OPERATING CLASSIFICATIONS OF TUBE AMPLIFIERS

While the discussion of amplifiers will be covered in detail in later NEETS modules, some discussion of the classes of operation of an amplifier is needed at this point. This is because their operation class is directly determined by the bias voltage of the tube.

The classification of amplifiers by operation is based on the percentage of the time that the tube conducts when an input signal is applied. Under this system amplifiers may be divided into four main classes: A, AB, B, and C.

CLASS A OPERATION

An amplifier biased into Class A operation, is one in which conduction through the tube occurs throughout the duration of the input signal. Such an amplifier is shown in figure 1-27, view A. This is the same type of circuit with which you are already familiar. Notice when you compare the input to the output that the tube is always conducting, and that the entire input signal is reproduced at the output.

Figure 1-27. - Classes of amplifier operation.

CLASS AB OPERATION

The Class AB amplifier is one in which the tube conducts for more than half, but less than the entire input cycle.

View B of figure 1-27 depicts an amplifier biased into Class AB operation. Notice that in this application, grid bias has been increased to -9 volts. We will assume that the tube reaches cutoff when the voltage on the grid is -10 volts. Under these conditions, when the input reaches -10 volts, the tube will cut off and stay cut off until the input goes above -10 volts. The tube conducts during the entire duration of the positive alternation and part of the negative alternation. If you remember back in the discussion of distortion, we pointed out that this represents distortion. In some amplifiers, faithful reproduction of the input is not an important requirement. Class AB amplifiers are used only where this distortion can be tolerated.

CLASS B OPERATION

A Class B biased amplifier is one in which the tube will conduct for only half of the input signal duration. This is done by simply biasing the amplifier at cutoff. View C of figure 1-27 depicts a class B biased amplifier.

As you can see, the tube conducts on the positive alternations. As soon as the input signal voltage reaches 0 volts, the tube cuts off. The tube will remain cut off until the input signal voltage climbs above zero volts on the next positive alternation. Because the tube conducts during the entire positive alternation, but not on the negative alternation, the tube conducts for only half the input cycle duration.

CLASS C

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.

TUBE CONSTANTS

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 tube's operation are collectively called TUBE CONSTANTS. Four of the most important of these tube constants are: TRANSIENT TIME, INTERELECTRODE CAPACITANCE, TRANSCONDUCTANCE, and AMPLIFICATION FACTOR.

TRANSIT TIME

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.

Q.25 Match each amplifier characteristic listed below with its class of amplification.