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Vacuum tubes were the first active elements and they had a tremendous role in the development of the field
of electronics
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The English physicist John Ambrose Fleming was the inventor of the first rectifying tube which was
developed in 1904. It was essentially an incandescent light bulb with an added electrode inside.
When the bulb's filament is heated white-hot, electrons are "boiled" off its surface and into the
vacuum inside the bulb. If the electrode -- called a "plate" or "anode" -- is made more positive than
the hot filament, a direct current flows through the vacuum to the electrode (a demonstration of the
Edison effect).
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The first amplifying tube was invented by the American electrical engineer Lee De Forest in 1906. It
differs form the diode only in the third electrode called the grid.
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Later was invented the tetrode to overcome stability problems and limited voltage gain due to the Miller
effect. One more grid was added between the grid and the anode, known as the screen grid.
The pentode was invented to solve a new problem in the tetrode called "tetrode kink" which caused
instability. The solution was to add another grid between the screen grid and the main anode, called the
suppressor grid.
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Vacuum tubes suffered a slow death during the 1950s and '60s thanks to the invention of the transistor—
specifically, the ability to mass-produce transistors by chemically engraving, or etching, pieces of silicon.
Transistors were smaller, cheaper, and longer lasting. They could also be packed into microchips to switch
on and off according to different, complex inputs, paving the way for smaller, more powerful computers.
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Why transistors are better
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Usually lower cost than tubes, especially in small-signal circuits
Smaller than equivalent tubes
Can be combined in one die to make integrated circuit
Lower power consumption than equivalent tubes, especially in small-signal circuits
Less waste heat than equivalent tubes
Can operate on low-voltage supplies, greater safety, lower component costs, smaller clearances
Matching transformers not required for low-impedance loads
more physical ruggedness than tubes (depends on chassis construction)
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Tube advantages
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Highly linear without negative feedback, specially some small-signal types
Clipping is smooth, which is widely considered more musical than transistors
Tolerant of overloads and voltage spikes
Characteristics highly independent of temperature, greatly simplifies biasing
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Wider dynamic range than typical transistor circuits, thanks to higher operating voltages
Device capacitances vary only slightly with signal voltages
Capacitive coupling can be done with low-value, high-quality film capacitors
Circuit designs tend to be simpler than semiconductor equivalents
Operation is usually in Class A or AB, which minimizes crossover distortion
Output transformer in power amp protects speaker from tube failure
Maintenance tends to be easier because user can replace tubes
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Tube sound
Audiophiles may agree or disagree on the relative merits of tube vs solid state amplification. Some say they
prefer the sound produced from tube amplifiers on the grounds that it is more natural and satisfying than the
sound from transistor amplifiers.
The key is in the harmonics distortion prodused by the active element. The tubes produce more low-order
even harmonics distortion which is perceived as pleasant by the listener. On the other hand the transistor
produces more higher order uneven harmonics which that are perceived as unpleasant.
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Modern vacuum tube application
One of the most popular applications is in the audio equipment.
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Vacuum fluorescent display is another popular application
VFDs have the advantages of being rugged, inexpensive, and easily configured to display a wide variety of
customized messages, and unlike LCDs, VFDs are not limited by the response time of rearranging liquid
crystals and are thus able to function normally in cold, even sub-zero, temperatures, making them ideal for
outdoor devices in cold climates. Early on, the main disadvantage of such displays was their use of
significantly more power (0.2 watts) than a simple LCD. This was considered a significant drawback for
battery-operated equipment like calculators, so VFDs ended up being used mainly in equipment powered by
an AC supply or heavy-duty rechargeable batteries.
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The cavity magnetron is a high-powered vacuum tube that generates microwaves using the interaction of a
stream of electrons with a magnetic field. The high power of pulses from the cavity magnetron made
centimeter-band radar practical, with shorter wavelength radars allowing detection of smaller objects. At
present, cavity magnetrons are commonly used in microwave ovens and in various radar applications.
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A klystron is a specialized used as an amplifier for high frequencies, from UHF radio frequencies up into the
microwave range. Klystron amplifiers have the advantage (over the magnetron) of coherently amplifying a
reference signal so its output may be precisely controlled in amplitude, frequency and phase.
Klystrons can produce far higher microwave power outputs than solid state microwave devices such as
Gunn diodes. In modern systems, they are used from UHF (hundreds of MHz) up through hundreds of
gigahertz. Klystrons can be found at work in radar, satellite and wideband high-power communication (very
common in television broadcasting and EHF satellite terminals), medicine (radiation oncology), and highenergy physics (particle accelerators and experimental reactors). At SLAC, for example, klystrons are
routinely employed which have outputs in the range of 50 megawatts (pulse) and 50 kilowatts (timeaveraged) at 2856MHz.
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A traveling-wave tube (TWT) is a specialised vacuum tube that is used in electronics to amplify radio
frequency (RF) signals to high power. The bandwidth of a broadband TWT can be as high as one octave,
although tuned (narrowband) versions exist, and operating frequencies range from 300 MHz to 50 GHz. The
power gain of the tube is on the order of 70 decibels.
TWTAs are commonly used as amplifiers in satellite transponders, where the input signal is very weak and
the output needs to be high power.
A TWTA whose output drives an antenna is a type of transmitter. TWTA transmitters are used extensively
in radar, particularly in airborne fire-control radar systems, and in electronic warfare and self-protection
systems.
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CRT
No native resolution; No input lag No ghosting and smearing artifacts during fast motion due to submillisecond response time Near zero color, saturation, contrast or brightness distortion. Excellent viewing
angle. Can be used or stored in both extreme hot and cold temperature conditions without harm to the
system.
CRTs are still popular in the printing and broadcasting industries as well as in the professional video,
photography, and graphics fields due to their greater color fidelity, contrast, and better viewing from offaxis.
CRT monitors are still widely used in the study of the brain's visual processing (e.g. in psychophysics). The
speed and fidelity of their response, combined with the simplicity of their design, makes them well-suited
for experiments where scientists need to have very fine control over stimuli which are presented to an
observer.