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handbook of modern sensors

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170 5 Interface Electronic Circuits<br />

(A)<br />

(B)<br />

Fig. 5.17. Zener diode: (A) a volt–ampere characteristic; (B) a shunting-type voltage reference.<br />

Zener diodes fall into three general classifications: regulator diodes, reference<br />

diodes, and transient voltage suppressors. Regulator diodes are normally employed<br />

in power supplies where a nearly constant dc output voltage is required despite relatively<br />

large changes in input voltage or load impedance. Such devices are available<br />

with a wide range <strong>of</strong> voltage and power ratings, making them suitable for a wide<br />

variety <strong>of</strong> electronic equipments. Regulator diodes, however, have one limitation:<br />

they are temperature sensitive. Therefore, in applications in which the output voltage<br />

must remain within narrow limits during input-voltage, load-current, and temperature<br />

changes, a temperature-compensated regulator diode, called a reference diode,<br />

is required.<br />

It makes sense to take advantage <strong>of</strong> the differing thermal characteristics <strong>of</strong><br />

forward- and reverse-biased silicon p-n junctions. Like any silicon diode (see Section<br />

16.3 <strong>of</strong> Chapter 16), a forward-biased junction has a negative temperature coefficient<br />

<strong>of</strong> approximately −2 mV/ ◦ C, whereas a reverse-biased junction has positive temperature<br />

coefficient ranging from about 2 to 6 mV/ ◦ C depending on the current and<br />

the diode type. Therefore, it is possible, by a selective combination <strong>of</strong> forward- and<br />

reverse-biased junctions, to fabricate a device with a very low overall temperature<br />

coefficient (Fig. 5.18). The voltage changes <strong>of</strong> the two junctions are equal and opposite<br />

only at the specified current. For any other value <strong>of</strong> current, the temperature<br />

compensation may not be ideally accomplished. Nevertheless, even a simple backto-back<br />

connection <strong>of</strong> two zener diodes <strong>of</strong> the same type may significantly improve<br />

the overall temperature stability over a rather broad range <strong>of</strong> currents and temper-<br />

Fig. 5.18. Temperature compensation <strong>of</strong> a zener diode.

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