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handbook of modern sensors

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14.2 Photodiodes 417<br />

Fig. 14.9. Response <strong>of</strong> a photodiode with an uncompensated<br />

circuit. (Courtesy <strong>of</strong> Hamamatsu<br />

Photonics K.K.)<br />

where C c = 1/(2πR L f c ), and f c is the unity-gain crossover frequency <strong>of</strong> the operational<br />

amplifier. The capacitor boosts the signal at the inverting input by shunting R L<br />

at higher frequencies.<br />

When using photodiodes for the detection <strong>of</strong> low-level light, the noise floor should<br />

be seriously considered. There are two main components <strong>of</strong> noise in a photodiode:<br />

shot noise and Johnson noise (see Section 5.9 <strong>of</strong> Chapter 5). In addition to the sensor,<br />

the amplifier’s and auxiliary component noise also should be taken into account [see<br />

Eq. (5.75) <strong>of</strong> Chapter 5].<br />

For the photoconductive (PC) operating mode, a reverse-bias voltage is applied<br />

to the photodiode. The result is a wider depletion region, lower junction capacitance<br />

C j , lower series resistance, shorter rise time, and linear response in photocurrent<br />

over a wider range <strong>of</strong> light intensities. However, as the reverse bias is increased, the<br />

shot noise increases as well due to the increase in dark current. The PC mode circuit<br />

diagram is shown in Fig. 14.10A and the diode’s load characteristic is in Fig. 14.10B.<br />

The reverse bias moves the load line into the third quadrant, where the response<br />

linearity is better than that for the PV mode (the second quadrant). The load lines<br />

(A)<br />

(B)<br />

Fig. 14.10. Photoconductive operating mode: (A) a circuit diagram; (B) a load characteristic.

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