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active voltage divider |
You need to provide a series of voltages to the dynodes of a photomultiplier to make it work. An active voltage divider ensures constant gain up to a mean anode current of 100 µA, the maximum permitted in most photomultiplier types. Power consumption is typically 100 mW making them particularly suited to portable instrumentation. An active divider consists of a string of high impedance resistors connected in parallel with a string of FETs. The resistors provide reference voltages for each stage of the active string to pick off and apply to the dynodes. An ideal active divider is one that can sense these voltages over the full dynamic range of anode currents without unwanted load effects. Active dividers are suitable for pulsed applications with the addition of decoupling capacitors. They can be constructed with FETs only in final stages and with resistors for earlier stages. However, Electron Tubes prefers to use FETs throughout, as shown in figure 1. There does have to be at least one resistor, R, in the active string in order to set the standing current. Reliability and temperature stability are excellent in active dividers and the improvement in linearity of performance at reduced cost to power consumption is a rare prize. Further information on voltage dividers can be found in our photomultiplier brochure, technical reprint Photomultiplier Voltage Dividers RP/085 or by contacting us. |
