Vacuum Tubes

Type of physical science: Classical physics

Field of study: Electromagnetism

Vacuum tubes, despite being replaced by transistors in many applications, are still in use today in high-power applications. With the recent success in developing vacuum tube microelectronic circuits, there is a need to study vacuum tubes in great detail.

Overview

Vacuum tubes are devices in which electrons flow in a complete or partial vacuum, and the flow is controlled for a variety of radio, microwave, and optical applications. Historically, the oldest vacuum tube is a diode. It consists of a glass or metallic tube with two electrodes in complete vacuum. One of the two electrodes is called the emitter, or cathode, and consists of a tungsten wire or an oxide-coated plate, which releases electrons when heated to a temperature greater than 1,900 degrees Celsius by passing a suitable current either through it or through a filament insulated from it. These electrons then flow toward the other electrode, called a plate or an anode, when a positive voltage is applied between the plate and the emitter. The tube is completely sealed, but pins are available externally for connecting proper voltages to the plate and the emitter and for supplying filament current to heat the emitter. A diode is useful for converting alternating currents to direct currents.

To study the current-voltage characteristics of a diode, its emitter is connected to the negative terminal of a direct current supply (battery), and its plate is connected to the positive terminal of the supply through a resistance. When the plate supply voltage is low, current flow is also low, because most of the electrons released by the emitter stay close to the emitter and constitute a space charge cloud. Only a few electrons will be attracted as a result of the positive charge on the plate. As the positive voltage on the plate is increased, current in the circuit will also increase as more electrons are attracted toward the plate. Eventually, there will be a plate voltage called "saturation voltage" at which all electrons released from the cathode will be pulled toward the plate, and further increase in the plate voltage does not lead to any increase in plate current. On the other hand, if the plate is at a negative voltage with respect to the emitter, all the electrons released from the emitter will be repelled by the plate, and there will be no current flow.

A great advance took place when a metallic mesh was inserted between the emitter and the plate of a diode tube. This mesh is called a "control grid," and the new device is called a triode, indicating it has three electrodes. In general, a negative direct voltage is applied between the control grid and the emitter. The control grid repels the electrons moving toward the plate and is more efficient in doing so because it is closer to the emitter than the plate is. Yet, since it is only a mesh, many electrons will slip through the mesh and reach the plate. To study the current voltage characteristics of a triode, the emitter is connected to the positive terminal of a fixed-voltage battery and the negative terminal is connected to control the grid. The plate is connected to the positive terminal of another battery through a resistance and the negative terminal of this battery is connected to the emitter. Now, plate current (current in the circuit connecting the plate and the cathode) in the triode circuit is a function of the voltage between the plate and the emitter (the plate voltage) and voltage between the control grid and the emitter (the grid voltage). For a given control-grid voltage, the higher the plate voltage, the higher the plate current will be. As in the case of the diode circuit, there is a saturation current, which now depends on the control-grid voltage. The more negative the control-grid voltage is, the less the corresponding saturation current will be. Plate current versus plate voltage for different values of control-grid voltage are plotted on a graph for use in designing circuits. On the other hand, for a given plate voltage, the more negative the grid voltage is, the less the plate current will be.

Therefore, another choice is to plot plate current versus control-grid voltage (negative) for different values of plate voltage.

A triode is more versatile than a diode. It can amplify small voltage signals applied between the control grid and the emitter. It can be used to design oscillators, wave-shaping circuits, modulators, and demodulators. The important characteristics of a triode are its plate resistance, amplification factor, and transconductance. The plate resistance of a vacuum tube is defined as the ratio of a small change in plate voltage to a corresponding small change in plate current and is, in general, on the order of 100,000 ohms. The amplification factor is given by the ratio of a change in plate voltage to a corresponding change in grid voltage to produce equal changes in plate current. It can range anywhere from 5 to 100 ohms or higher, and it is so important a design criterion that special physical constructions of tubes are adopted to achieve high, low, or medium amplification factors. The transconductance is defined as the ratio of change in plate current to a small change in the grid voltage. Its value may range anywhere from hundreds to thousands of micromhos. A micromho is a millionth of a mho (the reciprocal of the ohm). For a given tube, plate and control-grid voltages will also play a role in determining exact values of the amplification factor and the transconductance. The physical construction of the tube depends on the desired application. For example, when the tube is used in a receiver circuit, requirements are low power handling, large amplification factor, and least amount of distortion.

There are three capacitances in a triode. One capacitance is between the plate and the control grid, another is between the emitter and the grid, and the third capacitance is between the plate and the emitter. Of these three capacitances, at radio frequencies the capacitance between the grid and the plate feeds back energy from the plate to the grid, leading to oscillations and instability of circuits at times. Thus, a need arose to reduce this capacitance, which is achieved by putting another mesh electrode between the grid and the plate. This is called a "screen grid" and is maintained at a positive potential with respect to the emitter, but at a voltage less than the plate voltage. Such a device is called a tetrode and the device characteristics now depend on three voltages: those on the plate, the grid, and the screen grid. The screen grid is also a mesh and thus electrons can pass through it easily to the plate. A finite amount of current will flow in the circuit, connecting the emitter and the screen grid. A bypass capacitor, which offers a low impedance at the desired frequency of operation, is connected across the screen grid and the emitter to reduce an unwanted screen current.

At high plate voltages, electrons in the tube travel at very high speeds. When they strike the plate, electrons are emitted from the surface of the plate. These are called secondary electrons, and some of these electrons are collected by the screen grid in a tetrode, as it is at a positive voltage. This leads to a reverse current flow between the screen and the plate. Another grid in the form of mesh is inserted between the plate and the screen and is maintained at the potential of the cathode. This is called a "suppressor grid," which then shields the secondary electrons from reaching the screen. This five-electrode tube is called a pentode. Such a device has a very high plate resistance, as plate voltage now does not have much influence on current because of the presence of the screen grid. Nevertheless, the control grid has substantial influence on plate current, and this leads to the same level of transconductance as in an equivalent triode. A combination of both factors leads to a high amplification factor.

Most practical emitters in vacuum tubes are made of tungsten, thoriated tungsten, and oxide-coated materials. Tungsten is generally operated at 2,270 degrees Celsius to give maximum life for the filament. Thoriated tungsten gives good electron emission at 1,900 degrees Celsius. The oxide-coated emitters consist of a mixture of barium and strontium, coated on nickel or a nickel alloy and are typically operated at 900 degrees Celsius. The thorium layer on the surface of the thoriated tungsten emitter gets stripped off as a result of bombardment by positive ions from any residual gas in the tube. Oxide-coated emitters are most efficient in that they give maximum emission per watt of heating power, but they deteriorate more rapidly. They can give high instantaneous emission for a few microseconds, which is used for generating pulses for radar. They are generally used in small receiver tubes. For power tubes operated at extremely high voltages, tungsten or thoriated tungsten is preferable. In all cases, the cathode can be heated either directly when the filament current passes through the cathode, or indirectly when an insulated tungsten filament heater inside a thin nickel cylinder cathode with an external oxide coating heats it.

Major difficulties with vacuum tubes are the deteriorating effects of residual gas in the tube, transit time effects caused by finite electron velocities and capacitances between the electrodes. The residual gas gets ionized in the presence of heat and the electric field. The resulting positive ions may strike the emitting surface and then either mechanically or chemically degrade it. So, operating the tube at high voltages reduces the lifetime of the device. Note, however, that the tungsten filament cathode is immune to the gas effects. To reduce the gas effects, substances such as barium or magnesium are introduced into the tube. These materials, called getters, help in obtaining a high vacuum initially and combine with any gas that is released by the cathode materials or left as residue.

Electrons take a finite amount of time to travel from the cathode to the plate. At low frequencies up to a few megahertz, transit time of the electrons in a vacuum tube under normal operating voltages is small compared with the time period of the applied frequency. This time period is defined as one divided by the frequency in hertz. At low frequencies, electron flow, which constitutes the plate current, will be able to follow the fluctuations of the applied alternating voltage on the control grid or the plate. Yet, at very high frequencies, the applied voltages will change their magnitudes too rapidly and the current caused by electron flow will not be able to follow the voltages. This leads to distortion and reduced amplification of the signal. Further degradation of the basic characteristics of the vacuum tubes results if applied signals are large in magnitude. Some of these effects can be reduced by using closely spaced small electrodes, which reduce capacitance and leads with reduced inductance.

The failure of triodes, tetrodes, and pentodes at high frequencies caused by transit time effects led to alternate strategies, such as to making use of transit time in achieving high power and high-frequency operation. These are the high-power microwave tubes, such as klystron, magnetron, traveling wave tube, and backward wave oscillator. For example, in a traveling wave tube, a radio-frequency voltage to be amplified is applied near the cathode end of a long and loose metallic helix and guided by it. An electron beam is shot from the cathode end along the axis of the helix and collected by the anode at the other end of the helix. Under suitable conditions, an amplified signal of the applied microwave voltage appears at the other end of the helix.

There are other types of tubes, such as television picture tubes and photomultiplier tubes. The photomultiplier tube consists of a photocathode, several dynodes, and an anode. The photocathode, which is made up of a material such as gallium arsenide or indium phosphide, generates electrons when light falls on it. These electrons are accelerated by the voltage on a secondary electrode so that when the electrons strike it, more electrons are released by it. By using several secondary electrodes, good amplification of electrons can be achieved. These are collected by an anode to produce current in the plate circuit. A photomultiplier tube is capable of detecting extremely small optical signals. In television and cathode-ray tubes, the screen consists of phosphor materials such as zinc orthosilicate, which emits light when electrons strike it.

Applications

With the advent of vacuum tubes, radio engineering was born. A host of circuits for generation, amplification, and detection of radio waves were designed, and these gave birth to radio, television, radar, and modern communication engineering.

A simple diode circuit can be used as a detector, a rectifier, or a mixer. In a simple diode rectifier circuit, an emitter is connected to an alternating sinusoidal signal, and the other terminal of the source is connected to the plate along with a series resistance. In this configuration, the voltage of the plate with respect to the emitter is changing sinusoidally. For half of the sinusoidal cycle, the voltage on the plate is positive with respect to the cathode, and there will be a current flow in the circuit. There will be no current flow for the other half of the cycle, as the plate is negative with respect to the emitter for that period. This is called the half-wave rectifier, and the resulting voltage across the resistance contains a direct voltage, an alternate voltage of the same frequency as that of the applied voltage and harmonics, which are alternating voltages at frequencies that are integer multiples of the frequency of original applied voltage. There are other variations of this simple rectifier circuit, such as a full-wave rectifier, which is more efficient in converting alternating currents to direct currents and to currents at higher harmonics. Thus, in a radio receiver, a simple diode can be used as a detector to recover the applied signal. Also, the rectification property can be used to detect and demodulate the transmitted signal.

When diodes are used for detection, the resulting detected signal is very low. Yet, a triode can amplify low power signals for clarity and further uses. It is found that when an alternating sinusoidal signal is applied between the control grid and the emitter, a sinusoidal signal of the same frequency at a much larger voltage level is observed across resistance, connected between the emitter and the plate. This results from the fact that the control grid is more efficient in repelling electrons from the emitter than the plate is in collecting the electrons.

Thus, the signal applied between the control grid and the emitter has been amplified and appears across the plate and the emitter. This led to great advances in radio engineering, as very small signals can now be amplified and studied in detail. Other tubes are merely improvements of triodes to overcome some of the defects in their performance. The often-used design criteria for amplifiers are gain, bandwidth, center frequency, power, and distortion. By proper choice of bias supply and alternating voltage applied to the control grid, it is possible to control the amount of time the plate current is nonzero. This control can be used for linear amplification, power amplification, and harmonic generation. Several vacuum tube amplifier circuits were described in ELECTRONIC AND RADIO ENGINEERING (1948) by Frederick Emmons Terman.

When a part of the output signal (voltage between the plate and the cathode) of an amplifier is fed back to the input (voltage between the control grid and the cathode) with proper circuit design, it leads to oscillations. In actual practice, no input signal is needed. Depending on the actual details of feedback network and device, the oscillator circuit chooses a frequency from noise power of the device and keeps on amplifying until it reaches stable oscillations. Thus, oscillators, which can generate stable output voltages, are designed using feedback techniques.

When a sum of more than two alternating signals of different frequencies are used as a plate voltage, the rectification property of the diode generates frequencies that are the sum and difference of integer multiples of these frequencies. This property is called mixing, and it has several useful applications in instrument design, such as spectrum analyzers and intermediate frequency generation. Triode circuits can also be used for mixer operation. Special-purpose vacuum tubes, such as hexodes and octodes, are efficient mixers.

In some communication systems, amplitude or frequency of a sinusoidal wave, called a carrier, is varied according to the signal being transmitted; this is called modulation. For modulation, a carrier signal in series with the modulating voltage and a fixed bias is connected between the control grid to the cathode. This will produce a modulating plate current that carries information around both the carrier wave and the modulating voltage. By mixing it with a local oscillator at the carrier frequency, the modulating voltage can be reconstructed; this is called demodulation.

Digital vacuum-tube circuits played a major role in the development of the first large-scale computers. Also, photomultiplier tubes continue to be used for amplifying extremely small optical signals. Microwave tubes, such as magnetron and traveling wave tubes, are still in use today for generating large amounts of high power. Cathode-ray tubes, and its variation picture tube, continue to be used in television receivers and electronic instruments.

Since 1985, efforts have been under way to develop miniaturized vacuum-tube electronic circuits, using the concept of field emission, whereby electrons can be pulled from the tip of a metal by applying a strong electrical field. Using several such tips together, called field emitter arrays, engineers are developing large-scale integrated circuits. One of the motivations behind this goal is that the speed with which electrons can move in a transistor is limited by properties of semiconductor materials. No such limits exist for electron flow in a vacuum, and space-charge problems can be circumvented by proper arrangement of cathodes. Also, these devices can be switched very rapidly. Several prototype field emitter arrays and microminiature integrated circuits are being reported. This may revolutionize the design of flat-screen television sets, superfast computers, optically controlled computers, and communication equipment, besides several military applications such as electronic counter measures and light-weight radars.

Context

Sir William Crookes was the first scientist to study the flow of charges from one electrode to another electrode through a vacuum. This led to discoveries of X rays by Wilhelm Conrad Rontgen and electrons by Sir Joseph John Thomson. The Braun tube, introduced by Karl Ferdinard Braun in 1897, was the earliest vacuum tube and later evolved into the cathode-ray tube used in oscilloscopes. In 1883, the American inventor Thomas Alva Edison noticed an unexplained current flow between the two filaments of his electric lamp under certain conditions.

This current flow was in a direction opposite to that of the lamp current. This phenomenon was studied and identified as a thermionic emission. Sir John Ambrose Fleming got the idea of using the Edison effect to study radio waves and develop the vacuum-tube diode for detecting radio waves in 1904. It detected radio waves very well, but the resulting signals were weak. In 1907, an American engineer Lee de Forest and a little later, van der Bijl discovered the triode. This was a singularly important contribution to the entire development of radio engineering. The triode amplified extremely weak signals by converting the bias direct current power into alternating current power of the signal. Walter Hans Schottky invented the tetrode in 1919, and Jobst and B.

D. H. Tellegen designed the pentode in 1926 to improve the performance of the triode. The iconoscope, which converted an optical signal into an electrical signal, was invented by Vladimir Zworykin in the 1920's. In later years, the iconoscope was to become the television tube.

The study of electron emission from various materials was also under way simultaneously. Heinrich Rudolph Hertz discovered the photoelectric effect in 1887. Studies on oxide emission began in 1903 and culminated in the famous thermionic emission equation given by Sir Owen Willians Richardson and S. Dushman in 1923. Schottky introduced electron emission under intense fields in 1918. These resulted in good cathodes for vacuum tubes. Also, a theoretical understanding of noise in vacuum tubes such as Schottky noise, flicker noise, and thermal noise was achieved.

World War II gave great impetus to electronic and radio engineering. Many reliable and excellent electronic circuits for a vast variety of applications were designed during this time.

The war needs such as radar, electronic guidance for weapons, aerospace and naval communication equipment, and general communication equipment required the use of microwave frequencies. At microwave frequencies, the conventional vacuum tubes, such as the triode and the pentode, did not work well, and this led to the development of klystrons, magnetrons, and traveling wave tubes.

When the transistor was invented in the 1950's, the use of vacuum tubes in electronic circuitry rapidly diminished as the transistor was much smaller in size, lighter in weight, and required less power consumption. It also led to miniaturization and better integration of large electronic circuits. No cooling circuits were needed. Yet the transistor could not deliver high powers. Therefore, vacuum tubes are still being used today for high-power applications, such as radar, microwave ovens, and broadcasting stations.

It is fair to say that almost all major applications of electronics and principles of design were first conceived in the vacuum-tube era and later reused for transistor technology. Even the first all-electronics computer was designed using vacuum tubes.

Efforts to develop vacuum microelectronic circuits, which use the field-emission concept, are meeting with success. The future study of vacuum tubes will be directly related to the physics and engineering of vacuum microelectronics. Some of the topics that will be pursued are switching speeds of the devices, their ability to operate at high and variable temperatures, field configurations at such short distances, resistance to radiation, and noise mechanisms.

Principal terms

EMITTER: also referred to as a cathode; in vacuum tubes, it is an electrode that emits electrons when heated, a phenomenon often referred to as thermionic emission

GRIDS: all electrodes other than the emitter and the plate in a vacuum tube; they are generally in the form of mesh, and depending on their functions, they are classified as control grid, screen grid, suppressor grid, and the like

MICROWAVE FREQUENCY SIGNALS: alternating voltages whose frequencies are in the range of 300 megahertz to 30 gigahertz

PLATE: also called an anode; in vacuum tubes, it collects electrons, emitted by the cathode as it is held at a voltage that is positive with respect to an emitter

RADIO FREQUENCY SIGNALS: alternating voltages whose frequencies are in the range of a few kilohertz to 30 megahertz

Bibliography

DeMaw, Doug, ed. THE RADIO AMATEUR'S HANDBOOK. Newington, Conn.: American Radio Relay League, 1970. This is the standard manual of amateur radio communication. Provides information on radio engineering. The material can be used for practical training and also as a quick reference. The treatment of several radio topics is essentially nonmathematical.

Fink, Donald G., ed. ELECTRONICS ENGINEERS' HANDBOOK. New York: McGraw-Hill, 1975. This book covers a variety of topics in electronics engineering. Of particular interest are chapters on amplifiers, oscillators, modulators, demodulators, and frequency converters. Also, the chapter on ultrahigh-frequency and microwave devices is valuable. The mathematical treatment is easy to follow. Examines vacuum tube technology with reference to other aspects of electronics in detail.

Millman, Jacob, and Christos C. Halkias. ELECTRONIC DEVICES AND CIRCUITS. New York: McGraw-Hill, 1967. This can be used as a first course in electronics for serious students. Millman and Halkias provide extensive physics-based discussion for each topic and also provide a good mathematical treatment. A popular book for undergraduates in physics and engineering. Many chapters are relevant.

Port, Otis. "Taming Space and Time to Make Tomorrow's Chips." BUSINESS WEEK, March 13, 1989, 68-74. This is a popular article on vacuum microelectronics and quantum chips with regard to the semiconductor industry and is written in a style that any layperson can understand. Covers the current state of work at various research laboratories in the United States and discusses the implications for semiconductor industry in the future.

Spangenberg, Karl R. VACUUM TUBES. New York: McGraw-Hill, 1948. Spangenberg drew upon his extensive experience as a teacher at Stanford University to provide a comprehensive treatment of vacuum tube physics and engineering. It is perhaps the most referenced book on vacuum tubes. There are extensive discussions, which cannot be found elsewhere, on topics such as electron emission, determination of fields in the tubes, space-charge effects, noise in the tubes, and special tubes such as octodes, heptodes, and hexodes.

Terman, Frederick Emmons. ELECTRONIC AND RADIO ENGINEERING. New York: McGraw-Hill, 1948. An important book on vacuum-tube techniques that emphasizes circuit design. Written by one of the influential figures in electronics education and industry; it contains extensive information on designing vacuum-tube circuits. Easily understandable and gives many principles and techniques of radio engineering for students and practicing engineers. The mathematical treatment of the topics is kept to a minimum.

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Electron Emission from Surfaces

Radio and Television