Detectors and Preamplifiers
Matching the characteristics of detectors to a suitable preamplifier for a given application is something that is often given little consideration. The reason is often that the engineer specifying the combination of detector and preamplifier for a system is much more interested in either the optics side or the electronics side of the system, and the critical nature of this interface is not well understood. In addition there are several misconceptions about various detectors and preamplifiers, as well as misleading rules of thumb, that abound in the industry.
The usual approach is to select a preamplifier recommended for general purpose use by the manufacturer of the detector. For many applications, especially those requiring only narrow signal bandwidth, this approach is adequate, and one need not be concerned about it further. However, there are other applications where the signal to noise ratio is a critical parameter, where this signal to noise ratio is limited by the combination of the detector and preamplifier, and where the usual approach results in a lower than optimum signal to noise ratio. In these cases one must understand the various sources of noise and their transfer functions to the output of the preamplifier and how to select and adjust the preamplifier to achieve the optimum signal to noise ratio for the required signal bandwidth.
Simply understanding the sources of noise in the system is important. Once one does that, one can evaluate whether there are ways to improve the overall signal to noise ratio. Also, by comparing the theoretical noise performance to that actually being exhibited, one can immediately tell whether a physical system is working well, or whether there are problems that need tracking down.
I have developed preamplifiers for light levels ranging from laser illumination to photons per second, and for electrical bandwidths ranging from 1 Hz to 20 MHz. Most recently I have helped a client plan and manage a program to produce a series of cooled transimpedance preamplifiers to be used at high bandwidth with InSb photodiodes for mid-infrared lidar applications.
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Copyright © 2002, David F. Schaack. All Rights Reserved. |