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A triode is an electronic amplifier vacuum tube (or thermionic valve in British English) consisting of three inside an evacuated glass envelope: a heated filament or cathode, a control grid, and a Plate electrode (anode).
Developed from Lee De Forest's 1906 Audion, a partial vacuum tube that added a grid electrode to the thermionic diode (Fleming valve), the triode was the first practical electronic amplifier and the ancestor of other types of vacuum tubes such as the tetrode and pentode. Its invention helped make amplified radio technology and long-distance telephony possible. Triodes were widely used in consumer electronics devices such as radios and televisions until the 1970s, when replaced them. Today, their main remaining use is in high-power Radio frequency amplifiers in Transmitter 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-type audio systems by audiophiles who prefer the sound of tube-based electronics.
The first vacuum tube used in radio was the thermionic diode or Fleming valve, invented by John Ambrose Fleming in 1904 as a detector for . It was an evacuated glass bulb containing two electrodes, a heated filament (cathode) and a plate (anode).
De Forest's Audion did not see much use until its ability to amplify was recognized around 1912 by several researchers, who used it to build the first successful amplifying radio receivers and electronic oscillators.. Republished as The many uses for amplification motivated its rapid development. By 1913 improved versions with higher vacuum were developed by Harold Arnold at American Telephone and Telegraph Company, which had purchased the rights to the Audion from De Forest, and Irving Langmuir at General Electric, who named his tube the "Pliotron", These were the first vacuum tube triodes. The name "triode" appeared later, when it became necessary to distinguish it from other kinds of vacuum tubes with more or fewer elements (diodes, , , etc.). There were lengthy lawsuits between De Forest and von Lieben, and De Forest and the Marconi Company, who represented John Ambrose Fleming, the inventor of the diode.James A. Hijiya, Lee de Forest and the Fatherhood of Radio Political, and Economic Development Lehigh University Press, 1992. , pages 93-94
The triode served as the technological base from which later vacuum tubes developed, such as the tetrode (Walter Schottky, 1916) and pentode (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 in consumer electronics such as radios, televisions, and until it was replaced in the 1960s by the transistor, 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 state semiconductor 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.
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 tubes increases capacitances, but this increase is offset by their overall reduced dimensions compared to lower-frequency tubes.
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 in amplifier. 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 the transconductance. 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 in voltage 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-channel JFET; 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 the voltage 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 the power gain, or the output power obtained from a certain AC input voltage is often of greater interest. When these devices are used as (or ), they all have a voltage "gain" of just under 1, but with a large current gain.
The triode was the first non-mechanical device to provide power gain at audio and radio frequencies, and made radio practical. Triodes are used for and oscillators. 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-marketed consumer electronics, having been overtaken by less expensive transistor-based solid-state devices. However, more recently, vacuum tubes have been making somewhat of a comeback. Triodes continue to be used in certain high-power Radio frequency amplifiers and . While proponents of vacuum tubes claim their superiority in areas such as high-end audio and professional audio applications, the solid-state MOSFET has similar performance characteristics.
In the example characteristic shown on the image, suppose we wish to operate it at a quiescent anode voltage Va of 200 V and a grid voltage bias of −1 V. This implies a quiescent plate (anode) current of 2.2 mA (using the yellow curve on the graph). In a class-A triode amplifier, one might place an anode resistor (connected between the anode and the positive power supply). If we choose Ra = 10000 Ω, the voltage drop on it would be V+ − Va = Ia × Ra = 22 V for the chosen anode current of Ia = 2.2 mA. Thus we require a power supply voltage V+ = 222 V in order to obtain Va = 200 V on the anode.
Now suppose we impress on the −1 V bias voltage a signal of 1 V peak-peak, so that the grid voltage varies between −0.5 V and −1.5 V. When Vg = −0.5 V, the anode current will increase to 3.1 mA, lowering the anode voltage to Va = V+ − 10 kΩ × 3.1 mA = 191 V (orange curve). When Vg = −1.5 V, the anode current will decrease to 1.4 mA, raising the anode voltage to Va = 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 about 17 V.
Thus voltage amplification of the signal is obtained. The ratio of these two changes, the voltage amplification factor (or mu) is 17 in this case. It is also possible to use triodes as cathode followers in which there is no voltage amplification but a huge reduction in dynamic impedance; in other words, the current is greatly amplified (as it also is in the common-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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