FBH research: 11.10.2018

Highly efficient load tuning assisted RF power amplifier with discrete-level supply modulation

Insertion and return loss of the matching network tuned to various states
Fig. 1: Insertion and return loss of the matching network tuned to various states and the envelope of all tuning states combined
Mounted tunable PA module
Fig. 2: Photograph of the mounted tunable PA module, varactors marked red, GaN HEMT green

This year, FBH’s RF Power Lab presented two papers at the International Microwave Symposium in Philadelphia and one paper at the co-located ARFTG – Microwave Measurement Conference. All presented papers show future directions towards more agile telecom systems, operating at much larger instantaneous bandwidth and at a more limited power budget.

The first work [1] deals with a load tuning assisted RF power amplifier with class-G, i.e. discrete level supply, modulation. This very flexible topology allows to evaluate load as well as supply modulation in the same design. The load tuning is implemented using commercial ceramic thin-film barium-strontium-titanate (BST) varactors forming a π-type tunable matching network at the output. The power amplifier including output matching network is realized very compactly inside a modified RF transistor package, shown in Fig. 2. With load tuning the wideband performance of the RF power amplifier can be optimized. As can be seen in Fig. 1, improved matching, i.e. return loss, can be tuned from 1.4 GHz to 2.3 GHz by setting alternative control voltages on the varactors. The flexible hardware developed in this work shows that the PAEavg of a 60 MHz wide 11.9 dB PAPR signal is improved by 4 percentage points using load tuning and further 10 percentage points due to supply modulation. The improved PAE values reached by supply modulation can be supported by load tuning to obtain linearity. Despite the slow response of the commercial thin-film BST varactors the results shows that additional advantages can be achieved by implementing tunable matching networks based on this technology in RF power amplifiers. This work is an extension of a project financed by the German Research Foundation (DFG).

In the second topic presented, the impact of charge trapping effects in a GaN-HEMT power amplifier is analyzed and quantified using a low complexity model for drain-lag effects [2]. Charge trapping effects are inherent in GaN devices and cause a temporary shift in the intrinsic bias point, leading to non-linear effects in the device. The response times of the traps are in the same time range as the bandwidth of the modulated signal for modern telecom signals. In this work, the peak drain-source voltage under modulated operation is used to define a dynamic shift of the threshold voltage, resulting in a variation of the bias point. Quantification is based on continuous wave (CW) measurements, as shown in Fig. 3, but allows accurate modeling of the drain current under modulated operation. Therefore, it enables to accurately predict the efficiency for wideband-modulated operation. The theory extracted from the CW-based model is verified by wideband-modulated measurements using an eight-tone signal with 28 MHz instantaneous modulation bandwidth. The peak-to-average power ratio of the signal is swept in a range from 2 to 9 dB to vary the peak drain-source voltage at constant average output power. The findings bring further evidence that charging times for some traps in GaN-HEMTs are in the picosecond range and thus fast enough for the RF swing to charge the traps. Most interesting and much appreciated, this work also provided a comprehendible explanation for the well-known drain current valley shown in power sweeps of GaN-HEMT devices at medium power (see Fig. 3). This effect simply arises through combining the reduced current due to drain lag caused by increasing peak voltage with a power drive until the point where the drain current is dominated by the power-induced drain current increase, which can be observed for all RF power amplifiers biased in class-AB.   

The third paper was presented at the ARFTG microwave measurements conference, introducing a galvanically isolated probe for wideband drain current measurements of a supply modulated RF power amplifier [3]. Supply modulation of RF power amplifiers (PA) is a powerful efficiency enhancement technique. To optimize the RF PA and the supply modulator the dynamic low-frequency drain current is of large interest. However, measurements are difficult due to the large voltage variations at the PA drain bias supply terminal, the wide required bandwidth for modulated signals and the non 50 Ω, low impedance interface. A non-invasive technique is preferred to the use of bulky directional couplers. In principle, a sensor based on a shunt resistor is a favorable choice, but the extremely large common-mode voltage complicates its use. In the paper, a measurement technique shown in Fig. 4 is investigated. It is based on reflection measurements of an active reflector element using a 24 GHz Doppler radar interferometer. The system allows to conduct very wideband current measurements in the GHz range with high common-mode rejection and low parasitic loading of the shunt resistor. Thereby, the method has the potential to meet the requirements for the extreme wide bandwidth signals used in the future telecommunication infrastructure for 5G. The method was developed several years ago and has been patented.

Overall, these results presented at IMS and ARFTG underline the relevance of the topics investigated at FBH’s RF Power Lab, which lead to state-of-the-art results.

References

S. Preis, N. Wolff, F. Lenze, A. Wiens, R. Jakoby, W. Heinrich, O. Bengtsson, "Load Tuning Assisted Discrete-Level Supply Modulation Using BST and GaN Devices for Highly Efficient Power Amplifiers" IEEE MTT-S Int. Microw. Symp. Dig., Philadelphia, USA, Jun. 10-15, pp. 1230-1233 (2018).

[2] N. Wolff, T. Hoffmann, W. Heinrich and O. Bengtsson, "Impact of Drain-Lag Induced Current Degradation for a Dynamically Operated GaN-HEMT Power Amplifier", IEEE MTT-S Int. Microw. Symp. Dig., Philadelphia, USA, Jun. 10-15, pp. 168-171 (2018).

[3] N. Wolff, T. Hoffmann, W. Heinrich, O. Bengtsson, "Wideband Dynamic Drain Current Measurements with a Galvanically Isolated Probe Targeting Supply-Modulated RF Power Amplifiers for 5G Infrastructure", 91st ARFTG Microwave Measurement Conference (ARFTG), Philadelphia, USA, Jun. 15 (2018).