Triode
The triode, from the Greek τρίοδος, triodos, from tri (three) and hodós (ways), is an electronic amplification valve consisting of three electrodes arranged in the interior of an evacuated glass envelope: a heated filament or cathode, a grid, and a plate or anode. His invention ushered in the age of electronics, making amplified radio technology, long-distance telephony, and television possible. Triodes were widely used in consumer electronic devices such as radios and televisions until the 1960s, when transistors replaced them.
History
The first vacuum tube, the Fleming diode or valve, which had two electrodes (a glow filament, the cathode; and a collector plate, the anode), was invented by John Ambrose Fleming in 1904 as a detector for radio waves. The first three-element tube, a tube filled with mercury vapor with a control grid, was patented on March 4, 1906, by the Austrian physicist Robert von Lieben.
Independently, beginning in 1906, American engineer Lee De Forest invented and patented a three-element tube design, adding an electrode to the diode, which he initially called Device for Amplifying Weak Currents and then, for short, Audion, which is considered the first triode. The triode design, in which the grid sits between the filament and the plate, was patented on January 29, 1907. The Audion was a partial vacuum device with some low-pressure gas, as conceived by De Forest, but this gas caused erratic operation and shortened filament life. The Audion was initially used as a detector of wireless signals until in 1912, several people recognized that it could work as an amplifier, which led to its being used successfully to build the first radio receivers with amplification and electronic oscillators. One of the first successful inventors in the development of the device as an amplifier, it was the American inventor Fritz Lowenstein who applied for a patent on April 24, 1912 for his device which he called the "Telephone Relay". The many uses of this device for amplification motivated its rapid development. In 1913, improved versions with higher vacuum were developed at the American Telephone and Telegraph Company, which had acquired the rights to the Audion from Lee De Forest, and at General Electric, by inventor Irving Langmuir who named his tube "Pliotron". These were the first vacuum tube triodes. The name triode appeared later, when it became necessary to distinguish it from other types of vacuum tubes with more or fewer elements (diodes, tetrodes, pentodes, etc.)
The discovery of the amplifying ability of the triode in 1912 revolutionized electrical technology, creating the new field of electronics. "continuous wave" they replaced the inefficient 'damped wave' spark transmitters, allowing the transmission of sound by amplitude modulation (AM). Amplifying triode radio receivers, which could already drive loudspeakers, replaced weak galena crystal radios, which had to be listened to with high-impedance headphones. This led to the evolution of radio from a commercial message service to the first mass communication medium, with broadcasting beginning around 1920.
Triodes made transcontinental telephone service possible. Vacuum triode tube-based repeaters, invented at Bell Telephone after its purchase of the rights to the Audion tube, allowed telephone calls to travel beyond the unamplified limit of approximately 1,300 km. Bell's opening of the first transcontinental telephone line was celebrated on January 25, 1915. Other inventions made possible by the triode are television, public address systems, electric phonographs, and talkies.
The triode served as the technological base from which other vacuum tubes were later developed, such as the tetrode, invented by the German physicist Walter H. Schottky, of the Siemens company in 1916 and the pentode, of the Dutch inventors Gilles Holst and Bernardus Dominicus Hubertus Tellegen, of the Philips company in 1926, who corrected some of the deficiencies of the triode.
The triode was widely used in consumer electronics equipment such as radios, televisions, and audio systems until it was replaced in the 1960s by the transistor, invented in 1947, which ushered in the era of "vacuum tube" initiated by the triode will come to an end. Today, triodes are mainly used in high-power applications for which semiconductor devices are not suitable, such as radio transmitters and industrial heating equipment. However, in recent times, the triode and other vacuum tube devices have been experiencing a resurgence and comeback in hi-fi audio and music equipment thanks to manufacturing companies such as the South African Valve Audio, the American Pacific Creek and the Chinese Line Magnetic Audio among others.
Construction
All triodes have an electrode (cathode) heated by a filament, which releases electrons, and a flat metal plate electrode (anode) that attracts them, with a grid consisting of a screen of current-controlling wires. These are inside a sealed glass container in which an evacuation has been made, keeping the interior at a pressure of 10-9 atmospheres. Since the filament eventually melts down, the tube has a limited useful life and is designed as a replaceable unit; the electrodes are connected to terminal pins that plug into a receptacle. The operational life of a triode is around 2,000 hours for small tubes and 10,000 hours for power tubes.
Principle of operation
The voltage applied to the grid causes the flow of electrons from the cathode to the anode to be greater or less. This means that by applying a signal of very low intensity between the cathode and the grid, it is possible to achieve a very large variation in the flow of electrons between it and the anode. That is, with a small voltage a large current is controlled, which constitutes the phenomenon of amplification.
The element that emits electrons is called a cathode, but in doing so it acquires a positive polarity. In the simplest valves, this function is fulfilled by the same filament, which is the heating element. The third element, the grid, was introduced in 1906 by Lee de Forest.
The fundamental parameters of the triode and, in general, of all thermionic valves with three or more electrodes are:
- The Amplification Factor (μ) is defined as the quotient between the plate voltage and the grid voltage, keeping the plate current constant, when a signal is applied to the grid.
- Transconductance or Mutual Conductance (Gm) is the quotient between the plate current (Ia) and the grid voltage (Vg), keeping the plate voltage (Va) constant. This parameter is measured in mho, reverse unit of Oh.or siemens, although in practice the μmho or μsiemens. The value of the transductance depends on the point of the characteristic grid curve in which the valve is working. A high transconductance means that small modifications of the grid potential are translated into large variations of the plate current.
- Internal resistance (r)p), which is the quotient between the plate voltage (Va) and the plate current (Ia) while the grid voltage (Vg) is kept constant.
The three fundamental parameters of a triode are related by the expression:
μ μ =Gm⋅ ⋅ rp{displaystyle mu =G_{m}cdot r_{p}}}
These parameters are dynamic, that is, they depend on the polarization point and therefore they are constantly changing as the signal that is circulating through the device varies. The grid characteristic curve is also important, which is the diagram obtained with the plate or anode current intensity values as a function of the potentials applied to the grid.
Gallery of triodes
Several triodos.
Two ECC89 double triodes designed to amplify VHF.
Triodos assorted.
German oscillator triode.
