Compensation of Thermal Effects by Dynamic Bias in Low Noise Amplifiers
Examensarbete för masterexamen
Wireless, photonics and space engineering (MPWPS), MSc
There is an increasing need to understand how thermal effects affect the performance of amplifiers in radar systems. Increased chip power densities are to be expected as the integration of multiple transceivers in SiGe/BiCMOS and in GaN, increases. This will increase the electrical as well as thermal coupling between the transceivers, and the increased temperature is likely to impair the performance of amplifiers. Dynamic bias techniques are used today to increase the efficiency of transmitters. Therefore, it is important to investigate if these techniques can be used also to compensate for thermal effects which affect the elements in a multi transceiver chip. This thesis deals with the development of a GaN based temperature sensor as well as a study of the heat propagation properties in GaN on SiC structures. Furthermore, a study of thermal effects in low noise amplifiers has been carried out, and the use of dynamic bias to compensate for thermal performance deterioration, as well as other features, is demonstrated. A mesa resistor sensor and Schottky diode sensor were designed and evaluated. It was shown that a 15 μm mesa resistor works well as a temperature sensor when biased at an appropriate point. Models for predicting the temperature were developed based on measurements and a calibration method is proposed. It was shown that heat pulses can be detected by the sensors. A sensor area was designed and used to study heat propagation versus distance and temperature. A model describing the response of the sensor was proposed and evaluated. The model was used to study how heat is coupled to the sensor in the GaN and SiC layers. The thermal conductivity was seen to increase significantly in the GaN and SiC layers at lower temperatures. The layer time constants and propagation delay were observed to increase with temperature and distance. Light sources were also observed to impact the sensor current response. It was determined by measurements that thermal effects in general degrades the performance of three evaluated low noise amplifiers, and that dynamic bias control techniques can be used to cancel these effects for certain parameters. Increased power consumption levels was observed when applying dynamic bias control. In addition, it was demonstrated how dynamic bias can be used to eliminate gain recovery effects after high power pulses. Lastly, suggestions for different modes of operation, where dynamic bias is utilized differently, are presented.
Informations- och kommunikationsteknik , Elektronik , Information & Communication Technology , Electronics