Klasse-S Mikrowellen-Leistungsverstärker mit GaN-Transistoren (Volume 18) (English shop)

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This work presents a comprehensive analysis of the class-S concept in the microwave frequency range. Voltage-mode (VMCS) and current-mode (CMCS) class-S amplifiers based on GaN MMICs are designed, realized and characterized by measurement for the 450 MHz band. The contributions of the work are structured into three main topics.

First, a time-domain measurement setup is developed. This makes it possible to analyze the signal waveform of the digital power switch MMICs used in the class-S amplifiers. Various GaN power switch MMICs including their respective driver circuits were characterized. They achieve a drain efficiency of up to 90 % and broadband (digital) output powers of up to 20 W.

Subsequently, a switch-based transistor model is presented, which was developed for large-signal time-domain simulations of switching amplifiers. It offers direct access to the parasitic elements of the transistor such as the drain-source on-resistance Rds ON and the drain-source capacitance Cds. This allows, for example, loss mechanisms to be identified more precisely. Due to its simple structure it converges faster than the standard models at the same accuracy.

The focus of this work lies on the design, optimization and analysis of class-S amplifiers with particular consideration of the associated output networks. A detailed theoretical and practical investigation of the networks for the voltage-mode and current-mode configurations is carried out. They are key elements for the operation and the efficiency of the class-S concept, especially in current mode, where the filter has a differential input. Amplifiers were realized both in current-mode (CMCS) and in voltage-mode topology (VMCS) for the 450 MHz band. The achieved results represent a significant advance for the state of the art of class-S amplifiers in the microwave frequency range. In class-S operation with a 1-tone bandpass ΔΣ (BPDS) bit sequence, the VMCS and CMCS amplifiers reach peak efficiencies of 50 % and 41 % at maximum output powers (Pout) of 3.4 W and 9 W, respectively. However, if the coded signal input power is reduced by 10 dB, the drain efficiency falls below 10 %. Although the achieved results represent best values for microwave class-S amplifiers in this frequency range and for these output powers, they must be improved in order to make the concept competitive.

After identifying the main loss mechanisms, various approaches to improving the efficiency, especially at power back-off, were investigated. The results show that varying the oversampling ratio of the BPDS-modulated signal leads to no significant improvements. The coding efficiency of the modulation, however, plays a key role with regard to efficiency and output power. With a periodic square-wave signal at the input, which exhibits the maximum coding efficiency, Pout and efficiency can be increased considerably. The amplifiers deliver up to 19 W in the current-mode and 7 W in the voltage-mode configuration. This corresponds to approximately a doubling of the BPDS results. In the course of the investigations on the coding of the input signal, the class-S amplifiers were also tested with pulse-width modulation (PWM) for coding the amplitude in back-off operation. Here, a version of the voltage-mode configuration without freewheeling diodes now shows 83 % drain efficiency at full drive (Pout = 3 W) and an improvement at 10 dB power back-off from 11 % (BPDS) to 40 %. The efficiency of the current-mode amplifier increases with such a PWM signal to 20 % at 10 dB back-off (10 % for BPDS modulation). Moreover, the achieved results form an excellent basis for further improvements of the class-S concept with regard to efficiency, especially at high power back-offs.