Page 92 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Interlude: Alternative Circuits and Detection Techniques

Interlude: Alternative Circuits and Detection Techniques 85

integrate both an LED and a photodetector into the same package. The LED
was used as the primary sensor for track servoing to read out reflective tracks
printed onto flexible disk media.
If you try this by just biasing the LED and looking at the LED voltage AC
coupled to an oscilloscope input you are unlikely to see any signal. With proper
receiver design, however, signals can be detected. The rules are exactly the same
as those used in Chap. 3, although the requirements and design solutions
are rather different from the high-resistance, high-sensitivity designs of the
discussion up to now.
The forward-biased LED photodetector passes a static current of several mil-
liamps, and we have to contend with the resulting full shot noise. The pho-
tocurrent generator acts in parallel with the forward-biased perfect diode of
our photodiode model. Hence it acts as a generator with very low impedance,
approximately the few ohm dynamic impedance of the diode (25/I(mA)W),
and a receiver must be optimized to work with this low source impedance.
Similar requirements exist for low-noise detection of signals from moving-
coil record player heads, moving-ribbon microphones, and certain types of
photoconductors.
The dominant parameter determining noise performance here is the voltage
noise density of the input amplifier stage. Most operational amplifiers have
voltage noise spectral densities in the range 10 to 50nV/ Hz , but better per-
formance can be obtained with discrete transistors. As we saw in Chap. 3 the
input-equivalent noise sources of junction transistors are the shot noise of the
base current and the thermal noise of the effective base resistance. These are
given by:
2 qI e
i n = 2 qI b = (4.1)
2
b
kT
e n = 4 kT Ê Ë r b + r e ˆ ¯ : r e = qI e (4.2)
2
2
I b and I e are the transistor base and emitter currents, and r b , r e are the base
spreading resistance and emitter small-signal resistance. b is the transistor
current gain. Figure 4.6 shows the variation of i n and e n with transistor emitter
current.
For the ª1W source resistance of the forward-biased LED, the current shot
noise is negligible and thermal noise is dominant. The design clearly needs the
input transistor to be run at high current. When this is optimized, signal-to-
noise is limited by its base-spreading resistance r bb. This can be minimized by
connecting several transistors in parallel and/or by choosing devices specially
processed for low r bb. Large die power transistors are sometimes used for this
purpose, but specialist devices are better. Devices such as the 2SD786 and
2SB737 transistors are available with r bb of 4W and 2W respectively, which leads
to e n < 1nV/ Hz . Figure 4.5 shows a high-gain common emitter amplifier
designed around the LM394 bipolar supermatch pair. A balanced configuration
using a second LED reduces the effects of temperature drifts, but the circuit is

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