A vacuum tube consists of electrodes in a vacuum in an insulating heat-resistant envelope which is usually tubular. Many tubes have glass envelopes, though some types such as power tubes may have ceramic or metal envelopes. The electrodes are attached to leads which pass through the envelope via an airtight seal. On most tubes, the leads are designed to plug into a tube socket for easy replacement.
The simplest vacuum tubes resemble incandescent light bulbs in that they have a filament sealed in a glass envelope which has been evacuated of all air. When hot, the filament releases electrons into the vacuum: a process called thermionic emission. The resulting negatively charged cloud of electrons is called a space charge. These electrons will be drawn to a metal plate inside the envelope, if the plate (also called the anode) is positively charged relative to the filament (or cathode). The result is a flow of electrons from filament to plate. This cannot work in the reverse direction because the plate is not heated and does not emit electrons. This very simple example described can thus be seen to operate as a diode: a device that conducts current only in one direction. The vacuum tube diode conducts conventional current from plate (anode) to the filament (cathode); this is the opposite direction to the flow of electrons (called electron current).
Vacuum tubes require a large temperature difference between the hot cathode and the cold anode. Because of this, vacuum tubes are inherently power-inefficient; enclosing the tube within a heat-retaining envelope of insulation would allow the entire tube to reach the same temperature, resulting in electron emission from the anode that would counter the normal one-way current. Because the tube requires a vacuum to operate, convection cooling of the anode is not generally possible unless the anode forms a part of the vacuum envelope (in which case the cooling is by conduction through the anode material and then convection outside the vacuum envelope). Thus anode cooling occurs in most tubes through black-body radiation and conduction of heat to the outer glass envelope via the anode mounting frame. Cold cathode tubes do not rely on thermionic emission at the cathode and usually have some form of gas discharge as the operating principle; such tubes are used for lighting (neon lamps) or as voltage regulators.
Sometimes another electrode, called a control grid, is added between the cathode and the anode. The vacuum tube is then known as a "triode." A triode is a voltage-controlled device, in that a voltage that is applied as an input to the grid can be used to modulate the rate of electron flow between anode and cathode. The relationship between this input voltage and the output current is determined by a transconductance function. Control grid current is practically negligible in most circuits. The solid-state device most closely analogous to the vacuum tube is the JFET, although the vacuum tube typically operates at far higher voltage (and power) levels than the JFET.
[edit] History and development
The 19th century saw increasing research with evacuated tubes, such as the Geissler and Crookes tubes. Scientists who experimented with such tubes included Eugen Goldstein, Nikola Tesla, Johann Wilhelm Hittorf, Thomas Edison, and many others. These tubes were mostly for specialized scientific applications, or were novelties, with the exception of the light bulb. The groundwork laid by these scientists and inventors, however, was critical to the development of vacuum tube technology.
Though the thermionic emission effect was originally reported in 1873 by Frederick Guthrie, it is Thomas Edison's 1884 investigation of the Edison Effect that is more often mentioned. Edison patented what he found,[1] but he did not understand the underlying physics, or the potential value of the discovery. It wasn't until the early 20th century that this effect was put to use, in applications such as John Ambrose Fleming's diode used as a radio detector, and Lee De Forest's 1906 "audion" (soon improved by others as the triode in 1908) used in the first telephone amplifiers. These developments led to great improvements in telecommunications technology, particularly the first coast-to-coast telephone line in the US, and the birth of broadcast radio.
Diodes and triodes
The English physicist John Ambrose Fleming worked as an engineering consultant for firms, including Edison Telephone and the Marconi Company. In 1904, as a result of experiments conducted on Edison effect bulbs imported from the USA, he developed a device he called an "oscillation valve" (because it passes current in only one direction).
Later known as the Fleming valve, it could be used as a rectifier of alternating current and as a radio wave detector.
In 1906 Robert von Lieben filed[2] for a three-electrode amplifying vacuum tube. His invention also included a beam-focusing electromagnet.
In 1907 Lee De Forest placed a bent wire serving as a screen, later known as the "grid" electrode, between the filament and plate electrode. As the voltage applied to the grid was varied from negative to positive, the number of electrons flowing from the filament to the plate would vary accordingly. Thus the grid was said to electrostatically "control" the plate current. The resulting three-electrode device was therefore an excellent and very sensitive amplifier of voltages. De Forest called his invention the "Audion". In 1907, De Forest filed[3] for a three-electrode version of the Audion for use in radio communications. The device is now known as the triode. De Forest's device was not strictly a vacuum tube, but clearly depended for its action on ionisation of the relatively high levels of gas remaining after evacuation. The De Forest company, in its Audion leaflets, warned against operation which might cause the vacuum to become too hard. The Finnish inventor Eric Tigerstedt significantly improved on the original triode design in 1914, while working on his sound-on-film process in Berlin, Germany. The first true vacuum triodes were the Pliotrons developed by Irving Langmuir at the General Electric research laboratory (Schenectady, New York) in 1915. Langmuir was one of the first scientists to realize that a harder vacuum would improve the amplifying behaviour of the triode. Pliotrons were closely followed by the French 'R' Type which was in widespread use by the allied military by 1916. These two types were the first true vacuum tubes. Historically, vacuum levels in production vacuum tubes typically ranged between 10 µPa to 10 nPa.
The non-linear operating characteristic of the triode caused early tube audio amplifiers to exhibit harmonic distortions at low volumes. This is not to be confused with the overdrive that tube amplifiers exhibit at high volume levels (known as the tube sound). To remedy the low-volume distortion problem, engineers plotted curves of the applied grid voltage and resulting plate currents, and discovered that there was a range of relatively linear operation. In order to use this range, a negative voltage had to be applied to the grid to place the tube in the "middle" of the linear area with no signal applied. This was called the idle condition, and the plate current at this point the "idle current". Today this current would be called the quiescent or standing current. The controlling voltage was superimposed onto this fixed voltage, resulting in linear swings of plate current for both positive and negative swings of the input voltage. This concept was called grid bias.
[edit] Tetrodes and pentodes
When triodes were first used in radio transmitters and receivers, it was found that they had a tendency to oscillate due to parasitic anode-to-grid capacitance. Many circuits were developed to reduce this problem (e.g. the Neutrodyne amplifier), but proved unsatisfactory over wide ranges of frequencies. It was discovered that the addition of a second grid, located between the control grid and the plate and called a screen grid could solve these problems. ("Screen" implies shielding, not physical construction.) A positive voltage slightly lower than the plate voltage was applied to it, and the screen grid was bypassed (for high frequencies) to ground with a capacitor. This arrangement decoupled the anode and the first grid, completely eliminating the oscillation problem. An additional side effect of this second grid is that the Miller capacitance is also reduced, which improves gain at high frequency. This two-grid tube is called a tetrode, meaning four active electrodes, and was common by 1926.
However, the tetrode has some new problems. In any tube, electrons strike the anode hard enough to knock out secondary electrons. In a triode these (less energetic) electrons cannot reach the grid or cathode, and are re-captured by the anode. But in a tetrode, they can be captured by the second grid, reducing the plate current and the amplification of the circuit. Since secondary electrons can outnumber the primary electrons, in the worst case, particularly when the plate voltage dips below the screen voltage, the plate current can actually go down with increasing plate voltage. This is negative-resistance behavior.[4] This is the "tetrode kink" (see the reference for a plot of this effect in the RCA-235 tetrode). Another consequence of this effect is that under severe overload, the current collected by the screen grid can cause it to overheat and melt, destroying the tube.
Again, the solution was to add another grid, called a suppressor grid. This third grid was biased at either ground or cathode voltage and its negative voltage (relative to the anode) electrostatically suppressed the secondary electrons by repelling them back toward the anode. This three-grid tube is called a pentode, meaning five electrodes. The pentode was invented in 1928 by Bernard D. H. Tellegen.
A related type is the beam tetrode, discussed later in this article.
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