This work discusses the efficiency bandwidth constrains in Doherty amplifiers. An analysis of the bandwidth limitations imposed by the impedance inverter and output capacitance of the active devices is given.Alternative wideband matching and output connection schemes for Doherty amplifiers are evaluated for their efficiency performance both at full output power, as well as , in power back-off operation. The presented theory is verified using a Doherty demonstrator amplifier, which allows independent control of the input signals for the main and peak device. The related measurements show the largest high-efficiency bandwidth reported up to date for a Doherty amplifier.
Laboratory produced test specimens are usually obtained with devices like Marshall compaction, gyratory compaction, or roller compaction. However, with these methods it is difficult to control the sample-to-sample variation of the final density of the test specimens, which can strongly influence the results of performance testing. It is very important to have a repeatable and efficient production method of test specimens available in the laboratory. The shear box compactor was recently developed to simulate field compaction with a constant compressive force and a cyclic shear force with constant maximum shear angle applied to the asphalt mixture. The shear box compactor produces asphalt blocks with a size of 450 mm in length, 150 mm in width, and 145–185 mm in height. Test specimens like beams or cylinders can be obtained from the block for laboratory performance testing. In this paper, the compaction results with the shear box compactor are reported for asphalt mixtures with different gradations and binder types. Asphalt specimens with different mixture compositions, shapes, sizes, and sampling positions were investigated by volumetric properties. Finite element modeling was introduced to obtain more understanding of the compacting process of the shear box compactor. The results indicate that the decreasing of voids content of asphalt mixtures during compaction process is dependent on the gradation than the binder type. The asphalt mixture specimens obtained from the same asphalt mixture block has a variation in voids content of less than 1 %. Test specimens obtained from the upper part of the asphalt block are more compacted than specimens from the lower part. And the specimens obtained close to the side of the block are less compacted due to lack of shear stress. As a result, the shear box compactor provides a reliable means of sample preparation, making it very suitable for producing specimens with constant volumetric properties.
Durable and sustainable asphalt pavements require high quality asphalt materials. In pavement design, the tensile strength of asphalt concrete is a fundamental engineering property. Due to the characteristics of bitumen, the tensile strength is a function of temperature and loading rate. In the past some regression equations were developed to predict the tensile strength of asphalt concrete. However, these equations are only valid for a narrow temperature and strain rate ranges and do not relate to the material properties. This paper proposes a new general equation for the prediction of the tensile strength, which is based on tension tests performed at different test conditions and relates to the complex modulus of the bitumen. By means of the Dynamic Shear Rheometer (DSR), frequency sweeps were done to obtain a master curve for the complex modulus of the bitumen. Uniaxial tensile strength (UTS) tests were conducted to get the tensile strength of the asphalt mixture at different temperatures and loading rates, respectively. In this paper, an S-shaped prediction curve is used to link the tensile strength of the mixture and complex modulus of the bitumen to each other. The test results indicate that the proposed prediction model has a low relative error, which means that it gives a realistic description for the tensile strength of an asphalt mixture.
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