Page 41 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Amplified Detection Circuitry

34 Chapter Two

+15V +5V

PD
R L
R f

V o

R e

Figure 2.12 The highest speed transimpedance
amplifiers are often simple circuits made
with heavily reverse-biased, specially selected
low capacitance photodiodes and high GBW
discrete semiconductors.

heavily reverse biased chip-photodiode connected to an R f = 20k transimped-
ance amplifier. The first two transistors in common-collector, common-emitter
connections provide the gain, with overall shunt feedback. The last transistor
is just an emitter follower to drive the output load. Such circuits are useful up
to a few hundred megahertz bandwidth. Even here the stray capacitances and
inductances of the particular layout chosen start to dominate performance.
Beyond those frequencies, up to the 50GHz of the current top-end designs,
similar principles apply, although the search for special components such as
ultra-low-capacitance waveguide-coupled photodetectors becomes a key task.
See Umbach (2001) for a discussion of this high-bandwidth detection.

2.7 Big Problem 2: Limitations of the
Feedback Resistor

2.7.1 Alternative resistors
In Sec. 2.5.3 we looked at the bandwidth restriction of the parasitic capacitance
of a high value resistor. This can often be the dominant limitation. Even with
the photodiode represented by a pure, capacitance-free current source and an
ideal opamp of infinite gain, system bandwidth can be poor. As we know, the
output voltage is just the photocurrent multiplied by the feedback impedance.
A standard 0.4-W metal-film resistor typically has an intrinsic capacitance of
about 0.2pF, so if R f = 100MW, the photoreceiver will exhibit a low-pass filtered
output with a characteristic frequency f c = 8kHz. In practice, the bandwidth
will be even less than this owing to additional stray capacitance from wiring
the resistor to its amplifier. One way to improve bandwidth is to find another
make or type of resistor with lower parasitic capacitance. In general, the smaller
the resistor the better; chip devices perform best.

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.