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High-Vacuum Thermionic Three-Electrode Tubes

Triodes were first used as amplifiers in 1906 by De Forest. The first major commercial application was as an amplifier in the repeaters1 of the first transcontinental telephone circuit, opened officially in 1915. Credit is due to Arnold and his Bell System associates for perfecting the triode.14

The characteristics of a small triode are shown in Fig. 16. Because

Figure 15. A cold-cathode gas diode connected to stabilize the voltage impressed across the load RL.

The grid of a triode is in a strategic location between the plate and the cathode, the grid has greater influence in controlling the plate current than the plate does. If the plate voltage is eb2 and the grid voltage is ec2, the plate current will be ib. If the plate voltage is reduced to eb1, and the grid voltage is reduced to ec1, the plate current remains ib. The amplification factor is defined1 as "the ratio of the change in plate voltage to a change in control-grid voltage, under the conditions that the plate current remains unchanged and all other electrode voltages are maintained constant." Thus, the application factor is

For triodes the amplification factors may be as low as about 3, and as high as about 100.

When a triode is amplifying a signal, such as speech or music, an alternating current flows from plate to cathode within the tube. The opposition to this signal-current flow is the alternating-current plate resistance1 of the tube. The plate resistance rp of a tube is the reciprocal of the slope of the curves of Fig. 16, and

characteristics of a high-vacuum triode
Figure 16. Characteristics of a small high-vacuum triode.

For triodes the plate resistance may be as low as about 1000 ohms, and as high as about 100,000 ohms.

When the triode is operated as a non-distorting amplifier, an alternating signal voltage is impressed between the grid and the cathode, and a corresponding signal current flows in the plate circuit. The control-grid-plate transconductance,1 or mutual conductance, determines the magnitude of the plate current caused by grid-voltage changes. This factor can be found by dividing equation 4 by equation 5; hence,

and the unit of measure is the mho, or micromho.



Last Update: 2011-05-30