Voltage Amplification Constant

Author: J.B. Hoag

Because the grid is located in the middle of the space charge, its potential proves to be very effective in controlling the number of electrons which reach the plate; much more so in fact than the potential of the plate. The ratio of effectiveness of changes of the grid potential and of the plate potential in changing the plate current is called the voltage amplification constant of the tube.

As indicated by the line AB of Fig. 12 B, a change of 10 volts on the plate may be counteracted by a change of opposite polarity of only 1 volt on the grid (-3 to -4). The ratio of 10 to 1 is the voltage amplification constant in this particular case.

In order to be more accurate, let us now write down the equation for this important vacuum tube constant, namely,

Thus the accurate definition of μ is: the ratio of the change of plate voltage (E_p — E'_p) to the change in grid voltage (E'_g — E_g) for zero change in the plate current (I'_p —I_p = 0).

The amplification constant may be obtained from the characteristic curves or it may be measured directly by means of an instrument called a tube tester, whose details can be found in any of the more advanced treatises on radio.

The value of this constant is different for different tubes according to their structural details, and ranges from 1 to 100 for the various commercial triodes, having a usual value of about 10.