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CHAPTER
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Some uses of diodes
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THE TERM DIODE MEANS TWO ELEMENTS. IN THE EARLY YEARS OF ELECtronics and radio, most diodes were vacuum tubes. The cathode element emitted electrons, and the anode picked up electrons. Thus, current would flow as electrons through the tube from the cathode to the anode, but not the other way. Tubes had filaments to drive electrons from their cathodes. The filaments were heated via a low ac voltage, but the cathodes and anodes usually wielded hundreds or even thousands of dc volts. Today, you ll still hear about diodes, anodes, and cathodes. But rather than large, heavy, hot, high-voltage tubes, diodes are tiny things made from silicon or other semiconducting materials. Some diodes can handle voltages nearly as great as their tube counterparts. Semiconductor diodes can do just about everything that tube diodes could, plus a few things that people in the tube era probably never imagined.
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Rectification
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The hallmark of a rectifier diode is that it passes current in only one direction. This makes it useful for changing ac to dc. Generally speaking, when the cathode is negative with respect to the anode, current flows; when the cathode is positive relative to the anode, there is no current. The constraints on this behavior are the forward breakover and avalanche voltages, as you learned about in the last chapter. Suppose a 60-Hz ac sine wave is applied to the input of the circuit in Fig. 20-1A. During half the cycle, the diode conducts, and during the other half, it doesn t. This cuts off half of every cycle. Depending on which way the diode is hooked up, either the positive half or the negative half of the ac cycle will be removed. Figure 20-1B shows the output of the circuit at A. Remember that electrons flow from negative to positive, against the arrow in the diode symbol. The circuit and wave diagram of Fig. 20-1 show a half-wave rectifier circuit. This 370
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Detection 371
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20-1 At A, half-wave rectifier. At B, output of the circuit of A with sine-wave ac input.
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is the simplest possible rectifier. That s its chief advantage over other, more complicated rectifier circuits. You ll learn about the different types of rectifier diodes and circuits in the next chapter.
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Detection
One of the earliest diodes, existing even before vacuum tubes, was a semiconductor. Known as a cat whisker, this semiconductor consisted of a fine piece of wire in contact with a small piece of the mineral galena. This bizarre-looking thing had the ability to act as a rectifier for small radio-frequency (RF) currents. When the cat whisker was connected in a circuit like that of Fig. 20-2, the result was a receiver capable of picking up amplitude-modulated (AM) radio signals.
20-2 Schematic diagram of a crystal set radio receiver.
A cat whisker was a finicky thing. Engineers had to adjust the position of the fine wire to find the best point of contact with the galena. A tweezers and magnifying glass were invaluable in this process. A steady hand was essential. The galena, sometimes called a crystal, gave rise to the nickname crystal set for this low-sensitivity radio. You can still build a crystal set today, using a simple RF diode, a coil, a tuning capacitor, a headset, and a long-wire antenna. Notice that there s no battery! The audio is provided by the received signal alone.
372 Some uses of diodes The diode in Fig. 20-2 acts to recover the audio from the radio signal. This is called detection; the circuit is a detector. If the detector is to be effective, the diode must be of the right type. It should have low capacitance, so that it works as a rectifier at radio frequencies, passing current in one direction but not in the other. Some modern RF diodes are actually microscopic versions of the old cat whisker, enclosed in a glass case with axial leads. You have probably seen these in electronics hobby stores. Details about detector circuits are discussed in chapter 27. Some detectors use diodes; others do not. Modulation methods are examined in chapter 26.
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