How fast is a photoresistor when used in RF?

Capacitance

The equivalent circuit for the output photocell is a resistor in parallel with the capacitance. The capacitance arises from the topside metallization of the electrodes which form a coplanar capacitor. The value of this capacitance is largely determined by the size of the ceramic base. For lower capacitance, a smaller cell is needed. The capacitance is so small (3.0 pF, typical on catalog AOIs) that it is negligible in most applications. However, there are applications such as wideband or high frequency amplifiers in which the capacitance needs to be considered. At 4.5 MHz, the video baseband frequency, the photocell capacitive reactance is only 12 kilohms. If the phase shift of the signal is to be kept below 10°, the highest useful cell resistance is only 2.0 kilohms. At high AOI input drive, where the cell is drive below 1.0 kilohm, the capacitance can increase additionally from 2 to 10 times, possibly due to distributed effects.

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FvM said:

Please review this paragraph from the PerkinElmer Vatrol databook:


You can calculate that the LDR capacitance reduces the achievable control dynamic with increasing frequency.

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