
426 OPTICS FOR ENGINEERS
but not always, this dynode chain consists of resistors with identical values. If the currents
arising from electron ow in the PMT, shown as current sources in the gure, are small, then the
voltages between adjacent diodes will all be the same. To maintain this condition, the resistors
must be small enough so that the resistor current exceeds the current in the photomultiplier.
The smaller resistors, however, result in greater power dissipation in each stage, V
2
mn
/R
mn
, and
this power is all dissipated as heat. Alternative bias circuits including active ones that provide
more constant voltage with lower power consumption