Operational amplifier band width

Three types are used. (I) Barrier-layer cells. These are satisfactory only for simple filter instruments. (2) Vacuum phototubes. These tubes require an external power supply, unlike barrier-layer cells, and their output is usually amplified before measurement. (3) Photomultiplier tubes are easily the most satisfactory detectors for use in flame photometry. The photocurrent is amplified inside the tube in such a way that much lower light levels can be detected and measured accurately than is possible with vacuum phototubes with amplifiers. A stable source of high voltage up to perhaps 2000 volts is required to operate the photomultiplier tubes, but these tubes are almost universally used in high-performance instruments and are essential if the advantages of using narrow band width are to be obtained. 

The three curves for amplifier-limited operation have been calculated assuming typical amplifier normalized noise currents of 0.3,1, and 3 pA/a/Hz for the respective band-widths 1 kHz, I MHz, and I GHz. The applicable value depends upon the specifications of the chosen amplifier. Generally, the best performance is obtained by integrating the amplifier and detector on the chip. Avalanche multiplication can supply pre-amplification current gain and reduce the expected NEP by a factor from 10 to 100. At 1 GHz bandwidth, this can bring performance to within an order of magnitude of photon noise-limited behavior. 

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