Page 86 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Source: Photodetection and Measurement

Chapter
4

Interlude: Alternative Circuits and

Detection Techniques

4.1 Introduction

The majority of photoreceiver designs fall into a couple of simple configurations,
usually a reverse-biased photodiode with voltage follower or transimpedance
amplifier and resistive load. However, these are not the only ways to detect light
and design receivers, and we should be open to alternative approaches. In this
section we look at a handful of less common but occasionally very useful
photodetection configurations.

4.2 Optical Feedback Systems
We have pointed out the considerable difficulty of choosing the transimpedance
feedback resistor in some applications. Perhaps the high values required are
not available, they are too expensive, their temperature coefficients are too
large, they show excessively high parasitic capacitance, or they are physically
too large for the application. This begs the question: what is a resistor? We
could be pedantic and say that it is a two-port linear network for converting a
time-varying voltage into a proportional time-varying current! There are many
other ways to perform this function. Figure 4.1a shows a transimpedance
configuration drawn “upside down,” with its “two-port linear network”
underneath. Rather than considering the output voltage as the result of
the photocurrent, here we say that it is the voltage V o(t) that drives the
photocurrent I p (t) through the transimpedance network and photodiode.
Below the transimpedance are four circuit fragments that perform the same
function.
Figure 4.1b is just the transimpedance itself. Figure 4.1c uses the voltage V o(t)
to drive an LED, part of whose output generates a photocurrent I p (t). Under
the above definition the resistor, LED, photodiode combination could equally be

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