2-4
A third limitation caused by tube construction is TRANSIT TIME. Transit time is the time required
for electrons to travel from the cathode to the plate. While some small amount of transit time is required
for electrons to travel from the cathode to the plate, the time is insignificant at low frequencies. In fact,
the transit time is so insignificant at low frequencies that it is generally not considered to be a hindering
factor. However, at high frequencies, transit time becomes an appreciable portion of a signal cycle and
begins to hinder efficiency. For example, a transit time of 1 nanosecond, which is not unusual, is only
0.001 cycle at a frequency of 1 megahertz. The same transit time becomes equal to the time required for
an entire cycle at 1,000 megahertz. Transit time depends on electrode spacing and existing voltage
potentials. Transit times in excess of 0.1 cycle cause a significant decrease in tube efficiency. This
decrease in efficiency is caused, in part, by a phase shift between plate current and grid voltage.
If the tube is to operate efficiently, the plate current must be in phase with the grid-signal voltage and
180 degrees out of phase with the plate voltage. When transit time approaches 1/4 cycle, this phase
relationship between the elements does not hold true. A positive swing of a high-frequency grid signal
causes electrons to leave the cathode and flow to the plate. Initially this current is in phase with the grid
voltage. However, since transit time is an appreciable part of a cycle, the current arriving at the plate now
lags the grid-signal voltage. As a result, the power output of the tube decreases and the plate power
dissipation increases. Another loss of power occurs because of ELECTROSTATIC INDUCTION.
The electrons forming the plate current also electrostatically induce potentials in the grid as they
move past it. This electrostatic induction in the grid causes currents of positive charges to move back and
forth in the grid structure. This back and forth action is similar to the action of hole current in
semiconductor devices. When transit-time effect is not a factor (as in low frequencies), the current
induced in one side of the grid by the approaching electrons is equal to the current induced on the other
side by the receding electrons. The net effect is zero since the currents are in opposite directions and
cancel each other. However, when transit time is an appreciable part of a cycle, the number of electrons
approaching the grid is not always equal to the number going away. As a result, the induced currents do
not cancel. This uncancelled current produces a power loss in the grid that is considered resistive in
nature. In other words, the tube acts as if a resistor were connected between the grid and the cathode. The
resistance of this imaginary resistor decreases rapidly as the frequency increases. The resistance may
become so low that the grid is essentially short-circuited to the cathode, preventing proper operation of
the tube.
Several methods are available to reduce the limitations of conventional tubes, but none work well
when frequency increases beyond 1,000 megahertz. Interelectrode capacitance can be reduced by moving
the electrodes further apart or by reducing the size of the tube and its electrodes. Moving the electrodes
apart increases the problems associated with transit time, and reducing the size of the tube lowers the
power-handling capability. You can see that efforts to reduce certain limitations in conventional tubes are
compromises that are often in direct opposition to each other. The net effect is an upper limit of
approximately 1,000 megahertz, beyond which conventional tubes are not practical.
Q-1. What happens to the impedance of interelectrode capacitance as frequency increases?
Q-2. What undesirable effect is caused by the inductance of the cathode lead?
Q-3. How does transit time affect the relationship of the grid voltage and the plate current at high
frequencies?
Q-4. Moving tube electrodes apart to decrease interelectrode capacitance causes an increase in the
effect of what property?
