows that the purely conductive load modulation can enable high back-off efficiency operation for LM inverse Class-F PAs. As proof of concept, a demonstrator amplifier is fabricated and measured. The experimental results show that the power added efficiency (PAE) with optimum Vctrl is improved by 5% over an OPBO range of 6 dB compared with the same PA with fixed Vctrl.

Усилители мощности инверсного класса F – эффективное средство улучшения энергетических характеристик радиопередатчиков

This paper describes energy characteristics of inverse class-F power amplifier with limited quantity of harmonics and defines load characteristics for this type of amplifiers. As a result of the research, allowed ranges of parameter values for class and scheme are defined, where high energy characteristics are saved.

DC Polarity Control in Radio Frequency Synchronous Rectifier Circuits

In this letter, two different switching modes are identified for RF synchronous rectifier circuits which lead to a change in the polarity of the rectified output voltage. The operation of the two modes is defined in terms of the dynamic current-voltage (I-V) characteristics of the switching device and the polarity of the output load voltage. In one mode, the dc output voltage is positive and associated with a switching condition where the device is ON in quadrant-III and OFF in quadrant-I. This mode of operation has been reported widely in the literature. In the second mode, the quadrants associated with the on- and off-state are swapped leading to a polarity reversal across the load. Theoretical justification for the two modes is given and the experimental results are shown to confirm the two different switching modes in the synchronous rectifier. Experiments with a GaN inverse class-F rectifier were made to verify the theory, and both modes have similar measured peak rectification efficiencies of 77% at a frequency of 1.8 GHz.

Extended Continuous Inverse Class-F Power Amplifiers With Class-AB Bias Conditions

In this letter, the operation of continuous inverse Class-F (Class-F−1) mode with Class-AB bias conditions is modeled. The extended waveform expression of intrinsic drain current is derived for continuous Class-F−1 power amplifiers (PAs) biased above pinchoff point. Based on the modified current and voltage waveform formulas, the delivered power and efficiency performance is analyzed as a function of conduction angle. Moreover, the design space of fundamental and harmonic load admittances is generalized for any conduction angle between $\pi $ and $2\pi $ . For experimental verification, a broadband PA is implemented with a 6-W LDMOS device. Under a Class-AB bias condition, the fabricated PA exhibits more than 70% power added efficiency and 38-dBm output power with a fractional bandwidth of 62% from 500 to 950 MHz.

On Optimal Truncated Biharmonic Current Waveforms for Class-F and Inverse Class-F Power Amplifiers

In this paper, two-parameter families of periodic current waveforms for class-F and inverse class-F power amplifiers (PAs) are considered. These waveforms are obtained by truncating cosine waveforms composed of dc component and fundamental and either second(k=2)or third(k=3)harmonic. In each period, waveforms are truncated to become zero outside of a prescribed interval (so-called conduction angle). The considered families of waveforms include both discontinuous and continuous waveforms. Fourier series expansion of truncated waveform contains an infinite number of harmonics, although a number of harmonics may be missing. Taking into account common assumptions that for class-F PA the third(n=3)harmonic is missing in current waveform and for inverse class-F PA the second(n=2)harmonic is missing in current waveform, we consider the following four cases: (i)n=k=3,(ii)n=3,k=2,(iii)n=k=2,and (iv)n=2,k=3.We show that, in each of these cases, current waveform enabling maximal efficiency (optimal waveform) of class-F and inverse class-F PA is continuous for all conduction angles of practical interest. Furthermore, we provide closed-form expressions for parameters of optimal current waveforms and maximal efficiency of class-F (inverse class-F) PA in terms of conduction angle only. Two case studies of practical interest for PA design, involving suboptimal current waveforms, along with the results of nonlinear simulation of inverse class-F PA, are also presented.

Design of broadband inverse class-F power amplifier based on resistive-reactive series of inverse continuous modes

Design of broadband inverse class-F power amplifier based on resistive-reactive series of inverse continuous modes

Power-Efficiency Characteristics of Class-F and Inverse Class-F Synchronous Rectifiers

This paper presents analytic and experimental results for high-efficiency GaN HEMT class-F and inverse class-F (class-F−1) synchronous rectifiers. The rectifier designs are based on time-reversed duals of corresponding class-F amplifiers. A detailed analysis of the power efficiency of a class-F rectifier is presented. The analysis shows that, unlike the amplifier dual, the rectifier configuration does not have significant overlap loss because of device constraints in terms of operation in quadrant III. Consequently, the power efficiency of the class-F rectifier is slightly higher than the amplifier circuit under equal source power conditions. Experimental measurements are shown, which compare the rectification efficiency of a class-F rectifier with class-F−1 rectifiers. It is shown that operation of the rectifiers under back-off conditions mimics the power efficiency of an amplifier with envelope tracking. Based on this paper, it is concluded that the class-F−1 mode has better rectification efficiency compared with that of class-F, both at peak power and under back-off conditions.

Integrated Inverse Class-F Silicon Power Amplifiers for High Power Efficiency at Microwave and mm-Wave

This paper presents two 2-stage inverse class-F power amplifiers (PAs) at 24 GHz and 38 GHz, integrated in $0.13~\mu \text {m}$ SiGe BiCMOS technology. The PAs are composed of an inverse class-F output stage proceeded by a class-AB driving amplifier. An inter-stage matching network between the driver and output stage delivers an optimal inter-stage power to the output stage with a maximum PAE in the driver amplifier. The output stage’s load network based on multi-resonance LC resonators terminates harmonic impedance explicitly up to the third of the fundamental signal. By leveraging a native low capacitive reactance, the class- $\text {F}^{\mathrm {-1}}$ load network shapes a quasi-rectangular current waveform that effectively contains up to the fifth harmonic spectral component. A high impedance control is limited up to the second harmonic, shaping a half-sinusoid voltage peaking induced by DC, fundamental, and the second harmonic voltage spectra. The 24 GHz PA achieves 50% peak PAE, 16 dBm $\text {OP}_{\mathrm {-1dB}}$ , 18 dBm $\text {P}_{\mathrm {sat}}$ , and 19 dB saturated power gain with 2.3 V supply voltage at 24 GHz. The 24 GHz PA can maintain >45% PAE over 23.5–25.5 GHz and 1.5–2.4 V supply voltage range, manifesting a PAE robustness to the frequency and supply variations. For 38 GHz PA, measurements show 38.5% peak PAE, 15 dBm $\text {OP}_{\mathrm {-1dB}}$ , 17 dBm $\text {P}_{\mathrm {sat}}$ with 15 dB of saturated power gain at the $\text {OP}_{\mathrm {-1dB}}$ point when 2.4 V supply voltage is applied. The 38 GHz PA can sustain >35% PAE over 36–39 GHz and 1.5–2.5 V supply variation. The PAs also are tested under the input of band-limited signals modulated by various modulation schemes including 8-PSK, QAM, 16-QAM, 64-QAM, and 128-QAM, and test results are presented in this paper. The chip size is $0.95\times 0.6$ mm2 for 24 GHz PA and $0.93\times 0.55$ mm2 for 38 GHz PA, including all pads.

Two-Stage High-Efficiency Concurrent Dual-Band Harmonic-Tuned Power Amplifier

The design strategies and the implementation of a two-stage concurrent dual-band (DB) microwave harmonic-tuned power amplifier (PA) are presented in this paper. The power stage and the driver stage of this circuit are designed to operate under DB class-F conditions and DB inverse class-F conditions, respectively. The proposed DB harmonic control network of the power stage is implemented by combining a composite right/left-handed transmission line and a DB biasing line with harmonic tuning. A novel DB interstage matching network is developed to transfer the optimum fundamental output impedances of the driver stage to optimum fundamental input impedances of the power stage, as well as suppress the third harmonics at two frequency bands of interest. In the nonconcurrent mode (CM), the measured output power of the two-stage DB PA achieves two peaks of 40.2 and 40 dBm at 1.62 and 2.08 GHz with the maximum power-added efficiencies (PAEs) of 71.5% and 73%, respectively. In the CM, the maximum average PAE is up to 62%. To our best knowledge, the implementation of the two-stage concurrent DB harmonic-tuned PA is reported for the first time in the literature.

X-band inverse class-F GaN internally-matched power amplifier**Project supported by the National High Technology Research and Development Program of China (Grant No. 2015AA016801).

An X-band inverse class-F power amplifier is realized by a 1-mm AlGaN/GaN high electron mobility transistor (HEMT). The intrinsic and parasitic components inside the transistor, especially output capacitor Cds, influence the harmonic impedance heavily at the X-band, so compensation design is used for meeting the harmonic condition of inverse class-F on the current source plane. Experiment results show that, in the continuous-wave mode, the power amplifier achieves 61.7% power added efficiency (PAE), which is 16.3% higher than the class-AB power amplifier realized by the same kind of HEMT. To the best of our knowledge, this is the first inverse class-F GaN internally-matched power amplifier, and the PAE is quite high at the X-band.

Improved modeling of GaN HEMTs for predicting thermal and trapping-induced-kink effects

In this paper, an improved modeling approach has been developed and validated for GaN high electron mobility transistors (HEMTs). The proposed analytical model accurately simulates the drain current and its inherent trapping and thermal effects. Genetic-algorithm-based procedure is developed to automatically find the fitting parameters of the model. The developed modeling technique is implemented on a packaged GaN-on-Si HEMT and validated by DC and small-/large-signal RF measurements. The model is also employed for designing and realizing a switch-mode inverse class-F power amplifier. The amplifier simulations showed a very good agreement with RF large-signal measurements.

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

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 Methodology for Implementation of High-Efficiency Broadband Power Amplifiers With Second-Harmonic Manipulation

This brief describes a methodology for designing broadband power amplifiers (PAs) based on the manipulation of second-harmonic impedance. The proposed approach starts from the inverse class-F (F−1) design specification and extends it to a family of specifications that allow achieving better than class-B performance in terms of output power and drain efficiency. It is shown that the achievable performance can be maintained over a wide frequency range reaching 57% of fractional bandwidth. As such, the proposed approach permits higher flexibility and offers relaxed matching constraints in implementing broadband high-efficiency PAs. For the experimental implementation, the proposed approach is adopted to implement a broadband PA using a 10-W gallium-nitride transistor. Peak drain efficiencies greater than 75% are measured over carrier frequencies between 500 and 900 MHz. The saturated output power levels, on the other hand, are higher than 10 W with a maximum of 13 W. Measured results are confirmed to be in good agreement with theory and simulations.