Amplifier Potentials

Electronic amplifiers are characterized by the use of tubes having three or more elements. In triodes or three-element tubes, the addition of the third element, the grid, alters the voltage gradient between cathode and anode as shown in Fig. 102.

Fig. 102. Diode and triode voltage gradient.

The grid either aids or opposes the flow of electrons from cathode to anode, depending on whether the grid voltage is positive or negative respectively, compared to the cathode, which is shown at zero voltage in Fig. 102.

As the grid voltage is made more and more negative, electron flow is diminished and finally stops. At this point the anode current is zero; the condition is called anode current cut-off.

If the grid voltage is made more and more positive, eventually further increase in grid voltage causes no additional anode current increase. This condition is called grid saturation.

Tetrodes and pentodes have respectively two and three grids. The voltage gradient between cathode and anode is more complex than that indicated in Fig. 102. The advantages to be gained from the additional grids are mentioned below.