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| SEC. 5-1 THE ZENER DIODE This is a special diode optimized for operation in the breakdown region. Its main use is in voltage regulators—circuits that hold the load voltage constant. Ideally, a reverse-biased zener diode is like a perfect battery. To a second approximation, it has a bulk resistance that produces a small additional voltage. SEC. 5-2 THE LOADED ZENER REGULATOR When a zener diode is in parallel with a load resistor, the current through the current-limiting resistor equals the sum of the zener current and the load current. The process for analyzing a zener regulator consists of finding the series current, load current, and zener current (in that order). SEC. 5-3 SECOND APPROXIMATION OF A ZENER DIODE In the second approximation, we visualize a zener diode as a battery of VZ and a series resistance of RZ. The current through RZ produces an additional voltage across the diode, but this voltage is usually small. You need zener resistance in order to calculate ripple reduction. SEC. 5-4 ZENER DROP-OUT POINT A zener regulator will fail to regulate if the zener diode comes out of breakdown. The worst-case conditions occur for minimum source voltage, maximum series resistance, and minimum load resistance. For the zener regulator to work properly under all operating conditions, there must be zener current under the worst-case conditions. SEC. 5-5 READING A DATA SHEET The most important quantities on the data sheet of zener diodes are the zener voltage, the maximum power rating, the maximum current rating, and the tolerance. Designers also need the zener resistance, the derating factor, and a few other items. SEC. 5-6 TROUBLESHOOTING Troubleshooting is an art and a science. Because of this, you can learn only so much from a book. The rest has to be learned from direct experience with circuits in trouble. Because troubleshooting is an art, you have to ask “What if?” often and feel your way to a solution. SEC. 5-7 LOAD LINES The intersection of the load line and the zener diode graph is the Q point. When the source voltage changes, a different load line appears with a different Q point. Although the two Q points may havedifferent currents, the voltages are almost identical. This is a visual demonstration of voltage regulation. SEC. 5-8 OPTOELECTRONIC DEVICES The LED is widely used as an indicator on instruments, calculators, and other electronic equipment. By combining seven LEDs in a package, we get a sevensegment indicator. Another important optoelectronic device is the optocoupler, which allows us to couple a signal between two isolated circuits. SEC. 5-9 THE SCHOTTKY DIODE The reverse recovery time is the time it takes a diode to shut off after it is suddenly switched from forward to reverse bias. This time may be only a few nanoseconds, but it places a limit on how high the frequency can be in a rectifier circuit. The Schottky diode is a special diode with almost zero reverse recovery time. Because of this, the Schottky diodeis useful at high frequencies where short switching times are needed. SEC. 5-10 THE VARACTOR The width of the depletion layer increases with the reverse voltage. This is why the capacitance of a varactor can be controlled by the reverse voltage. A common application is remote tuning of radio and television sets. SEC. 5-11 OTHER DIODES Varistors are useful as transient suppressors. Constant-current diodes hold the current, rather than the voltage, constant. Step-recovery diodes snap off and produce a step voltage that is rich in harmonics. Back diodes conduct better in the reverse direction than in the forward direction. Tunnel diodes exhibit negative resistance, which can be used in highfrequency oscillators. PIN diodes use a forward-biased control current to change its resistance in RF and microwave communication circuits. |