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FUNDAMENTAL
PROPERTIES OF VACUUM TUBES
The
major operating characteristics of a vacuum tube can be
expressed in terms of the amplification factor (µ),
the dynamic plate resistance (Rr) and the mutual conductance
(GM). When these are known one can
make quantitative calculations of the tube performance under
many conditions.
The Amplification Factor is defined as the
ratio of a small increment in plate voltage to the corresponding
change in grid voltage necessary to maintain constant plate
current. In other words, it is the ratio of the effectiveness
of the grid and plate voltages in producing electrostatic
forces at the surface of the cathode. The amplification
factor depends upon the configuration of the electrode system,
especially the grid structure, and the electrode voltages.
Changes which cause the grid to more completely shield the
plate from the cathode will increase the value of µ.
The dynamic Plate Resistance may be defined
as the ratio of a small change in plate voltage to the corresponding
change in plate current produced. The value will depend
upon the grid and plate voltages at the operating point
under consideration. It will not be equal to the ratio of
total plate voltage to total plate current. The dimensions
and relative positions of the tube electrodes will largely
determine the value of plate resistance.
The Mutual Conductance (GM),
sometimes called control grid-plate transconductance (SM
), is the ratio of the amplification factor to the plate
resistance and represents the rate of change in plate current
with respect to the change in grid voltage when the other
voltages remain constant.
Interelectrode Capacities: The electrodes
of a vacuum tube form a complicated electrostatic system,
and each element may be considered as forming one plate
of a small condenser. In a three-element tube the capacitance
between the cathode and grid, between the grid and plate,
and between the plate and cathode, are known as the interelectrode
capacitances of the tube. Of these, the grid-plate capacity
is generally the most important. The effect of these capacitances
depends upon the relationship between their reactances and
the associated external circuit impedances. Their effect
is, therefore, a function of frequency and external load.
In multi-electrode tubes the number of separate interelectrode
capacitances is larger than for a triode. Fortunately, only
three of these direct interelectrode capacitances are of
great importance in most applications. These are:
1. Grid-plate capacity (CGP).
2. Direct input capacity from control grid to cathode plus
all other electrodes except output plate.
3. Direct output capacity from plate to cathode plus all
other electrodes except the input grid.
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