Figure 8. Photoconductive mode of operation circuit example: Low Light Level / Wide Bandwidth
Where GBP is the Gain Bandwidth Product of amplifier (A1) and CA is the amplifier input capacitance.
In low speed applications, a large gain, e.g. >10MΩ can be achieved by introducing a large value (RF) without the need for the second stage.
Typical components used in this configuration are:
In high speed, high light level measurements, however, a different approach is preferred. The most common example is pulse width measurements of short pulse gas lasers, solid state laser diodes, or any other similar short pulse light source. The photodiode output can be either directly connected to an oscilloscope (Figure 9) or fed to a fast response amplifier. When using an oscilloscope, the bandwidth of the scope can be adjusted to the pulse width of the light source for maximum signal to noise ratio. In this application the bias voltage is large. Two opposing protection diodes should be connected to the input of the oscilloscope across the input and ground.
Figure 9. Photoconductive mode of operation circuit example: High Light Level / High Speed Response