Mohammadhassan Akbarpour

Analytical Design Methodology for Generic Doherty Amplifier Architectures Using Three-Port Input/Output Networks

In this paper, a new Doherty amplifier architecture along with an analytical based design methodology is proposed. The proposed architecture uses novel three-port network as the output matching/combining network (OMCN). The three-port OMCN performs the power combining for any arbitrary output power ratios from the two transistors of the Doherty amplifier. It also performs the impedance matching from any arbitrary complex load impedance to the optimum impedances for both transistors at peak output power. Commonly in Doherty amplifiers, an optimum performance at peak power and a sub-optimum performance at power back-off are often obtained. Using the proposed output network, optimum performance can be reached at power back-off, as well as at peak power. Another three-port network is proposed for input matching/dividing network (IMDN) at the input of the proposed Doherty amplifier. The three-port IMDN is designed to perform the power division with any arbitrary division ratio, adjust the arbitrary phase difference between the input signals to the two transistors, and provide the impedance matching from any arbitrary complex source impedance to the optimal source impedances for the two transistors. To verify the provided theory, two prototype amplifiers are designed and tested. A 12-W amplifier is designed for a 50-Ω source and load impedances at 1 GHz. Another 12-W amplifier is designed at 1 GHz for complex source and load impedances. Both amplifiers have efficiency of higher than 50% at 7-dB output power range.

Broadband Doherty power amplifiers

In this paper, the transformer-less load-modulated (TLLM) architecture for designing broadband Doherty amplifiers is presented. In this architecture, the quarter-wave impedance transformers of the Doherty amplifier are not used. As a result, the design will be much more compact and large operational bandwidths can be obtained. The simulation results for an amplifier implemented using this architecture shows that drain efficiency of higher than 47% is obtainable at 6dB power back-off in 1.7-2.8GHz frequency range (50% fractional bandwidth).

Current-Biasing of Power-Amplifier Transistors and Its Application for Ultra-Wideband High Efficiency at Power Back-Off

A new biasing scheme is proposed for transistors used in power-amplifier applications. In the proposed biasing scheme, a constant current source is used as the power supply for the transistor’s output terminal. A whole new family of amplifier classes can be defined using this biasing scheme. Analytical equations are obtained and verified for current and voltage waveforms of a current-biased transistor for both resistive and tuned load impedances. Using the proposed current biasing scheme, the transistor presents completely different behaviors compared with the conventional voltage biasing scheme. These properties can be utilized for new design concepts and can provide new possibilities in the future designs and applications. Some of the differences between current-biased and voltage-biased amplifiers are discussed. One of the different behaviors shown by the current-biased transistor amplifiers is the reversed load modulation. Using this property of the current-biased amplifiers, a reversed modulation dual branch (RMDB) amplifier structure is proposed for ultra-wideband high efficiency at power back-off. Due to the reversed load modulation of current-biased transistors, a multibranch amplifier can be implemented to obtain high efficiency at power back-off without the need for an impedance inverter at the output of the current-biased amplifier. By using the proposed amplifier structure, a wideband RMDB amplifier was fabricated and tested exhibiting higher than 37% efficiency for long-term evolution (LTE) signals in 0.8–2.2-GHz bandwidth, which is equivalent to 93% fractional bandwidth.

Sequential Load-Pull Technique for Multioctave Design of RF Power Amplifiers

A design methodology for multioctave RF power amplifiers (PAs) is presented in this brief using sequential harmonic characterization. The investigation of in-band harmonics on the multioctave performance of the RF PA shows that a more optimal performance can be achieved using the proposed technique compared to the conventional load-pull. To demonstrate the capability of this technique, a broadband multioctave PA prototype was designed and fabricated using sequential second harmonic characterization. The fabricated prototype uses a GaN HEMT from Cree Inc. and exhibits a drain efficiency of 53%-64% in the range of 0.7-4.0 GHz while having an output power of 37.5-39.1 dBm corresponding to the fractional bandwidth of 140%. Measurement results validate the proposed method as a robust PA design procedure for broadband and multistandard applications.

A Theorem for Multi-Frequency DC-Feed Network Design

In this letter, a novel and rigorous analytical design approach formulated as a theorem for the exact design of arbitrary multi-frequency DC-feed network is presented. While the conventional method necessitates optimization to arrive at a near exact solution, the proposed theorem discards the need of optimization. Moreover, by blending the theorem with the power of todays RF/microwave CAD tools, a CAD assisted design methodology is proposed that is able to provide with exact design parameters very quickly. To demonstrate the effectiveness of the proposed theorem, several design cases have been worked out and compared with the existing design.

Efficiency optimized 60 GHz CMOS Power amplifier for high PAPR signals

In this paper, a 60 GHz power amplifier is presented in 65nm CMOS technology based on the transformerless load-modulated (TLLM) amplifier architecture. It is designed to obtain high efficiency at output power back-off. The amplifier has 12.5 dB small signal gain and 11.5 dBm saturated output power. Measurement results show that this amplifier has more than 8% drain efficiency (7% power-added efficiency) in the last 6 dB output power range.

Broadband high efficiency GaN RF power amplifier for multi-band applications

This paper presents the broadband power amplifier design based on characterized behavior of the active device. Compared with other broadband design techniques, this approach is not limited to waveform manipulating; hence, multi octave performance is achievable. GaN HEMT package and die device models are used to verify this approach showing high efficiency performance over very large bandwidth. Simulation results show PAE of higher than 60% over 0.5-4 GHz for die and higher than 50% over 0.3-3.5 GHz for package device models.

Two-Dimensional Piecewise Behavioral Model for Highly Nonlinear Dual-Band Transmitters

This paper presents a two-dimensional (2-D) piecewise model for the dynamic behavioral modeling and digital predistortion of highly nonlinear dual-band radio frequency (RF) power amplifiers (PAs). The model performance was validated in two cases: an envelope-tracking dual-band PA and a current-controlled wide-band PA, using carrier-aggregated long-term evolution signals. The proposed model is partitioned according to different functions of composite multi-input amplitudes rather than the single input amplitude. Measurement and simulation results show that this model is much more flexible in modeling highly nonlinear RF transmitters with inconsistent behavior at different multi-input levels. The experimental results for dual-band cases confirm that the 2-D piecewise model can produce better performance than conventional models that have limited performance in characterization of these hard nonlinearities.

On the Dual-Frequency Impedance/Admittance Characteristic of Multisection Commensurate Transmission Line

In this brief, first an interesting dual-frequency feature of a section commensurate transmission line (TL) is reported. The property concerns the impedance (or admittance) looking into a multisection TL terminated with real impedance. A rigorous mathematical analysis is provided to prove the existence of the property. Second, as an application, a novel multisection dual-frequency impedance transformer for the real impedance is presented. The proposed transformer can be used to provide matching over a much wider transformation ratio that is otherwise impossible to realize using the current state-of-the-art of dual-frequency transformers. The electromagnetic (EM) simulated and measured results exhibit good performance.

Chapter 4 – mmW Doherty

One of the important specifications of the power amplifiers used in transmitter systems is the energy efficiency. The Doherty amplifier is a well-known structure used in RF and microwave transmitter systems to improve efficiency for transmitters with large peak-to-average power ratio (PAPR) signals. The Doherty amplifier works based on active load modulation to provide high efficiency over large output power back-off. In millimeter-wave (mmW) frequencies, however, the design of the Doherty amplifier is challenging because of the requirements in the Doherty structure and relatively poor performance of active and passive components at the mmW frequency range. In this chapter, the Doherty amplifier, its working principles, and its design considerations are described. Then, the design challenges and considerations for the design of mmW Doherty amplifiers in silicon technologies are described.