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Show tributing to low 1/ f noise. 4.3 HBT Circuit Applications HBTs can be used for the same applications as a silicon bipolar transi tor. However it has higher speed and frequency of circuit operation. Digital functions attractive for the HBT technology are components for high-speed switching with low phase noise as required for phase-locked loops and direct digital frequency synthesizers, such as fixed and variable dividers, multiplexers and demultiplexers and A/D and D /A converters. They are also being exploited for high-performance analog circuits such as operational amplifiers. The excellent device matching, high intrinsic gain, high linearity and high speed of the GaAs HBT make it ideally suited for A/D conversion applications. Its main limitation is its lower maximum integration level compared to the advanced silicon BJT technologies, which are demonstrating increasing A/D converter performance capabilities. For ultra-high-speed, low-to-moderate resolution A/D converters, the GaAs HBT is particularly attractive. Its superior device matching and higher intrinsic gain allow the HBT to outperform the GaAs FETs, and its higher intrinsic speed provides an advantage over silicon BJTs. For high-speed and high-resolution A/D converters, the GaAs HBT also has an advantage over competing technologies. Lower parasitic capacitances and higher intrinsic device speed and linearity give HBT an advantage over silicon bipolar transistors. The main limiting factor in achieving monolithic high-resolution HBT A/D converters is the level of integration. However, multichip HBT A/Ds will be able to outperform Silicon BJT versions if power dissipation is not a major factor. The poor device matching, low intrinsic device gain, and high 1/ f noise of GaAs FETs make them much more difficult to use in high-resolution A/D converters. HBT's advantages for linear, wide-band A/Ds will result in reduced developmental costs as compared to GaAs FET approaches. With such attractive advantages over other technologies, HBT is the rightful choice for our high speed sigma-delta A/D converter. |
