It behaves as an almost constant current source because of its very high output voltage coupled with its very high output resistance and so it supplies the same few microamperes at any output voltage up to hundreds of thousands of volts ( or even tens of megavolts ) for large laboratory versions.
22.
The real reason for driving an LED from a voltage source through a resistor, rather than directly from a hypothetical voltage source of the exact same voltage as the LED's forward voltage drop, is that the resistor-plus-voltage-source approximates a constant current source.
23.
For this purpose, some circuits are driven by a constant current source or by a real voltage source with internal impedance : current-to-voltage converter ( transimpedance amplifier ), capacitive integrator ( named also current integrator or charge amplifier ), resistance-to-voltage converter ( a resistive sensor connected in the place of the impedance " Z " ).
