Page 150 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Control of Ambient Light

Control of Ambient Light 143

and low-frequency interference, flows through the load resistor R L . However,
the high-pass filter action of C suppresses the signal seen by the opamp with a
low-frequency cutoff at f c = 1/2pC(R L + R 1 ). The signal modulation frequency
should be >f c. The rate of cutoff below f c is -20dB/decade of frequency, so with
f c = 1kHz, approximately 20dB suppression (to 10 percent in voltage) will be
obtained at the dominant 100/120Hz interference frequency. Slower intensity
variations will be suppressed further. If higher suppression is needed, the mod-
ulation could perhaps be moved to 10kHz or beyond and f c increased propor-
tionately. Alternatively further RC networks can be cascaded for a greater than
-20dB/decade slope. If 100/120Hz interference is the main problem, modula-
tion at a frequency several decades above 100Hz will greatly ease filtration by
AC coupling.
While effective, this circuit can still be overloaded by high-intensity low-
frequency illumination. This is because low-frequency photocurrents see the
full load resistor R L, and the voltage on it may increase toward the bias voltage
V b , removing the reverse bias condition. The load resistor must be chosen small
enough to avoid this, which might mean that it is not as large as we would like
for best S/N. We have also seen that the voltage follower configuration is some-
what restricting for high-speed detection.

7.2.2 AC Coupling: transimpedance amplifier
When a transimpedance amplifier is used, AC coupling cannot be added in series
with the photodiode. Without a DC path, the anode will charge positively and
forward bias it. Instead RC coupling can be used only after the front-end ampli-
fier (Fig. 7.2b). While this again gives a high-pass response and suppression of
DC and low-frequency photocurrents before all later amplifier stages, it does
not stop the first amplifier from being overloaded by low-frequency light. Again
R L must be small enough to avoid overload, and the receiver’s dynamic range
will be limited just as with the bias box or Fig. 7.2a.

7.2.3 Inductive coupling
An apparently attractive alternative to series capacitive coupling is shunt induc-
tive coupling. Figure 7.3 shows voltage follower and transimpedance configu-
rations with inductive loads. At DC the inductor will provide a low-impedance
path for photocurrent, giving a low output voltage. At high frequencies the
inductor will exhibit a high impedance, allowing the full load resistor sensitiv-
ity to be obtained. This is what is wanted. It leads to a high-pass response to
photocurrent, with a cutoff frequency f c occurring when the impedance of the
inductor (2pfL) equals that of the resistor R L. The difficulty with the technique
lies in obtaining suitable inductive components.

For example, for a 1kHz cutoff and R L = 1MW, we can calculate L = R L/2pf c
= 159H. Obtaining an inductor of such a high value, in a convenient package,
whose impedance is not dominated by interwinding capacitance, is very diffi-

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