Overview of vacuum electron triode

Forrest's vacuum triode is based on the technology of vacuum diode invented by his predecessors. In 1904, Fleming of London University invented the vacuum diode. The vacuum diode can only conduct electricity in one direction, and can rectify AC current or detect signals, but it cannot amplify signals. Without devices that can amplify signals, electronic technology cannot continue to develop.

Fleming's vacuum diode
In order to improve the sensitivity of vacuum diode detection, Forrest added a fence-like metal mesh inside the glass tube to form the third pole of the electron tube. He was surprised to see that this "grid" was like a shutter, which could control the flow of electrons between the cathode and the screen; as long as a weak current passed through the grid, a larger current could be obtained on the screen, and the waveform was exactly the same as the grid current. In other words, adding an electrode to Fleming's vacuum diode made it a new device that could amplify the electrons. He named this new device triode.

Principle of vacuum triode

In addition to being in the "amplification" state, the vacuum triode can also be in the "saturation" and "cut-off" states. "Saturation" means that the current from the cathode (or emitter) to the screen (evelope) is fully conductive, which is equivalent to turning on the switch; "cut-off" means that no current flows from the cathode to the screen, which is equivalent to turning off the switch. The two states can be controlled by adjusting the voltage on the gate. Therefore, the vacuum triode can act as a switching device, and its speed is thousands of times faster than that of a relay.

After Forrest successfully developed the vacuum triode, he also made improvements to the vacuum tetrode and vacuum pentode, which, together with vacuum diodes and vacuum triodes, are collectively called electron tubes.

Various electron tubes

The vacuum triode paved the way for the birth of computers. In the world's first electronic computer ENIAC, the vacuum tube was its most basic component. The huge size and huge power consumption of the vacuum tube are two fatal flaws, so it will be replaced by small and exquisite semiconductor devices. But in analog circuits, the high-fidelity amplification characteristics of the vacuum tube still make it pale in comparison with transistors and integrated circuits. To this day, tube amplifiers with vacuum tubes as the core components are still the goal pursued by audiophiles. Just a few years ago, we could still see traces of vacuum tubes on feverish motherboards.

Aopen audiophile motherboard AX4B-533 Tube with dual triode tubes (September 2002)

Forrest loved to do things with his hands since he was a child. He made many small productions and inventions. After he started working, he became a prolific inventor and obtained more than 300 patents in his lifetime. Among all of Forrest's inventions, the vacuum triode had the greatest impact. It not only became the beginning of vacuum electronics, but also the beginning of the history of electronics, and promoted the progress of human civilization.

On the occasion of the centenary of the vacuum triode, can we, engineers who make a living from electronic design, gain some inspiration from the invention of the vacuum triode?

PS: Like vacuum tubes, the display tube of a CRT monitor is also an electric vacuum device. In order to prevent the filament from oxidizing and burning at high temperatures, the air inside must be evacuated. Incandescent lamps and daylight lamps are filled with inert gas to prevent the filament from oxidizing quickly.