Christer M. Andersson

A Modified Doherty Power Amplifier With Extended Bandwidth and Reconfigurable Efficiency

This paper derives the theory and presents measurements of a new power amplifier based on the Doherty power amplifier topology. It is theoretically shown that the proposed amplifier can simultaneously provide high efficiency at both full output power and at output power back-off, over a much improved bandwidth compared to the conventional Doherty power amplifier. It is also shown that the proposed amplifier allows reconfiguration of the efficiency in power back-off without the need of tunable elements.

High-Efficiency RF Pulsewidth Modulation of Class-E Power Amplifiers

A new switch-mode power-amplifier (SMPA) topology particularly suitable for energy efficient amplification of RF pulsewidth modulation (RF-PWM) signals is derived. It is analytically shown that high efficiency can be maintained over a wide power dynamic range if the imaginary part of the Class-E load impedance is varied along with the duty cycle (pulsewidth). Using the theory developed, an explicit design procedure is presented that allows practical realization of the proposed topology from the circuit and component specifications. Following the design procedure, and using in-house (Chalmers University, Göteborg, Sweden) SiC varactor diodes to implement the tunable imaginary load impedance, a 2-GHz 10-W peak output power GaN HEMT circuit demonstrator is realized. RF-PWM input signals for characterization of the prototype power amplifier (PA) is generated with a dedicated 65-nm CMOS modulator. The measurements show that a drain efficiency >; 70% can be obtained over an 6.5-dB dynamic range, which verifies the theory presented and demonstrates the feasibility of the proposed PA topology.

A SiC Varactor With Large Effective Tuning Range for Microwave Power Applications

SiC Schottky diode varactors have been fabricated for use in microwave power applications, specifically the dynamic load modulation of power amplifiers. A custom doping profile has been employed to spread the C(V) over a large bias voltage range, thereby increasing the effective tuning range under large voltage swing conditions. The small-signal tuning range is approximately six, and punch through is reached at a bias voltage of -60 V, while the breakdown voltage is on the order of -160 V. An interdigitated layout is utilized together with a self-aligned Schottky anode etch process to improve the Q-factor at 2 GHz, which is 20 at zero bias and approximately 160 at punch through.

Theory and Design of Class-J Power Amplifiers With Dynamic Load Modulation

A theory for class-J microwave amplifier operation as a function of drive level and fundamental load impedance is derived. Calculations show that, under appropriate operating conditions, it is sufficient to modulate the transistor load reactance to enable high-efficiency operation (>;70%) over a large output power dynamic range (>;10 dB) with high transistor power utilization. Such dynamic load modulation (DLM) networks are an ideal application of continuously tunable varactor technologies. Multiharmonic load-pull measurements are performed on a GaN HEMT and experimentally verify the theory of operation. A demonstrator amplifier using an SiC varactor technology is then designed and characterized by static measurements. The amplifier has a peak power of 38 dBm at 2.08 GHz and maintains efficiencies above 45% over 8 dB of power dynamic range. An analysis of the load network losses is performed to show the potential of the class-J DLM transmitter concept.

A novel wideband and reconfigurable high average efficiency power amplifier

This work presents theory and measurements of a new power amplifier based on the Doherty topology. It is theoretically shown that, by using a new set of design parameters with a Doherty power amplifier topology, the proposed amplifier can provide high efficiency, both at full output power and at power back-off, over a much improved bandwidth compared to the conventional Doherty power amplifier configuration. It is also shown that the proposed amplifier allows for simple reconfiguration of the efficiency in power back-off. A demonstrator circuit using individually controlled main and peak amplifier input signals is designed to validate the theoretical findings. Measurements show state-of-the art efficiency bandwidth at both full output power and at power back-off, as well as reconfigurable efficiency, thus validating the theory and demonstrating the potential of the proposed PA for use in future wireless transmitter applications.

Decade bandwidth high efficiency GaN HEMT power amplifier designed with resistive harmonic loading

The use of resistive loading at higher harmonics in wideband power amplifier design is proposed. Although the theoretical efficiency of such operation is lower than other classes the significantly simplified load network design potentially allows for multi-octave realizations. A decade bandwidth (0.4–4.1 GHz) GaN HEMT power amplifier was thereby designed, delivering more than 40 dBm output power with 10–15 dB gain and 40–62% drain efficiency. Linearized modulated signal amplification was then successfully demonstrated at multiple frequencies (0.9 to 3.5 GHz), using various downlink signals (LTE, WCDMA, WiMAX), with resulting ACLR lower than −46 dBc.

High efficiency RF pulse width modulation with tunable load network class-E PA

In this paper, a 10 W peak power 2 GHz highly efficient RF pulse width modulation (RF-PWM) based transmitter is presented. RF-PWM signals are generated with a dedicated 65 nm CMOS modulator and subsequently amplified with a GaN Class-E power amplifier (PA). The modulator use extended drain MOS (EDMOS) high voltage transistors to provide the required voltage swing to drive the GaN used as a switch. The imaginary load impedance of the Class-E is electronically tunable and is implemented with in-house high breakdown voltage SiC varactors. The tunable imaginary load impedance enables optimization of the Class-E versus the duty cycle (pulse width). The peak efficiency is therefore preserved over a wide range of output power levels. The measured drain efficiency of the Class-E output stage is above 70% over a 6.5 dB output power dynamic range. The overall transmitter efficiency is above 60% for the same dynamic range.

Nonlinear Characterization of Varactors for Tunable Networks by Active Source–Pull and Load–Pull

Varactors are key components in the realization of tunable networks, for instance, in high-efficiency power-amplifier architectures. This paper presents a method to measure the varactor quality factor (Q-factor) in the presence of nonlinear distortion. The importance of correctly choosing the loading condition at the second harmonic is illustrated by multiharmonic active source- and load-pull measurements. Furthermore, the method also allows for accurate extraction of the bias-dependent series resistance without the need of area consuming resonant structures. The proposed method is applied to characterize a silicon-carbide (SiC) Schottky diode varactor at 3 GHz. The measured results reemphasize that varactors are not linear tunable, but inherently nonlinear components that require proper consideration of the higher order harmonics.

A class-J power amplifier with varactor-based dynamic load modulation across a large bandwidth

A novel class-J operated power amplifier (PA) utilizing varactor-based dynamic load modulation is presented. It is theoretically shown that the proposed PA can maintain high average efficiency across more than 35% RF bandwidth by means of a purely reactive load modulation after the transistor output capacitance. The theory is experimentally verified by a 15 W GaN HEMT PA operating from 1.80 to 2.20 GHz, using SiC varactors as dynamically tunable load elements. In the band, the PA presents a power added efficiency (PAE) higher than 39% at 6 dB output power back-off. For a 3.84 MHz W-CDMA signal with 6.7 dB peak to average power ratio, an average PAE higher than 39% and an adjacent channel leakage ratio below -45.8 dBc are obtained across the entire band after linearization.

A 44 dBm 1.0–3.0 GHz GaN power amplifier with over 45% PAE at 6 dB back-off

In this paper we present a 1.0-3.0 GHz GaN power amplifier with significant back-off power added efficiency (PAE) enhancement. The design of the PA, which has two digitally controlled RF inputs, was derived from a practical combining network without assuming any idealized conditions at the higher harmonics. By using linear multi-harmonic calculations, a GaN PA design was optimized for high efficiency with a 6.7 dB peak to average power ratio (PAPR) WCDMA signal over 108% fractional bandwidth. Measurements on the assembled circuit demonstrated 44 ± 0.9 dBm maximum output power, and a drain efficiency and PAE at 6 dB output power back-off (OPBO) larger than 48% and 45%, respectively.