 Examples of low power triodes from 1918(left) to miniature tubes of the 1960s(right) | |
| Component type | Active |
|---|---|
| Working principle | Thermionic emission |
| Inventor | Lee de Forest |
| Invention year | 1908 |
| Pin names | Plate,grid, andcathode |
| Electronic symbol | |
 | |
Atriode is an electronicamplifyingvacuum tube (orthermionic valve inBritish English) consisting of threeelectrodes inside an evacuated glass envelope: a heatedfilament orcathode, agrid, and aplate (anode).
Developed fromLee De Forest's 1906Audion, a partial vacuum tube that added a grid electrode to thethermionic diode (Fleming valve), the triode was the first practicalelectronic amplifier and the ancestor of other types of vacuum tubes such as thetetrode andpentode. Its invention helped make amplifiedradio technology and long-distancetelephony possible.[1] Triodes were widely used inconsumer electronics devices such as radios and televisions until the 1970s, whentransistors replaced them. Today, their main remaining use is in high-powerRF amplifiers inradio transmitters and industrial RF heating devices. In recent years there has been a resurgence in demand for low power triodes due to renewed interest in tube-typeaudio equipment by audiophiles who prefer the sound of tube-based electronics.[2]
The name "triode" was coined by British physicistWilliam Eccles[3][4] some time around 1920, derived from theGreek τρίοδος,tríodos, fromtri- (three) andhodós (road, way), originally meaning the place where three roads meet.
Before thermionic valves were invented,Philipp Lenard used the principle of grid control while conducting photoelectric experiments in 1902.[5]
The firstvacuum tube used in radio[6][7] was thethermionic diode orFleming valve, invented byJohn Ambrose Fleming in 1904 as adetector forradio receivers. It was an evacuated glass bulb containing two electrodes, a heated filament (cathode) and a plate (anode).
Triodes came about in 1906 when American engineerLee de Forest[8] and Austrian physicistRobert von Lieben[9] independently patented tubes that added a third electrode, acontrol grid, between the filament and plate to control current.[10][11] Von Lieben's partially-evacuated three-element tube, patented in March 1906, contained a trace ofmercury vapor and was intended to amplify weak telephone signals.[12][13][14][9] Starting in October 1906[10] De Forest patented a number of three-element tube designs by adding an electrode to the diode, which he calledAudions, intended to be used as radio detectors.[15][8] The one which became the design of the triode, in which the grid was located between the filament and plate, was patented January 29, 1907.[16][8][17] Like the von Lieben vacuum tube, De Forest's Audions were incompletely evacuated and contained some gas at low pressure.[18][19] von Lieben's vacuum tube did not see much development due to his death seven years after its invention, shortly before the outbreak of theFirst World War.[20]
De Forest's Audion did not see much use until its ability to amplify was recognized around 1912 by several researchers,[19][1] who used it to build the first successful amplifying radio receivers andelectronic oscillators.[21][22] The many uses for amplification motivated its rapid development. By 1913 improved versions with higher vacuum were developed by Harold Arnold atAmerican Telephone and Telegraph Company, which had purchased the rights to the Audion from De Forest, andIrving Langmuir atGeneral Electric, who named his tube the "Pliotron",[19][1] These were the firstvacuum tube triodes.[18] The name "triode" appeared later, when it became necessary to distinguish it from other kinds of vacuum tubes with more or fewer elements (diodes,tetrodes,pentodes, etc.). There were lengthy lawsuits between De Forest and von Lieben, and De Forest and theMarconi Company, who representedJohn Ambrose Fleming, the inventor of the diode.[23]
The discovery of the triode's amplifying ability in 1912 revolutionized electrical technology, creating the new field ofelectronics,[1] the technology ofactive (amplifying) electrical devices. The triode was immediately applied to many areas of communication. During World War I,AM voicetwo way radio sets were made possible in 1917 (seeTM (triode)) which were simple enough that the pilot in a single seat aircraft could use it while flying. Triode "continuous wave"radio transmitters replaced the cumbersome inefficient "damped wave"spark-gap transmitters, allowing the transmission of sound byamplitude modulation (AM). Amplifying trioderadio receivers, which had the power to driveloudspeakers, replaced weakcrystal radios, which had to be listened to withearphones, allowing families to listen together. This resulted in the evolution of radio from a commercial message service to the firstmass communication medium, with the beginning ofradio broadcasting around 1920. Triodes made transcontinental telephone service possible. Vacuum tube trioderepeaters, invented atBell Telephone after its purchase of the Audion rights, allowed telephone calls to travel beyond the unamplified limit of about 800 miles. The opening by Bell of the first transcontinental telephone line was celebrated 3 years later, on January 25, 1915. Other inventions made possible by the triode weretelevision,public address systems, electricphonographs, andtalking motion pictures.
The triode served as the technological base from which later vacuum tubes developed, such as thetetrode (Walter Schottky, 1916) andpentode (Gilles Holst and Bernardus Dominicus Hubertus Tellegen, 1926), which remedied some of the shortcomings of the triode detailed below.
The triode was very widely used inconsumer electronics such as radios, televisions, andaudio equipment until it was replaced in the 1960s by thetransistor, invented in 1947, which brought the "vacuum tube era" introduced by the triode to a close. Today triodes are used mostly in high-power applications for which solid statesemiconductor devices are unsuitable, such as radio transmitters and industrial heating equipment. However, more recently the triode and other vacuum tube devices have been experiencing a resurgence and comeback in high fidelity audio and musical equipment. They also remain in use as vacuum fluorescent displays (VFDs), which come in a variety of implementations but all are essentially triode devices.
All triodes have a hotcathode electrode heated by afilament, which releases electrons, and a flat metalplate electrode (anode) to which the electrons are attracted, with agrid consisting of a screen of wires between them to control the current. These are sealed inside a glass container from which the air has been removed to a high vacuum, about 10−9 atm. Since the filament eventually burns out, the tube has a limited lifetime and is made as a replaceable unit; the electrodes are attached to terminal pins which plug into a socket. The operating lifetime of a triode is about 2000 hours for small tubes and 10,000 hours for power tubes.
Low power triodes have a concentric construction(see drawing right), with the grid and anode as circular or oval cylinders surrounding the cathode. Thecathode is a narrow metal tube down the center. Inside the cathode is afilament called the "heater" consisting of a narrow strip of high resistancetungsten wire, which heats the cathode red-hot (800 - 1000 °C). This type is called an "indirectly heated cathode". The cathode is coated with a mixture ofalkaline earth oxides such as calcium andthorium oxide which reduces itswork function so it produces more electrons. The grid is constructed of a helix or screen of thin wires surrounding the cathode. The anode is a cylinder or rectangular box of sheet metal surrounding the grid. It is blackened to radiate heat and is often equipped with heat-radiating fins. The electrons travel in a radial direction, from cathode through the grid to the anode. The elements are held in position bymica or ceramicinsulators and are supported by stiff wires attached to the base, where the electrodes are brought out to connecting pins. A "getter", a small amount of shinybarium metal evaporated onto the inside of the glass, helps maintain the vacuum by absorbing gas released in the tube over time.
High-power triodes generally use afilament which serves as the cathode (a directly heated cathode) because the emission coating onindirectly heated cathodes is destroyed by the higher ion bombardment in power tubes. Athoriated tungsten filament is most often used, in whichthorium added to the tungsten diffuses to the surface and forms a monolayer which increases electron emission. As the monolayer is removed by ion bombardment it is continually renewed by more thorium diffusing to the surface. These generally run at higher temperatures than indirectly heated cathodes. The envelope of the tube is often made of more durable ceramic rather than glass, and all the materials have higher melting points to withstand higher heat levels produced. Tubes with anode power dissipation over several hundred watts are usually actively cooled; the anode, made of heavy copper, projects through the wall of the tube and is attached to a large external finned metalheat sink which is cooled by forced air or water.
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A type of low power triode for use atultrahigh frequencies (UHF), the "lighthouse" tube, has a planar construction to reduce interelectrodecapacitance and leadinductance, which gives it the appearance of a "lighthouse". The disk-shaped cathode, grid and plate form planes up the center of the tube - a little like a sandwich with spaces between the layers. The cathode at the bottom is attached to the tube's pins, but the grid and plate are brought out to low inductance terminals on the upper level of the tube: the grid to a metal ring halfway up, and the plate to a metal button at the top. These are one example of "disk seal" design. Smaller examples dispense with the octal pin base shown in the illustration and rely on contact rings for all connections, including heater and D.C. cathode.
As well, high-frequency performance is limited by transit time: the time required for electrons to travel from cathode to anode. Transit time effects are complicated, but one simple effect is input conductance, also known as grid loading. At extreme high frequencies, electrons arriving at the grid may become out of phase with those departing towards the anode. This imbalance of charge causes the grid to exhibit a reactance that is much less than its low-frequency "open circuit" characteristic.
Transit time effects are reduced by reduced spacings in the tube. Tubes such as the 416B (a Lighthouse design) and the 7768 (an all-ceramic miniaturised design) are specified for operation to 4 GHz. They feature greatly reduced grid-cathode spacings of the order of 0.1 mm.
These greatly reduced grid spacings also give a much higher amplification factor than conventional axial designs. The 7768 has an amplification factor of 225, compared with 100 for the 6AV6 used in domestic radios and about the maximum possible for an axial design.
Anode-grid capacitance is not especially low in these designs. The 6AV6 anode-grid capacitance is 2 picofarads (pF), the 7768 has a value of 1.7 pF. The close electrode spacing used in microwave tubesincreases capacitances, but this increase is offset by their overall reduced dimensions compared to lower-frequency tubes.
In the triode,electrons are released into the tube from the metalcathode by heating it, a process calledthermionic emission. The cathode is heated red hot by a separate current flowing through a thin metalfilament. In some tubes the filament itself is the cathode, while in most tubes there is a separate filament which heats the cathode but is electrically isolated from it. The interior of the tube is wellevacuated so that electrons can travel between the cathode and the anode without losing energy in collisions with gas molecules. A positive DC voltage, which can be as low as 20V or up to thousands of volts in some transmitting tubes, is present on the anode. The negative electrons are attracted to the positively chargedanode (or "plate"), and flow through the spaces between the grid wires to it, creating a flow of electrons through the tube from cathode to anode.
The magnitude of this current can be controlled by a voltage applied on the grid (relative to the cathode). The grid acts like a gate for the electrons. A more negative voltage on the grid will repel more of the electrons, so fewer get through to the anode, reducing the anode current. A less negative voltage on the grid will allow more electrons from the cathode to reach the anode, increasing the anode current. Therefore, an input AC signal on the grid of a few volts (or less), even at a very high impedance (since essentially no current flows through the grid) can control a much more powerful anode current, resulting inamplification. When used in its linear region, variation in the grid voltage will cause an approximately proportional variation in the anode current; this ratio is called thetransconductance. If a suitable load resistance is inserted in the anode circuit, although the transconductance is somewhat lowered, the varying anode current will cause a varying voltage across that resistance which can be much larger than the input voltage variations, resulting involtage gain.
The triode is a normally "on" device; and current flows to the anode with zero voltage on the grid. The anode current is progressively reduced as the grid is made more negative relative to the cathode. Usually a constant DC voltage ("bias") is applied to the grid along with the varying signal voltage superimposed on it. That bias is required so that the positive peaks of the signal never drive the grid positive with respect to the cathode which would result in grid current and non-linear behaviour. A sufficiently negative voltage on the grid (usually around 3-5 volts in small tubes such as the 6AV6, but as much as –130 volts in early audio power devices such as the '45), will prevent any electrons from getting through to the anode, turning off the anode current. This is called the "cutoff voltage". Since beyond cutoff the anode current ceases to respond to the grid voltage, the voltage on the grid must remain above the cutoff voltage for faithful (linear) amplification as well as not exceeding the cathode voltage.
The triode is somewhat similar in operation to the n-channelJFET; it is normally on, and exhibits progressively lower and lower plate/drain current as the grid/gate is pulled increasingly negative relative to the source/cathode. Cutoff voltage corresponds to the JFET's pinch-off voltage (Vp) or VGS(off); i.e., the voltage point at which output current essentially reaches zero. This similarity is limited, however. The triode's anode current is highly dependent on anode voltage as well as grid voltage, thus limiting thevoltage gain. Because, in contrast, the JFET's drain current is virtually unaffected by drain voltage, it appears as a constant-current device, similar in action to a tetrode or pentode tube (high dynamic output impedance). Both the JFET and tetrode/pentode valves are thereby capable of much higher voltage gains than the triode which seldom exceeds 100. However thepower gain, or the output power obtained from a certain AC input voltage is often of greater interest. When these devices are used ascathode followers (orsource followers), they all have a voltage "gain" of just under 1, but with a largecurrent gain.
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Although S.G. Brown's Type G Telephone Relay (using a magnetic "earphone" mechanism driving a carbon microphone element) was able to give power amplification and had been in use as early as 1914, it was a purely mechanical device with limited frequency range and fidelity. It was suited only to a limited range of audio frequencies - essentially voice frequencies.[24]
The triode was the first non-mechanical device to provide power gain at audio and radio frequencies, and maderadio practical. Triodes are used foramplifiers andoscillators. Many types are used only at low to moderate frequency and power levels. Large water-cooled triodes may be used as the final amplifier in radio transmitters, with ratings of thousands of watts. Specialized types of triode ("lighthouse" tubes, with low capacitance between elements) provide useful gain at microwave frequencies.
Vacuum tubes are obsolete in mass-marketedconsumer electronics, having been overtaken by less expensive transistor-basedsolid-state devices. However, more recently, vacuum tubes have been making somewhat of a comeback. Triodes continue to be used in certain high-powerRF amplifiers andtransmitters. While proponents of vacuum tubes claim their superiority in areas such ashigh-end andprofessional audio applications, the solid-state MOSFET has similar performance characteristics.[25]
In triode datasheets, characteristics linking the anode current (Ia) to anode voltage (Va) and grid voltage (Vg) are usually given. From here, a circuit designer can choose theoperating point of the particular triode. Then the output voltage and amplification of the triode can be evaluated graphically by drawing aload line on the graph.
In the example characteristic shown on the image, suppose we wish to operate it at a quiescent anode voltageVa of200 V and a gridvoltage bias of−1 V. This implies a quiescent plate (anode) current of2.2 mA (using the yellow curve on the graph). In aclass-A triode amplifier, one might place an anode resistor (connected between the anode and the positive power supply). If we chooseRa = 10000 Ω, the voltage drop on it would beV+ −Va =Ia ×Ra = 22 V for the chosen anode current ofIa = 2.2 mA. Thus we require a power supply voltageV+ = 222 V in order to obtainVa = 200 V on the anode.
Now suppose we impress on the−1 V bias voltage a signal of1 V peak-peak, so that the grid voltage varies between−0.5 and −1.5 V. When Vg = −0.5 V, the anode current will increase to 3.1 mA, lowering the anode voltage toVa =V+ − 10 kΩ × 3.1 mA = 191 V (orange curve). WhenVg = −1.5 V, the anode current will decrease to1.4 mA, raising the anode voltage toVa =V+ − 10 kΩ × 1.4 mA = 208 V (green curve). Therefore a 1 V peak-peak signal on the input (grid) causes an output voltage change of about17 V.
Thus voltage amplification of the signal is obtained. The ratio of these two changes, thevoltage amplification factor (ormu) is 17 in this case. It is also possible to use triodes ascathode followers in which there is no voltage amplification but a huge reduction in dynamicimpedance; in other words, thecurrent isgreatly amplified (as it also is in thecommon-cathode configuration described above). Amplifying either the voltage or current results in power amplification, the general purpose of an amplifying tube (after all, either the current or voltage alone could be increased by decreasing the other just using a transformer, a passive device).
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