Maximum power is transferred to a load when the internal resistance (or impedance) of the power is equal to the load resistance (or imped- ance). This condition is valid for both ac and dc circuits.
T
ubes and semiconductor devices form the basis for all electronic cir- cuits involving rectification, amplification, oscillation, digital, and other related functions. Tubes (such as triodes and pentodes) and tran- sistors are called active devices because they are amplifying devices capable of boosting very weak signals to high-level amplitudes. For example, audio amplifiers can be used to amplify low-level audio sig- nals from microphones or phonograph pick-up devices, to high signal levels capable of driving loudspeakers. Radio frequency amplifiers (RF amplifiers) are used in radio receivers to amplify extremely weak RF sig- nals (in the microvolt range) from an antenna for subsequent signal pro- cessing and ultimate conversion to audio signals for driving speakers or headphones.The vacuum tube, once the mighty workhorse in radio communica- tions, has been relegated to the back seat by the smaller, more efficient transistor. Most all new radio equipment on today’s market employs solid-state technology. Only a few specialized applications, such as high-power final amplifiers in the range of 1000 watts and above still continue to use vacuum tubes.
The triode vacuum tube was invented by Lee De Forest in 1906.
One of the most important inventions of the twentieth century, the vacuum tube spurred the development of radio communications,
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radar, television, space-age electronics, computers, and the list goes on.
The present investment in vacuum tube electronic equipments, particularly amateur radios, is enormous. For example, vacuum tube HF transceivers will continue to give excellent service for years to come. You can usually find some of these old, reliable ham radios at local swap-fests and amateur radio conventions. Also, many major electronic companies continue to manufacture quality HF “1 kilowatt final power amplifiers” using rugged power tubes. The FCC requires that all amateur radio operators develop a working knowledge of vac- uum tubes and includes questions about this subject on all amateur examinations. Some introductory material on vacuum tubes is pro- vided here to help you pass the FCC examinations as well as to set up and operate vacuum tube ham radios. Also, from time to time, you will find some interesting construction articles involving home-built vac- uum tube transmitters and receivers in amateur radio magazines such as CQ, QST, and 73.
Like the vacuum tube, the invention of the transistor in the late 1940s and the integrated circuit in the late 1950s produced a major impact on all fields of electronics. Although the first semiconductor devices (crystal detectors using silicon or carborundum) were invented about 1906, scientists at that time could not explain the the- ory behind the crystal rectifier. The major breakthrough came in 1947 when scientists at the Bell Telephone Laboratories developed the present-day semiconductor theory. This immediately led to the fabrication of the transistor, the first solid-state amplifying device;
junction diodes; and thyristors. The disadvantages of the vacuum tube—fragility, bulkiness, requiring high-voltage power supplies, and use of filaments that require warm-up time and excessive amounts of power—are eliminated by the rugged and compact tran- sistor.
Let’s examine the tube and semiconductor devices in more detail.
You won’t need all this theory and practical information for the Tech- nician Class examinations; only one question on vacuum tubes is included in these examinations. However, a good background on tubes and semiconductors will help you understand the operation of your amateur radio equipment as well as prepare you for upgrade to higher class amateur licenses.
Definitions
Semiconductor A class of materials that are neither good conduc- tors nor good insulators of electricity. These materials, such as germa- nium and silicon, possess relatively few free electrons for conducting electrical current. In their pure form, semiconductors have little use in electronic applications.
PN junction diode The pn junction diode, a two-terminal semi- conductor device, is the simplest form of semiconductor used in elec- tronics. This device consists of two tiny blocks of semiconductor material referred to as n-type and p-type. These two blocks are, in effect, joined together as one continuous block with leads connected to each end. The pn junction diode allows current to flow easily in one direction but literally blocks current flow in the opposite direction.
The two leads or terminals of the pn junction diode are referred to as the anode and cathode. A pn junction diode can be constructed from either germanium or silicon. PN junction diodes are used as rectifiers in power supplies, detectors in radio receivers, and in other applica- tions where rectification action is required.
Zener diode The zener diode, a two-terminal device, uses the reverse-voltage characteristics of a pn junction to cause a voltage breakdown that results in a high current through the diode. This action produces a constant voltage drop across the diode and exhibits voltage-regulating characteristics. Zener diodes are extremely useful as voltage regulators in power supplies.
Bipolar transistor The bipolar transistor consists of three layers of n-type and p-type semiconductor material arranged either in a npn or pnp configuration. The three leads from the npn or pnp transistor are called the emitter, base, and collector. The bipolar transistor is a cur- rent-amplifying device and is used in amplifier, oscillator, and other types of electronic circuits.
Field-effect transistor Field-effect transistors, or FETs, are con- structed from various layers of n-type and p-type semiconductor mate- rials. FETs are available in two general types: the junction, field-effect transistor (JFET), and the insulated-gate field-effect transistor (IGFET).
Sometimes the IGFET is referred to as a metal-oxide semiconductor field-effect transistor, or MOSFET. The FET transistor is a voltage- amplifying device, and as such possesses a very high input resistance.
FETs are useful in amplifier, oscillator, and other types of electronic circuits where an active device is required.
Vacuum tube The vacuum or electron tube is a device that con- sists of a number of electrodes mounted in an evacuated enclosure of glass or metal. The vacuum tube employs a controlled flow of electron current to perform functions such as rectification or amplification.
A vacuum tube can contain one or more of the following electrodes or elements:
■ Cathode The cathode produces a stream of electrons that are directed towards the anode electrode. In most tubes, the cathode is heated indirectly, resulting in thermionic emission of elec- trons.
■ Anode The anode, or plate, is the electrode to which the stream of electrons flow.
■ Grid A grid is an electrode normally placed between the cath- ode and plate to control the flow of electrons. The grid has one or more openings to permit passage of the electrons. A vacuum tube can use one or more grids for controlling the flow of electrons.
The grids are called control, screen, suppressor or space-charge, depending on the function to be performed.
Diode The diode is the simplest form of a vacuum tube and con- sists of two electrodes, a cathode and plate. The word diode is derived from di, meaning two, and ode, for electrode. A positive potential, or voltage applied to the plate electrode, will attract the electrons emitted from the cathode. This results in one-way flow of current through the diode tube. Diodes are used primarily to rectify ac signals.
Triode A major step in vacuum tube development, the triode pro- vides a capability to amplify weak audio and radio-frequency signals.
The triode is a three-electrode device having a control grid positioned between the cathode and plate. The grid controls the flow of electrons to the plate. In addition to amplification, we can use the triode for other important functions in electronic circuits, such as frequency generation, conversion, modulation, and demodulation.
Pentode The pentode, a five-element vacuum tube, can be described as the ideal amplifier in vacuum tube development. Scientists had
invented the tetrode, a four-element vacuum tube to overcome the limi- tations of the triode. Although an improvement over the triode, the tetrode still possessed limitations. The pentode, possessing a cathode, control grid, screen grid, suppressor grid, and plate provides higher sig- nal amplification and more stable operating characteristics.