Most design engineers know that capacitance is the enemy of low noise design, and that the smallest distance between detector and amplifier the better. Therefore, mounting the amplifier in the same package as the detector offers a good noise advantage, in terms of capacitance and shielding. An example of a hybrid photodiode amplifier is shown in figure 1. This device consists of a large area photodiode, very low noise amplifier, and a 10 giga-ohm feedback resistor. The small size of the hybrid allows the device to be easily mounted on a thermal-electric cooler.
Monolithic devices offer the highest level of integration and compactness.
In its simplest form, the photo IC might consist of a photodiode and very simple current amplifier. Hamamatsu's S7815, which was designed to replace CdS photocells, is such a device. The CdS photocell is a very low cost device often used in auto dimming, twilight detection, and exposure meter applications. CdS has the disadvantage of using Cadmium, a heavy metal that is considered harmful to the environment. An amplifier is generally used to increase current, which in turn increases the cost and parts count.