Chapter Switching

Teach Yourself Electricity and Electronics Third Edition Book
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Summary of Contents

Teach Yourself Electricity and Electronics Third Edition Book

  • SwitchingThe ability of diodes to conduct with forward bias, and to insulate with reverse bias,makes them useful for switching in some electronic applications. Diodes can switch atextremely high rates, much faster than any mechanical device.One type of diode, made for use as an RF switch, has a special semiconductor layersandwiched in between the P-type and N-type material. This layer, called an intrinsicsemiconductor, reduces the capacitance of the diode, so that it can work at higher fre-quencies than an ordinary diode. The intrinsic material is sometimes called I type. Adiode with I-type semiconductor is called a PIN diode (Fig. 20-5).374 Some uses of diodes20-5The PIN diode has alayer of intrinsic (I-type) semiconductorat the P-N junction.Direct-current bias, applied to one or more PIN diodes, allows RF currents to be ef-fectively channeled without using complicated relays and cables. A PIN diode alsomakes a good RF detector, especially at frequencies above 30 MHz.Voltage regulationMost diodes have avalanche breakdown voltages much higher than the reverse biasever gets. The value of the avalanche voltage depends on how a diode is manufactured.Zener diodes are made to have well-defined, constant avalanche voltages.Suppose a certain Zener diode has an avalanche voltage, also called the Zener volt-age, of 50 V. If a reverse bias is applied to the P-N junction, the diode acts as an opencircuit below 50 V. When the voltage reaches 50 V, the diode starts to conduct. Themore the reverse bias tries to increase, the more current flows through the P-N junc-tion. This effectively prevents the reverse voltage from exceeding 50 V.The current through a Zener diode, as a function of the voltage, is shown in Fig.20-6. The Zener voltage is indicated by the abrupt rise in reverse current as the reversebias increases. A typical Zener-diode voltage-limiting circuit is shown in Fig. 20-7.There are other ways to get voltage regulation besides the use of Zener diodes, butZener diodes often provide the simplest and least expensive alternative. Zener diodesare available with a wide variety of voltage and power-handling ratings. Power suppliesfor solid-state equipment commonly employ Zener diode regulators.Details about power supply design are coming up in chapter 21.Amplitude limitingThe forward breakover voltage of a germanium diode is about 0.3 V; for a silicon diodeit is about 0.6 V. In the last chapter, you learned that a diode will not conduct until theforward bias voltage is at least as great as the forward breakover voltage. The “flip side”