Audio Amplification (part ii)
The origin & evolution of electrical amplifiers
In the previous lecture we explored the trilling heritage of the early inventions and amplification cum recording devices invented. While they did their job pretty well but were inherently limited in their scope when it came to extent & quality of amplification or playback of sound.
During the same era of early 20th century, in 1906, just one year after Einstein revolutionary 4 papers on space time, Lee de Forest an American inventor, self-described “Father of Radio“, and a pioneer in the development of sound-on-film recording used for motion pictures also holding over 180 patents invented first practical device that could amplify audio signal in the form of the triode vacuum tube.
This further led to the development of first amplifiers around 1912.
The development of audio communication technology in form of the telephone, first patented in 1876, created the need to increase the amplitude of electrical signals to extend the transmission of signals over increasingly long distances.
This was the need of the moment which drove the development of amplifiers.
The Mystic Triode
The Novel Device which changed everything… of course in a good way!
Triode- as the name indicates, triode is an electronic amplifying vacuum tube (or valve in British English) consisting of three electrodes inside an evacuated glass envelope: a heated filament or cathode, a grid, and a plate (anode).
The hows and whats of Vacuum tubes
Given about a century of wealth of knowledge driven by countless experiments and research, despite getting too verbose, lets try to understand how these beautiful devices work.
The Fundamental Concept (Thermionic Emission)
In 1853, Edmond Becquerel[1][2], subsequently in 1873 Frederick Guthrie in Britain[3] while doing work on charged objects, Guthrie discovered that a red-hot iron sphere with a negative charge would lose its charge (by somehow discharging it into air). He also found that this did not happen if the sphere had a positive charge.[4]
The effect was rediscovered again by Thomas Edison on February 13, 1880, while he was trying to discover the reason for breakage of lamp filaments and uneven blackening (darkest near the positive terminal of the filament) of the bulbs in his incandescent lamps.
Edison built several experimental lamp bulbs with an extra wire, metal plate, or foil inside the bulb that was separate from the filament and thus could serve as an electrode. If the foil was put at a negative potential (Voltage) relative to the filament, there was no measurable current between the filament and the foil. When the foil was raised to a positive potential relative to the filament, there could be a significant current between the filament through the vacuum to the foil if the filament was heated sufficiently (by its own external power source).
This observation suggested that the filament was emitting electrons, which were attracted to a positively charged foil, but not a negatively charged one. This one-way current was called the Edison effect (although the term is occasionally used to refer to thermionic emission itself). He found that the current emitted by the hot filament increased rapidly with increasing voltage, and filed a patent application for a voltage-regulating device using the effect on November 15, 1883 (U.S. patent 307,031,[8] the first US patent for an electronic device).
This was a remarkable discovery/invention which actually is at play when we talk about TRIODE VACUUM TUBES.
Digging deeper into Triodes
We learned that metals when heated, emit electrons given the fact that in metals which are good conductors of electricity because of the availability of free (loosely held) electrons in them and this behavior is seen more naturally and easily in some metals and alloys than the other, we know that today given an array of experiments and successful inventions.
In a triode out of the three electrodes there is one which is made up of such a material which when heated by an external heater or by itself emits electrons and these electrons move out from the surface of it and enter the space above it which is essentially vacuum. (A Triode Vacuum tube is basically an assembly of three electrodes within an enclosure, typically glass, with vacuum inside). These emitted electrons assemble in the free space and as intuitively appears they crowd up the space and the region becomes negatively charged. Since electron emissions is sequential process in time, the earlier emitted electrons are pushed ahead by the electrons emitted later and the further emission becomes more an more difficult because, each electron repels each other, we know that like charges repel each other. Just like humans in a metro trains during rush hour. Unless there is an exit, it will become increasingly difficult for more electrons to get emitted from the heated electrode. A point is reached when there is saturation and if one electron is emitted one goes back to the cathode, hence the charge is limited and reaches to a saturation stage. This electrode since it emits negatively charged electrons is termed as CATHODE.
Space Charge & Crowding Phenomenon
Courtesy: John F. Rider (Inside Vacuum Tubes) 1945, Illustration by Baxter Rowe
Saturation of Space Charge
Courtesy: John F. Rider (Inside Vacuum Tubes) 1945, Illustration by Baxter Rowe
We haven’t yet talked about the other guy in the tube. One of the electrodes, typically placed at the opposite end of Cathode, if somehow is charged positively then it will start attracting the emitted electrons accumulated in the space between. This is what is actually done and this electrode is called as ANODE. A positive voltage is applied to this electrode and this causes the electrons to flow into this anode and thereby creating space in the space between the electrodes for the occupation by freshly emitted electrons and that’s how a stream of electrons flow. Obviously, its very easy to visualize that a negative charge on Anode will not lead to any current as when the electron reach very near, the repulsive electrostatic forces become infinitely strong as suggested by Coulomb’s Law (force between 2 charged bodies varies inversely to the square of distance between them)
Anode Current
Courtesy: John F. Rider (Inside Vacuum Tubes) 1945, Illustration by Baxter Rowe
We can at this point of time also term the Cathode as Emitter and Anode as Collector of electrons basis their jobs.
in 1906 De Forest, introduced the third Electrode calling it as Control Grid, this small sounding modification had far reaching effects and revolutionized everything.
Control Grid & Triode Structure
Courtesy: John F. Rider (Inside Vacuum Tubes) 1945, Illustration by Baxter Rowe
Looking closely at the illustration of triode above, we see that the Grid is in the form of a wire spiral sitting between the Cathode and the Plate (Anode is also called the Plate). Let us understand what it does and why it is called the Control grid, what does it control?
Answering just these 2 questions will clear some of the most profound inventions.
We have learned and visualized the space charge phenomenon happening within the tube due to thermionic emissions and also that anode when charge positively attracts these space charge. Imagine the situation when the grid is charged positively as well. What it will do is to pull or attract the higher density of space charge accumulated near the cathode and since the grid itself doesn’t have enough surface and is like a net, majority the accelerated electrons will move past it with a higher velocity and land into anode eventually. The grid will act like an accelerator and significantly increased flow of electrons and hence current emanating from the Anode. The converse will also happen, if the grid is made negatively charge by applying a negative voltage, it will further push the electrons back towards the cathode and will significantly reduce the number of electrons reaching out to the Anode.
We saw how, by applying a positive or negative voltage at the grid controlled the flow of electrons and hence current. This is the reason why its called a control grid.
There is a remarkable mechanical analogy to this entire operation. Imagine a pipe connected to a water filled tank with a valve fitted in between. As you turn the Valve Slowly on either direction the flow of water drastically changes. This is in the same manner as the action of Control Grid. Here water is the current equivalent, the tank is the cathode and the flow of water is the current, perhaps a bucket on the open end of pipe is Anode. This is the precise reason why tubes are also called as Valves.
Obviously you can design numerous types of such valves, some require more turns for closing and opening, some very sensitive which may open and close by just a little turn, such sensitive valves will lead to more change in flow by a small change in valve action. Similarly there are numerous Vacuum Tubes or Valves with different properties designed for various levels of Current change at anode at various levels of changes on the grid.
Simply Put, the ratio of change in the Anode current to the corresponding change in Grid is what is called as Gain of the Tube.
As we shall see subsequently that, this gain typically isn’t constant and actually varies at different levels of Grid and Anode voltage.
In a nutshell, we observed the remarkable capability exhibited by the triode. A small change (at grid) is greatly accentuated and should we say now “Amplified” (at anode).
In our Subsequent Lectures, we shall examine and learn in a deeper and practical operational aspects of Triodes.
