Nikolai Wolff

A three-level class-G modulated 1.85 GHz RF power amplifier for LTE applications with over 50% PAE

A highly efficient three-level class-G modulated RF power amplifier (PA) is presented. The PA is designed to operate as downlink amplifier in the 1800-1900 MHz LTE-band. At 1.85 GHz the maximum output power under continuous wave excitation is 48.2 dBm (66 W). The system achieves an overall efficiency of more than 50% when amplifying a 20 MHz OFDM signal with 9 dB peak-to-average power ratio at 40 dBm (10 W) average output power and over 15 dB gain. In combination with digital predistortion the class-G modulated PA can achieve an ACLR of -36.2 dB and an EVM of 2.1%. The class-G modulation enables excellent efficiency due to absence of a linear amplifier in the modulator stage. The efficiency improvement due to the three-level class-G modulation reaches 18.7 percentage points.

Highly efficient class-G supply-modulated amplifier with 75 MHz modulation bandwidth for 1.8–1.9 GHz LTE FDD applications

This paper presents a broadband and highly efficient class-G supply modulated amplifier system. The system covers the full LTE band 1805–1880 MHz. The core element is the class-G supply modulator which switches the supply voltage of the RF power amplifier between three discrete levels with a minimum pulse duration of 2.5 ns. Measurement results are presented for a carrier aggregated signal using five subcarriers with modulation bandwidths from 5 MHz to 20 MHz each. In combination with digital predistortion the system achieves 38.5% PAE with −41 dB EVM and −45 dB ACLR for a 75 MHz multi-carrier signal with a PAPR of 10.4 dB at 38.5 dBm average output power. The total efficiency enhancement achieved by the class-G modulation is 13 percentage points.

Challenges in the design of wideband GaN-HEMT based class-G RF-power amplifiers

In GaN-based power amplifiers (PA) the gain under continuous wave (CW) excitation compresses slowly over a wide power range. Also, due to memory-effects in the GaN-HEMT, the gain for modulated excitation with varying envelope differs from the CW value. On the other hand, design approaches for supply voltage modulated systems based on post-processing of CW measurements are in widespread use. This paper scrutinizes these approaches in a case study for a class-G modulated PA at 2.7 GHz, i.e. a PA with discrete supply modulation. The errors related to the CW design approach are quantified by comparison with dynamic measurements. Test-signals with equal power distributions but varying modulation bandwidths in a range of 4 MHz to 32 MHz are used. It is found that the dynamic measurement results deviate from the expected CW performance by up to 5% in PAE and 1.5 dB in gain. Responsible for this are on one hand differences in CW and modulated gain. On the other hand the efficiency degrades with increasing supply voltage switching frequency, which is related to the modulation bandwidth and the supply voltage levels.

Complexity of DPD linearization in the full RF-band for a WiMAX power amplifier

In this paper, the influence of channel frequency in the RF-band on digital predistortion (DPD) is investigated empirically. A class-AB GaN-HEMT power amplifier targeting the 3.5 GHz band for WiMAX is used with a 20 MHz OFDM signal. It is found that frequency dependent memory effects in the band have a strong impact on the amount of memory taps needed for the DPD predistorter model. The overall characteristic is dominated by the non-linear behavior of the transistor, the memory effects, and the IQ imbalance introduced by the low cost modulation hardware. Using DPD models with memory that can handle IQ imbalance the EVM is improved by at least 12 dB in backed-off operation.

A 14 W wideband supply-modulated system with reverse buck converter and floating-ground rf power amplifier

This paper presents a wideband supply-modulated system with a floating ground RF power amplifier and a reverse buck topology DC/DC converter. The power amplifier and the reverse buck converter are based on microwave GaN technology. The system is operating at 1620 MHz and 40 V supply and shows 39% overall power-added efficiency at an average output power of 14.6 W for an 8 MHz OFDM modulated signal with 8.6 dB PAPR. The implemented floating-ground RF power amplifier accommodates signals with up to 40 MHz bandwidth. The reverse buck converter switches at 45 MHz with a PAE of 80–91% over duty cycles from 40–100% equivalent to supply voltages of 16–40 V. For the first time a reverse buck topology system enabling GaN switching operation referred to ground is shown in dynamic operation with performance similar to or exceeding classical systems.

Thick-film MIM BST varactors for GaN power amplifiers with discrete dynamic load modulation

Due to their extremely low static current consumption, varactors based on BST are perfect devices for realization of tunable and re-configurable components. This work presents fully screen-printed MIM thick-film BST varactors that are used to tune the load impedance for GaN HEMTs. The varactor tuning voltage is supplied in discrete steps using a high-speed GaN-based modulator. Modulated measurements with LTE and WCDMA signals show, for the first time, the functionality of a BST-based load modulation system and the power consumption of the load-modulation in dynamic operation. Using discrete dynamic load modulation, an average PAE of 27.3% was measured for the LTE signal with an ACLR below −45 dB.

A 56 W power amplifier with 2-level supply and load modulation

Dynamic load modulation is proposed to improve the efficiency of an RF power amplifier with discrete supply modulation. A 47.5 dBm, 2.7 GHz power amplifier is realized to show the potential of the method. The amplifier design is based on continuous wave load-pull measurements to determine the optimum load impedances, using the statistics of a 10.2 dB PAPR OFDM signal. The output network contains a single SiC varactor for adaptive output matching. The supply voltage is adjusted by a 2-level supply modulator. Static evaluation of the amplifier shows an improvement in PAE of approximately 9.7 percentage points at 10 dB back-off due to load modulation.

Analysis of the switching threshold in dual-level class-G modulated power amplifiers

A two-level class-G RF power amplifier system is analyzed. Measurements show that for low output powers there is an optimum switching threshold valid for both PAE and linearity while for higher output powers they diverge and there is a trade-off between PAE and linearity. This creates a region where the output power can be reduced from maximum to 2 dB (58%) back-off in which the PAE and linearity can be kept constant. Furthermore the power amplifier properties can be dynamically adjusted by changing the switching threshold voltage of the class-G modulator.