Oscillation and amplification

Chapter Oscillation and amplification

Teach Yourself Electricity and Electronics Third Edition Book
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Summary of Contents

Teach Yourself Electricity and Electronics Third Edition Book

  • Oscillation and amplificationUnder certain conditions, diodes can be made to produce microwave radio signals.There are three types of diodes that do this: Gunn diodes, IMPATT diodes, and tun-nel diodes.Gunn diodesA Gunn diode can produce up to 1 W of RF power output, but more commonly it worksat levels of about 0.1 W. Gunn diodes are usually made from gallium arsenide.A Gunn diode oscillates because of the Gunn effect, named after J. Gunn of Inter-national Business Machines (IBM) who observed it in the sixties. A Gunn diode doesn’twork anything like a rectifier, detector, or mixer; instead, the oscillation takes place asa result of a quirk called negative resistance.Gunn-diode oscillators are often tuned using varactor diodes. A Gunn-diode oscil-lator, connected directly to a microwave horn antenna, is known as a Gunnplexer.These devices are popular with amateur-radio experimenters at frequencies of 10 GHzand above.IMPATT diodesThe acronym IMPATT comes from the words impact avalanche transit time. This, likenegative resistance, is a phenomenon the details of which are rather esoteric. An IM-PATT diode is a microwave oscillating device like a Gunn diode, except that it uses sil-icon rather than gallium arsenide.An IMPATT diode can be used as an amplifier for a microwave transmitter that em-ploys a Gunn-diode oscillator. As an oscillator, an IMPATT diode produces about thesame amount of output power, at comparable frequencies, as the Gunn diode.Tunnel diodesAnother type of diode that will oscillate at microwave frequencies is the tunnel diode,also known as the Esaki diode. It produces only a very small amount of power, but itcan be used as a local oscillator in a microwave radio receiver.Tunnel diodes work well as amplifiers in microwave receivers, because theygenerate very little unwanted noise. This is especially true of gallium arsenide devices.The behavior of Gunn, IMPATT, and tunnel diodes is a sophisticated topic and is be-yond the scope of this book. College-level electrical-engineering texts are good sourcesof information on this subject. You will want to know about how these devices work ifyou plan to become a microwave engineer.Energy emissionSome semiconductor diodes emit radiant energy when a current passes through theP-N junction in a forward direction. This phenomenon occurs as electrons fall fromhigher to lower energy states within atoms.Energy emission377