Valve amplifier thermionic valves. The simplest valve was invented by John Ambrose Fleming while working for the Marconi Company in London in 1904 and named the diode, as it had two electrodes. The diode conducted electricity in one direction only and was used as a radio detector and a rectifier. In 1906 Lee De Forest added a third electrode and invented the first electronic amplifying device, the triode, which he named the "Audion". This additional "control grid" modulates the current that flows between cathode and anode. The relationship between current flow and plate and grid voltage is often represented as a series
Triode 22.5V if an anode current of I = 2.25mA is chosen. If the input voltage amplitude (at the grid) changes from −1.5V to −0.5V (difference of 1V), the anode current will change from 1.2 to 3.3mA (see image). This will change the resistor voltage drop from 12 to 33V (a difference of 21V). Since the grid voltage changes from −1.5V to −0.5V, and the anode resistor voltage drops from 12 to 33 V, an amplification of the signal resulted. The amplification factor is 21: output voltage amplitude divided by input voltage amplitude. Triode A triode is an electronic amplifying vacuum