CLASS C amplifiers are biased below cutoff, so that the tube will conduct for less than half of the
input signal cycle duration. View D of figure 1-27 depicts a Class C amplifier. Notice that the tube is
biased one-half volt below cutoff. The tube will only conduct on that part of the positive alternation that is
above +.5 volts. Therefore, the tube conducts for less than one-half cycle of the input. Again, this class
can be applied only where severe distortion can be tolerated.
In the discussion of triodes, we only considered the effects of the external circuit on the passage of
current through the tube. The behavior of the electron stream in a conducting tube is also influenced by
the physical structure of the tube. The effects that the physical structure of a tube has on the tubes
operation are collectively called TUBE CONSTANTS. Four of the most important of these tube
constants are: TRANSIENT TIME, INTERELECTRODE CAPACITANCE,
TRANSCONDUCTANCE, and AMPLIFICATION FACTOR.
Unlike electron flow in a conductor, electrons in a vacuum tube do not move at the speed of light.
Their velocity is determined by the potential difference between the plate and the cathode. The amount of
time the electrons take to travel from the cathode to the plate is called TRANSIT TIME. As a result of
this time difference, the appearance of a signal at the end of a tube is not followed instantaneously by a
change in current flow in the tube. Under normal conditions, the effect of this small time lag between the
input signal and a change in tube current is unnoticed. However, at frequencies such as those used in radar
equipment, this is not the case. Transit time at these frequencies has a very marked effect on tube
operation. It is a major factor that limits the use of a given tube at higher frequencies.
Match each amplifier characteristic listed below with its class of amplification.
a. Current flows through the tube for one-half cycle.
b. Current flows through the tube for less than one-half cycle.
Current flows through the tube for the entire cycle.
MU AND TRANSCONDUCTANCE
In your study of triodes so far, you have seen that the output of a triode circuit is developed across
the tube. The output is caused by the voltage dropped across RL due to current flow from tube conduction.
In all the demonstrations of gain, we assumed that RL was held constant and current through the tube was
varied. In this manner we achieved a voltage gain. If the resistance of RL is changed by the designer, the
gain of a triode circuit can be either increased or decreased. This is fairly easy to understand. Assume that
a circuit is composed of a triode with a plate-load resistor of 100 kohms. If a +2 volt signal causes 2
additional milliamperes to conduct through the tube, the voltage drop across RL (the output) will be: