Page 38 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Amplified Detection Circuitry

Amplified Detection Circuitry 31

quency values. While 15.8k from the E96 series and precision capacitors will
reduce the error such effort is probably wasted on this application.
When building transimpedance amplifiers with high value resistors, say,
10MW and up, often no parallel capacitance is needed, not even a few mil-
limeters of twisted wire-wrap wire. Even without it the circuit is overdamped.
This may be because the capacitance of the resistor itself is sufficient. Axial
geometry 0.4-W metal-film resistors typically show a terminal capacitance of
the order of 0.1 to 0.2pF. This means that with a 100-MW resistor, the imped-
ance of the combination will drop above about 8 to 15kHz due to this effect
alone. Add to this variables such as circuit strays and PCB tracks, and it will
be a struggle to get even this bandwidth. This is discussed in more detail later
in this chapter.
As we have seen, the transimpedance amplifier can be used with a completely
unbiased detector and still give a linear response. Without deliberate bias, the
voltage across the photodiode is just the offset voltage of the amplifier. We will
assume that this is zero, although even a few millivolts here can significantly
affect noise performance, especially if the photodiode has a low shunt
resistance.

2.6 Increasing Bandwidth with Loop-Gain
2.6.1 Opamp choice
We have seen that one key to overall photoreceiver detection speed is “gain
bandwidth” of the amplifier used in the transimpedance configuration. In
an earlier section the bandwidth of a 720-pF/1-GW receiver was increased from
0.22Hz to 363Hz through the use of a 0.6-MHz GBW opamp, the improvement
going approximately as the square root of GBW. Many other devices with higher
gain bandwidths are available. For example many common junction FET
(JFET) opamp families such as LF353, LF412, TL081, OPA121, AD711 have
GBWs around 4MHz. The OPA604 offers 20MHz. This should increase our
example system’s bandwidth to 2kHz. A few specialist FET amplifiers are avail-
able with even greater GBWs. The Burr-Brown (now part of Texas Instruments)
OPA657 JFET opamp has GBW = 1.6GHz, and a unity gain stable device has
230MHz. In theory this would push our detection bandwidth to 7kHz.

2.6.2 TRY IT! Opamp choice
Set up a 100-MW transimpedance receiver with a small photodiode with about
100-pF capacitance. If this is not available, use a small photodiode with a 100-pF
capacitance across it. In a voltage follower configuration we would obtain a detection
bandwidth on the order of 16Hz, or a transient rise time of about 0.35/16 = 22ms.
Use an integrated circuit socket for the opamp. Now take a selection of opamps and
observe the variation of transient response for a 5-kHz square-wave input. Figure 2.9
shows my results with a few opamps from the collection. It is clear that the majority
of devices from this collection perform similarly, with rise times of 20 to 50ms, whereas

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